Stacking Free Energies of All DNA and RNA Nucleoside Pairs and Dinucleoside-Monophosphates Computed Using Recently Revised AMBER Parameters and Compared with Experiment

Sensing the structural and conformational properties of single-stranded nucleic acids using electrometry and molecular simulations

Inferring the 3D structure and conformation of disordered biomolecules, e.g., single stranded nucleic acids (ssNAs), remains challenging due to their conformational heterogeneity in solution. Here, we use escape-time electrometry (ETe) to measure with sub elementary-charge precision the effective electrical charge in solution of short to medium chain length ssNAs in the range of 5-60 bases. We compare measurements of molecular effective charge with theoretically calculated values for simulated molecular conformations obtained from Molecular Dynamics simulations using a variety of forcefield descriptions. We demonstrate that the measured effective charge captures subtle differences in molecular structure in various nucleic acid homopolymers of identical length, and also that the experimental measurements can find agreement with computed values derived from coarse-grained molecular structure descriptions such as oxDNA, as well next generation ssNA force fields. We further show that comparing the measured effective charge with calculations for a rigid, charged rod-the simplest model of a nucleic acid-yields estimates of molecular structural dimensions such as linear charge spacings that capture molecular structural trends observed using high resolution structural analysis methods such as X-ray scattering. By sensitively probing the effective charge of a molecule, electrometry provides a powerful dimension supporting inferences of molecular structural and conformational properties, as well as the validation of biomolecular structural models. The overall approach holds promise for a high throughput, microscopy-based biomolecular analytical approach offering rapid screening and inference of molecular 3D conformation, and operating at the single molecule level in solution.

Competition between Stacking and Divalent Cation-Mediated Electrostatic Interactions Determines the Conformations of Short DNA Sequences

Interplay between divalent cations (Mg2+ and Ca2+) and single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA), as well as stacking interactions, is important in nucleosome stability and phase separation in nucleic acids. Quantitative techniques accounting for ion-DNA interactions are needed to obtain insights into these and related problems. Toward this end, we created a sequence-dependent computational TIS-ION model that explicitly accounts for monovalent and divalent ions. Simulations of the rigid 24 base-pair (bp) dsDNA and flexible ssDNA sequences, dT30 and dA30, with varying amounts of the divalent cations show that the calculated excess number of ions around the dsDNA and ssDNA agree quantitatively with ion-counting experiments. Using an ensemble of all-atom structures generated from coarse-grained simulations, we calculated the small-angle X-ray scattering profiles, which are in excellent agreement with experiments. Although ion-counting experiments mask the differences between Mg2+ and Ca2+, we find that Mg2+ binds to the minor grooves and phosphate groups, whereas Ca2+ binds specifically to the minor groove. Both Mg2+ and Ca2+ exhibit a tendency to bind to the minor groove of DNA as opposed to the major groove. The dA30 conformations are dominated by stacking interactions, resulting in structures with considerable helical order. The near cancellation of the favorable stacking and unfavorable electrostatic interactions leads to dT30 populating an ensemble of heterogeneous conformations. The successful applications of the TIS-ION model are poised to confront many problems in DNA biophysics.

RmsdXNA: RMSD prediction of nucleic acid-ligand docking poses using machine-learning method

Small molecule drugs can be used to target nucleic acids (NA) to regulate biological processes. Computational modeling methods, such as molecular docking or scoring functions, are commonly employed to facilitate drug design. However, the accuracy of the scoring function in predicting the closest-to-native docking pose is often suboptimal. To overcome this problem, a machine learning model, RmsdXNA, was developed to predict the root-mean-square-deviation (RMSD) of ligand docking poses in NA complexes. The versatility of RmsdXNA has been demonstrated by its successful application to various complexes involving different types of NA receptors and ligands, including metal complexes and short peptides. The predicted RMSD by RmsdXNA was strongly correlated with the actual RMSD of the docked poses. RmsdXNA also outperformed the rDock scoring function in ranking and identifying closest-to-native docking poses across different structural groups and on the testing dataset. Using experimental validated results conducted on polyadenylated nuclear element for nuclear expression triplex, RmsdXNA demonstrated better screening power for the RNA-small molecule complex compared to rDock. Molecular dynamics simulations were subsequently employed to validate the binding of top-scoring ligand candidates selected by RmsdXNA and rDock on MALAT1. The results showed that RmsdXNA has a higher success rate in identifying promising ligands that can bind well to the receptor. The development of an accurate docking score for a NA-ligand complex can aid in drug discovery and development advancements. The code to use RmsdXNA is available at the GitHub repository https://github.com/laiheng001/RmsdXNA.

van der Waals Parameter Scanning with Amber Nucleic Acid Force Fields: Revisiting Means to Better Capture the RNA/DNA Structure through MD

Molecular dynamics simulations can be used in combination with experimental techniques to uncover the intricacies of biomolecular structure, dynamics, and the resulting interactions. However, many noncanonical nucleic acid structures have proven to be challenging to replicate in accurate agreement with experimental data, often attributed to known force field deficiencies. A common force field criticism is the handling of van der Waals (vdW) parameters, which have not been updated since the regular use of Ewald's methods became routine. This work dives into the effects of minute vdW radii shifts on RNA tetranucleotide, B-DNA, and Z-DNA model systems described by commonly used Amber force fields. Using multidimensional replica exchange molecular dynamics (M-REMD), the GACC RNA tetranucleotide demonstrated changes in the structural distribution between the NMR minor and anomalous structure populations based on the O2' vdW radii scanning. However, no significant change in the NMR Major conformation population was observed. There were minimal changes in the B-DNA structure but there were more substantial improvements in Z-DNA structural descriptions, specifically with the Tumuc1 force field. This occurred with both LJbb vdW radii adjustments and incorporation of the CUFIX nonbonded parameter modifications. Though the limited vdW modifications tested did not provide a universal fix to the challenge of simulating the various known nucleic acid structures, they do provide direction and a greater understanding for future force field development efforts.

Structural Mapping of the Base Stacks Containing Post-transcriptionally Modified Bases in RNA

Post-transcriptionally modified bases play vital roles in many biochemical processes involving RNA. Analysis of the non-covalent interactions associated with these bases in RNA is crucial for providing a more complete understanding of the RNA structure and function; however, the characterization of these interactions remains understudied. To address this limitation, we present a comprehensive analysis of base stacks involving all crystallographic occurrences of the most biologically relevant modified bases in a large dataset of high-resolution RNA crystal structures. This is accompanied by a geometrical classification of the stacking contacts using our established tools. Coupled with quantum chemical calculations and an analysis of the specific structural context of these stacks, this provides a map of the stacking conformations available to modified bases in RNA. Overall, our analysis is expected to facilitate structural research on altered RNA bases.

RNAJP: enhanced RNA 3D structure predictions with non-canonical interactions and global topology sampling

RNA 3D structures are critical for understanding their functions. However, only a limited number of RNA structures have been experimentally solved, so computational prediction methods are highly desirable. Nevertheless, accurate prediction of RNA 3D structures, especially those containing multiway junctions, remains a significant challenge, mainly due to the complicated non-canonical base pairing and stacking interactions in the junction loops and the possible long-range interactions between loop structures. Here we present RNAJP ('RNA Junction Prediction'), a nucleotide- and helix-level coarse-grained model for the prediction of RNA 3D structures, particularly junction structures, from a given 2D structure. Through global sampling of the 3D arrangements of the helices in junctions using molecular dynamics simulations and in explicit consideration of non-canonical base pairing and base stacking interactions as well as long-range loop-loop interactions, the model can provide significantly improved predictions for multibranched junction structures than existing methods. Moreover, integrated with additional restraints from experiments, such as junction topology and long-range interactions, the model may serve as a useful structure generator for various applications.

Toward Force Fields with Improved Base Stacking Descriptions

Recent DNA force fields indicate good performance in describing flexibility and structural stability of double-stranded B-DNA. However, it is not clear how accurately base stacking interactions are represented that are critical for simulating structure formation processes and conformational changes. Based on the equilibrium nucleoside association and base pair nicking, we find that the recent Tumuc1 force field improves the description of base stacking compared to previous state-of-the-art force fields. Nevertheless, base pair stacking is still overstabilized compared to experiment. We propose a rapid method to reweight calculated free energies of stacking upon force field modifications in order to generate improved parameters. A decrease of the Lennard-Jones attraction between nucleo-bases alone appears insufficient; however, adjustments in the partial charge distribution on base atoms could help to further improve the force field description of base stacking.

Dissecting the energetic architecture within an RNA tertiary structural motif via high-throughput thermodynamic measurements

Structured RNAs and RNA/protein complexes perform critical cellular functions. They often contain structurally conserved tertiary contact "motifs," whose occurrence simplifies the RNA folding landscape. Prior studies have focused on the conformational and energetic modularity of intact motifs. Here, we turn to the dissection of one common motif, the 11nt receptor (11ntR), using quantitative analysis of RNA on a massively parallel array to measure the binding of all single and double 11ntR mutants to GAAA and GUAA tetraloops, thereby probing the energetic architecture of the motif. While the 11ntR behaves as a motif, its cooperativity is not absolute. Instead, we uncovered a gradient from high cooperativity amongst base-paired and neighboring residues to additivity between distant residues. As expected, substitutions at residues in direct contact with the GAAA tetraloop resulted in the largest decreases to binding, and energetic penalties of mutations were substantially smaller for binding to the alternate GUAA tetraloop, which lacks tertiary contacts present with the canonical GAAA tetraloop. However, we found that the energetic consequences of base partner substitutions are not, in general, simply described by base pair type or isostericity. We also found exceptions to the previously established stability-abundance relationship for 11ntR sequence variants. These findings of "exceptions to the rule" highlight the power of systematic high-throughput approaches to uncover novel variants for future study in addition to providing an energetic map of a functional RNA.

The binding model of adenosine-specific DNA aptamer: Umbrella sampling study

We report a novel model of the selective binding mechanism of adenosine-specific DNA aptamer. Our theoretical investigations of AMP (Adenosine monophosphate) dissociation from aptamer-AMP complexes reveals new details of aptamer molecular specificity and stabilisation factors. Umbrella sampling MD calculations using parmbsc1 force field shows that the disordered structure of the internal loop of the unbound aptamer hairpin has a characteristic packing of guanines, which prevents barrier-free penetration of ligands into the site cavity. Also, this disordered structure of the unbound aptamer has a network of hydrogen bonds stabilising the cavity near the target guanines within the binding sites during the whole binding process. We suggested that the first AMP molecule binds to the disordered structure of the site closest to the aptamer hairpin stem and spends some free energy on ordering of the internal loop. Then the second AMP molecule binds to the ordered site closest to the aptamer hairpin loop with a lower energy gain. As a result, the induced-fit binding model is the most applicable for this aptamer and does not contradict the modern experimental NMR and calorimetry data.

Brassinosteroid-induced gene repression requires specific and tight promoter binding of BIL1/BZR1 via DNA shape readout

BRZ-INSENSITIVE-LONG 1 (BIL1)/BRASSINAZOLE-RESISTANT 1 (BZR1) and its homologues are plant-specific transcription factors that convert the signalling of the phytohormones brassinosteroids (BRs) to transcriptional responses, thus controlling various physiological processes in plants. Although BIL1/BZR1 upregulates some BR-responsive genes and downregulates others, the molecular mechanism underlying the dual roles of BIL1/BZR1 is still poorly understood. Here we show that BR-responsive transcriptional repression by BIL1/BZR1 requires the tight binding of BIL1/BZR1 alone to the 10 bp elements of DNA fragments containing the known 6 bp core-binding motifs at the centre. Furthermore, biochemical and structural evidence demonstrates that the selectivity for two nucleobases flanking the core motifs is realized by the DNA shape readout of BIL1/BZR1 without direct recognition of the nucleobases. These results elucidate the molecular and structural basis of transcriptional repression by BIL1/BZR1 and contribute to further understanding of the dual roles of BIL1/BZR1 in BR-responsive gene regulation.

Duplex DNA Retains the Conformational Features of Single Strands: Perspectives from MD Simulations and Quantum Chemical Computations

Molecular dynamics simulations and geometry optimizations carried out at the quantum level as well as by quantum mechanical/molecular mechanics methods predict that short, single-stranded DNA oligonucleotides adopt conformations very similar to those observed in crystallographic double-stranded B-DNA, with rise coordinates close to ≈3.3 Å. In agreement with the experimental evidence, the computational results show that DNA single strands rich in adjacent purine nucleobases assume more regular arrangements than poly-thymine. The preliminary results suggest that single-stranded poly-cytosine DNA should also retain a substantial helical order in solution. A comparison of the structures of single and double helices confirms that the B-DNA motif is a favorable arrangement also for single strands. Indeed, the optimal geometry of the complementary single helices is changed to a very small extent in the formation of the duplex.

Acylation probing of "generic" RNA libraries reveals critical influence of loop constraints on reactivity

The reactivity of RNA 2'-OH acylation is broadly useful both in probing structure and in preparing conjugates. To date, this reactivity has been analyzed in limited sets of biological RNA sequences, leaving open questions of how reactivity varies inherently without regard to sequence in structured contexts. We constructed and probed "generic" structured RNA libraries using homogeneous loop sequences, employing deep sequencing to carry out a systematic survey of reactivity. We find a wide range of RNA reactivities among single-stranded sequences, with nearest neighbors playing substantial roles. Remarkably, certain small loops are found to be far more reactive on average (up to 4,000-fold) than single-stranded RNAs, due to conformational constraints that enhance reactivity. Among loops, we observe large variations in reactivity based on size, type, and position. The results lend insights into RNA designs for achieving high-efficiency local conjugation and provide new opportunities to refine structure analysis.

Evaluating Geometric Definitions of Stacking for RNA Dinucleoside Monophosphates Using Molecular Mechanics Calculations

RNA modulation via small molecules is a novel approach in pharmacotherapies, where the determination of the structural properties of RNA motifs is considered a promising way to develop drugs capable of targeting RNA structures to control diseases. However, due to the complexity and dynamic nature of RNA molecules, the determination of RNA structures using experimental approaches is not always feasible, and computational models employing force fields can provide important insight. The quality of the force field will determine how well the predictions are compared to experimental observables. Stacking in nucleic acids is one such structural property, originating mainly from London dispersion forces, which are quantum mechanical and are included in molecular mechanics force fields through nonbonded interactions. Geometric descriptions are utilized to decide if two residues are stacked and hence to calculate the stacking free energies for RNA dinucleoside monophosphates (DNMPs) through statistical mechanics for comparison with experimental thermodynamics data. Here, we benchmark four different stacking definitions using molecular dynamics (MD) trajectories for 16 RNA DNMPs produced by two different force fields (RNA-IL and ff99OL3) and show that our stacking definition better correlates with the experimental thermodynamics data. While predictions within an accuracy of 0.2 kcal/mol at 300 K were observed in RNA CC, CU, UC, AG, GA, and GG, stacked states of purine-pyrimidine and pyrimidine-purine DNMPs, respectively, were typically underpredicted and overpredicted. Additionally, population distributions of RNA UU DNMPs were poorly predicted by both force fields, implying a requirement for further force field revisions. We further discuss the differences predicted by each RNA force field. Finally, we show that discrete path sampling (DPS) calculations can provide valuable information and complement the MD simulations. We propose the use of experimental thermodynamics data for RNA DNMPs as benchmarks for testing RNA force fields.

Nearest-Neighbor dsDNA Stability Analysis Using Alchemical Free-Energy Simulations

The thermodynamic stability of double-stranded (ds)DNA depends on its sequence. It is influenced by the base pairing and stacking with neighboring bases along DNA molecules. Semiempirical schemes are available that allow us to predict the thermodynamic stability of DNA sequences based on empirically derived nearest-neighbor contributions of base pairs formed in the context of all possible nearest-neighbor base pairs. Current molecular dynamics (MD) simulations allow one to simulate the dynamics of DNA molecules in good agreement with experimentally obtained structures and available data on conformational flexibility. However, the suitability of current force field methods to reproduce dsDNA stability and its sequence dependence has been much less well tested. We have employed alchemical free-energy simulations of whole base pair transversions in dsDNA and in unbound single-stranded partner molecules. Such transversions change the sequence context but not the nucleotide content or base pairing in dsDNA and allow a direct comparison with the empirical nearest-neighbor dsDNA stability model. For the alchemical free-energy changes in the unbound single-stranded (ss)DNA partner molecules, we tested different setups assuming either complete unstacking or unrestrained simulations with partial stacking in the unbound ssDNA. The free-energy simulations predicted nearest-neighbor effects of similar magnitude, as observed experimentally but showed overall limited correlation with experimental data. An inaccurate description of stacking interactions and other possible reasons such as the neglect of electronic polarization effects are discussed. The results indicate the need to improve the realistic description of stacking interactions in current molecular mechanic force fields.

Conjugation of RNA via 2'-OH acylation: Mechanisms determining nucleotide reactivity

The acylation reactivity of RNA 2'-OH groups has proven broadly useful for labeling and mapping RNA. Here we perform kinetics studies to test the mechanisms governing this reaction, and we find strong steric and inductive effects modulating reactivity. The results shed light on new strategies for improved conjugation and mapping.

Direct detection of RNA modifications and structure using single-molecule nanopore sequencing

Modifications are present on many classes of RNA, including tRNA, rRNA, and mRNA. These modifications modulate diverse biological processes such as genetic recoding and mRNA export and folding. In addition, modifications can be introduced to RNA molecules using chemical probing strategies that reveal RNA structure and dynamics. Many methods exist to detect RNA modifications by short-read sequencing; however, limitations on read length inherent to short-read-based methods dissociate modifications from their native context, preventing single-molecule modification analysis. Here, we demonstrate direct RNA nanopore sequencing to detect endogenous and exogenous RNA modifications on long RNAs at the single-molecule level. We detect endogenous 2'-O-methyl and base modifications across E. coli and S. cerevisiae ribosomal RNAs as shifts in current signal and dwell times distally through interactions with the helicase motor protein. We further use the 2'-hydroxyl reactive SHAPE reagent acetylimidazole to probe RNA structure at the single-molecule level with readout by direct nanopore sequencing.

Stephenson et al. employ direct RNA nanopore sequencing to detect endogenous and exogenous modifications on single RNA molecules. The authors demonstrate detection of endogenous 2'-O-methylation (Nm) on native ribosomal RNAs, confirming known modification patterns. They describe the development of nanoSHAPE, a method that involves exogenously labeling RNA with a small-adduct-generating chemical probe that can reveal RNA structure using long-read sequencing.

A Primer on the oxDNA Model of DNA: When to Use it, How to Simulate it and How to Interpret the Results

The oxDNA model of Deoxyribonucleic acid has been applied widely to systems in biology, biophysics and nanotechnology. It is currently available via two independent open source packages. Here we present a set of clearly documented exemplar simulations that simultaneously provide both an introduction to simulating the model, and a review of the model's fundamental properties. We outline how simulation results can be interpreted in terms of-and feed into our understanding of-less detailed models that operate at larger length scales, and provide guidance on whether simulating a system with oxDNA is worthwhile.

Hydrogen Bonding in Natural and Unnatural Base Pairs-A Local Vibrational Mode Study

In this work hydrogen bonding in a diverse set of 36 unnatural and the three natural Watson Crick base pairs adenine (A)-thymine (T), adenine (A)-uracil (U) and guanine (G)-cytosine (C) was assessed utilizing local vibrational force constants derived from the local mode analysis, originally introduced by Konkoli and Cremer as a unique bond strength measure based on vibrational spectroscopy. The local mode analysis was complemented by the topological analysis of the electronic density and the natural bond orbital analysis. The most interesting findings of our study are that (i) hydrogen bonding in Watson Crick base pairs is not exceptionally strong and (ii) the N-H⋯N is the most favorable hydrogen bond in both unnatural and natural base pairs while O-H⋯N/O bonds are the less favorable in unnatural base pairs and not found at all in natural base pairs. In addition, the important role of non-classical C-H⋯N/O bonds for the stabilization of base pairs was revealed, especially the role of C-H⋯O bonds in Watson Crick base pairs. Hydrogen bonding in Watson Crick base pairs modeled in the DNA via a QM/MM approach showed that the DNA environment increases the strength of the central N-H⋯N bond and the C-H⋯O bonds, and at the same time decreases the strength of the N-H⋯O bond. However, the general trends observed in the gas phase calculations remain unchanged. The new methodology presented and tested in this work provides the bioengineering community with an efficient design tool to assess and predict the type and strength of hydrogen bonding in artificial base pairs.

Temperature-Responsive Peptide-Nucleotide Coacervates

Coacervates are a type of liquid–liquid phase separated (LLPS) droplets that can serve as models of membraneless organelles (MLOs) in living cells. Peptide–nucleotide coacervates have been widely used to mimic properties of ribonucleoprotein (RNP) granules, but the thermal stability and the role of base stacking is still poorly understood. Here, we report a systematic investigation of coacervates formed by five different nucleoside triphosphates (NTPs) with poly-l-lysine and poly-l-arginine as a function of temperature. All studied combinations exhibit an upper critical solution temperature (UCST), and a temperature-dependent critical salt concentration, originating from a significant nonelectrostatic contribution to the mixing free energy. Both the enthalpic and entropic parts of this nonelectrostatic interaction decrease in the order G/A/U/C/T, in accordance with nucleobase stacking free energies. Partitioning of two dyes proves that the local hydrophobicity inside the peptide–nucleotide coacervates is different for every nucleoside triphosphate. We derive a simple relation between the temperature and salt concentration at the critical point based on a mean-field model of phase separation. Finally, when different NTPs are mixed with one common oppositely charged peptide, hybrid coacervates were formed, characterized by a single intermediate UCST and critical salt concentration. NTPs with lower critical salt concentrations can remain condensed in mixed coacervates far beyond their original critical salt concentration. Our results show that NTP-based coacervates have a strong temperature sensitivity due to base stacking interactions and that mixing NTPs can significantly influence the stability of condensates and, by extension, their bioavailability.

Reaction Coordinate and Thermodynamics of Base Flipping in RNA

Base flipping is a key biophysical event involved in recognition of various ligands by ribonucleic acid (RNA) molecules. However, the mechanism of base flipping in RNA remains poorly understood, in part due to the lack of atomistic details on complex rearrangements in neighboring bases. In this work, we applied transition path sampling (TPS) methods to study base flipping in a double-stranded RNA (dsRNA) molecule that is known to interact with RNA-editing enzymes through this mechanism. We obtained an ensemble of 1000 transition trajectories to describe the base-flipping process. We used the likelihood maximization method to determine the refined reaction coordinate (RC) consisting of two collective variables (CVs), a distance and a dihedral angle between nucleotides that form stacking interactions with the flipping base. The free energy profile projected along the refined RC revealed three minima, two corresponding to the initial and final states and one for a metastable state. We suggest that the metastable state likely represents a wobbled conformation of nucleobases observed in NMR studies that is often characterized as the flipped state. The analyses of reactive trajectories further revealed that the base flipping is coupled to a global conformational change in a stem-loop of dsRNA.

Base-Pairing and Base-Stacking Contributions to Double-Stranded DNA Formation

Double-stranded (ds)DNA formation and dissociation are of fundamental biological importance. The negative DNA charge influences the dsDNA stability. However, the base pairing and the stacking between neighboring bases are responsible for the sequence-dependent stability of dsDNA. The stability of a dsDNA molecule can be estimated from empirical nearest-neighbor models based on contributions assigned to base-pair steps along the DNA and additional parameters because of DNA termini. In efforts to separate contributions, it has been concluded that base stacking dominates dsDNA stability, whereas base pairing contributes negligibly. Using a different model for dsDNA formation, we reanalyze dsDNA stability contributions and conclude that base stacking contributes already at the level of separate ssDNAs but that pairing contributions drive the dsDNA formation. The theoretical model also predicts that stability contributions of base-pair steps that contain only guanine/cytosine, mixed steps, and steps with only adenine/thymine follow the order 6:5:4, respectively, as expected based on the formed hydrogen bonds. The model is fully consistent with the available stacking data and the nearest-neighbor dsDNA parameters. It allows assigning a narrowly distributed value for the effective free energy contribution per formed hydrogen bond during dsDNA formation of -0.72 kcal·mol^-1 based entirely on the experimental data.

Improving and fine-tuning the properties of peptide-based hydrogels via incorporation of peptide nucleic acids

Peptide self-assemblies have attracted intense research interest over the last few decades thanks to their implications in key biological processes (e.g., amyloid formation) and their use in biotechnological and (bio)material fields. In particular, peptide-based hydrogels have been highly considered as high potential supramolecular materials in the biomedical domain and open new horizons in terms of applications. To further understand their self-assembly mechanisms and to optimize their properties, several strategies have been proposed with the modification of the constituting amino acid chains via, per se, the introduction of d-amino acids, halogenated amino acids, pseudopeptide bonds, or other chemical moieties. In this context, we report herein on the incorporation of DNA-nucleobases into their peptide nucleic acid (PNA) forms to develop a new series of hybrid nucleopeptides. Thus, depending on the nature of the nucleobase (i.e., thymine, cytosine, adenine or guanine), the physicochemical and mechanical properties of the resulting hydrogels can be significantly improved and fine-tuned with, for instance, drastic enhancements of both the gel stiffness (up to 70-fold) and the gel resistance to external stress (up to 40-fold), and the generation of both thermo-reversible and uncommon red-edge excitation shift (REES) properties. To decipher the actual role of each PNA moiety in the self-assembly processes, the induced modifications from the molecular to the macroscopic scales are studied thanks to the multiscale approach based on a large panel of analytical techniques (i.e., rheology, NMR relaxometry, TEM, thioflavin T assays, FTIR, CD, fluorescence, NMR chemical shift index). Thus, such a strategy provides new opportunities to adapt and fit hydrogel properties to the intended ones and pushes back the limits of supramolecular materials.

Recognition of Poly(A) RNA through Its Intrinsic Helical Structure

The polyadenosine (poly(A)) tail, which is found on the 3’ end of almost all eukaryotic messenger RNAs (mRNAs), plays an important role in the posttranscriptional regulation of gene expression. Shortening of the poly(A) tail, a process known as deadenylation, is thought to be the first and rate-limiting step of mRNA turnover. Deadenylation is performed by the Pan2–Pan3 and Ccr4–Not complexes that contain highly conserved exonuclease enzymes Pan2, and Ccr4 and Caf1, respectively. These complexes have been extensively studied, but the mechanisms of how the deadenylase enzymes recognize the poly(A) tail were poorly understood until recently. Here, we summarize recent work from our laboratory demonstrating that the highly conserved Pan2 exonuclease recognizes the poly(A) tail, not through adenine-specific functional groups, but through the conformation of poly(A) RNA. Our biochemical, biophysical, and structural investigations suggest that poly(A) forms an intrinsic base-stacked, single-stranded helical conformation that is recognized by Pan2, and that disruption of this structure inhibits both Pan2 and Caf1. This intrinsic structure has been shown to be important in poly(A) recognition in other biological processes, further underlining the importance of the unique conformation of poly(A).

Molecular Dynamics Simulations of DNA Adsorption on Graphene Oxide and Reduced Graphene Oxide-PEG-NH2 in the Presence of Mg2+ and Cl− ions

Graphene and its functionalised derivatives are transforming the development of biosensors that are capable of detecting nucleic acid hybridization. Using a Molecular Dynamics (MD) approach, we explored single-stranded or double-stranded deoxyribose nucleic acid (ssDNA or dsDNA) adsorption on two graphenic species: graphene oxide (GO) and reduced graphene oxide functionalized with aminated polyethylene glycol (rGO-PEG-NH2). Innovatively, we included chloride (Cl−) and magnesium (Mg2+) ions that influenced both the ssDNA and dsDNA adsorption on GO and rGO-PEG-NH2 surfaces. Unlike Cl−, divalent Mg2+ ions formed bridges between the GO surface and DNA molecules, promoting adsorption through electrostatic interactions. For rGO-PEG-NH2, the Mg2+ ions were repulsed from the graphenic surface. The subsequent ssDNA adsorption, mainly influenced by electrostatic forces and hydrogen bonds, could be supported by π–π stacking interactions that were absent in the case of dsDNA. We provide a novel insight for guiding biosensor development.

Stacking geometry between two sheared Watson-Crick basepairs: Computational chemistry and bioinformatics based prediction

BACKGROUND

Molecular modeling of RNA double helices is possible using most probable values of basepair parameters obtained from crystal structure database. The A:A w:wC non-canonical basepair, involving Watson-Crick edges of two Adenines in cis orientation, appears quite frequently in database. Bimodal distribution of its Shear, due to two different H-bonding schemes, introduces the confusion in assigning most the probable value. Its effect is pronounced when the A:A w:wC basepair stacks on Sheared wobble G:U W:WC basepairs.

METHODS

We employed molecular dynamics simulations of three possible double helices with GAG, UAG and GAU sequence motifs at their centers and quantum chemical calculation for non-canonical A:A w:wC basepair stacked on G:U W:WC basepair.

RESULTS

We noticed stable structures of GAG motif with specifically negative Shear of the A:A basepair but stabilities of the other motifs were not found with A:A w:wC basepairing. Hybrid DFT-D and MP2 stacking energy analyses on dinucleotide step sequences, A:A w:wC::G:U W:WC and A:A w:wC::U:G W:WC reveal that viable orientation of A:A::G:U prefers one of the H-bonding modes with negative Shear, supported by crystal structure database. The A:A::U:G dinucleotide, however, prefers structure with only positive Shear.

CONCLUSIONS

The quantum chemical calculations explain why MD simulations of GAG sequence motif only appear stable. In the cases of the GAU and UAG motifs "tug of war" situation between positive and negative Shears of A:A w:wC basepair induces conformational plasticity.

GENERAL SIGNIFICANCE

We have projected comprehensive reason behind the promiscuous nature of A:A w:wC basepair which brings occasional structural plasticity.

Liquid crystal ordering of nucleic acids

Several analytical calculations and computer simulations propose that cylindrical monodispersive rods having an aspect ratio (ratio of length to diameter) greater than 4 can exhibit liquid crystal (LC) ordering. But, recent experiments demonstrated the signature of LC ordering in systems of 4- to 20-base pair (bp) long nucleic acids (NAs) that do not satisfy the shape anisotropy criterion. Mechanisms of end-to-end adhesion and stacking have been proposed to explain this phenomenon. In this study, using all-atom molecular dynamics (MD) simulation, we explicitly verify the end-to-end stacking of double-stranded RNA (dsRNA) and demonstrate the LC ordering at the microscopic level. Using umbrella sampling (US) calculation, we quantify the potential of mean force (PMF) between two dsRNAs for various reaction coordinates (RCs) and compare our results with previously reported PMFs for double-stranded DNA (dsDNA). The PMF profiles demonstrate the anisotropic nature of inter-NA interaction. We find that, like dsDNA, dsRNA also prefers to stack on top of each other while repelling sideways, leading to the formation of supra-molecular-columns that undergo LC ordering at high NA volume fraction (φ). We also demonstrate and quantify the nematic ordering of the RNAs using several hundred nanosecond-long MD simulations that remain almost invariant for different initial configurations and under different external physiological conditions.

Controlling aggregation of cholesterol-modified DNA nanostructures

DNA nanotechnology allows for the design of programmable DNA-built nanodevices which controllably interact with biological membranes and even mimic the function of natural membrane proteins. Hydrophobic modifications, covalently linked to the DNA, are essential for targeted interfacing of DNA nanostructures with lipid membranes. However, these hydrophobic tags typically induce undesired aggregation eliminating structural control, the primary advantage of DNA nanotechnology. Here, we study the aggregation of cholesterol-modified DNA nanostructures using a combined approach of non-denaturing polyacrylamide gel electrophoresis, dynamic light scattering, confocal microscopy and atomistic molecular dynamics simulations. We show that the aggregation of cholesterol-tagged ssDNA is sequence-dependent, while for assembled DNA constructs, the number and position of the cholesterol tags are the dominating factors. Molecular dynamics simulations of cholesterol-modified ssDNA reveal that the nucleotides wrap around the hydrophobic moiety, shielding it from the environment. Utilizing this behavior, we demonstrate experimentally that the aggregation of cholesterol-modified DNA nanostructures can be controlled by the length of ssDNA overhangs positioned adjacent to the cholesterol. Our easy-to-implement method for tuning cholesterol-mediated aggregation allows for increased control and a closer structure-function relationship of membrane-interfacing DNA constructs - a fundamental prerequisite for employing DNA nanodevices in research and biomedicine.

Adenosine and lipids: A forced marriage or a love match?

Adenosine is a fascinating compound, crucial in many biochemical processes: this ubiquitous nucleoside serves as an essential building block of RNA, is also a component of ATP and regulates numerous pathophysiological mechanisms via binding to four extracellular receptors. Due to its hydrophilic nature, it belongs to a different world than lipids, and has no affinity for them. Since the 1970's, however, new discoveries have emerged and prompted the scientific community to associate adenosine with the lipid family, especially via liposomal preparations and bioconjugation. This seems to be an arranged marriage, but could it turn into a true love match? This review considered all types of unions established between adenosine and lipids. Even though exciting supramolecular structures were observed with adenosine-lipid conjugates, as well as with liposomal preparations which resulted in promising pre-clinical results, the translation of these technologies to the clinic is still limited.

Investigating the sequence-dependent mechanical properties of DNA nicks for applications in twisted DNA nanostructure design

DNA nick can be used as a design motif in programming the shape and reconfigurable deformation of synthetic DNA nanostructures, but its mechanical properties have rarely been systematically characterized at the level of base sequences. Here, we investigated sequence-dependent mechanical properties of DNA nicks through molecular dynamics simulation for a comprehensive set of distinct DNA oligomers constructed using all possible base-pair steps with and without a nick. We found that torsional rigidity was reduced by 28–82% at the nick depending on its sequence and location although bending and stretching rigidities remained similar to those of regular base-pair steps. No significant effect of a nick on mechanically coupled deformation such as the twist-stretch coupling was observed. These results suggest that the primary structural role of nick is the relaxation of torsional constraint by backbones known to be responsible for relatively high torsional rigidity of DNA. Moreover, we experimentally demonstrated the usefulness of quantified nick properties in self-assembling DNA nanostructure design by constructing twisted DNA origami structures to show that sequence design of nicks successfully controls the twist angle of structures. Our study illustrates the importance as well as the opportunities of considering sequence-dependent properties in structural DNA nanotechnology.

Hierarchical groove discrimination by Class I and II aminoacyl-tRNA synthetases reveals a palimpsest of the operational RNA code in the tRNA acceptor-stem bases

Class I and II aaRS recognition of opposite grooves was likely among the earliest determinants fixed in the tRNA acceptor stem bases. A new regression model identifies those determinants in bacterial tRNAs. Integral coefficients relate digital dependent to independent variables with perfect agreement between observed and calculated grooves for all twenty isoaccepting tRNAs. Recognition is mediated by the Discriminator base 73, the first base pair, and base 2 of the acceptor stem. Subsets of these coefficients also identically compute grooves recognized by smaller numbers of aaRS. Thus, the model is hierarchical, suggesting that new rules were added to pre-existing ones as new amino acids joined the coding alphabet. A thermodynamic rationale for the simplest model implies that Class-dependent aaRS secondary structures exploited differential tendencies of the acceptor stem to form the hairpin observed in Class I aaRS•tRNA complexes, enabling the earliest groove discrimination. Curiously, groove recognition also depends explicitly on the identity of base 2 in a manner consistent with the middle bases of the codon table, confirming a hidden ancestry of codon-anticodon pairing in the acceptor stem. That, and the lack of correlation with anticodon bases support prior productive coding interaction of tRNA minihelices with proto-mRNA.

Exploring ultraviolet photoinduced charge-transfer dynamics in a model dinucleotide of guanine and thymine

An understanding of the initial photoexcited states of DNA is essential to unravelling deleterious photoinduced chemical reactions and the intrinsic ultrafast photoprotection of the genetic code for all life. In our combined experimental and theoretical study, we have elucidated the primary non-radiative relaxation dynamics of a model nucleotide of guanine and thymine (2'-deoxyguanosine 3'-monophosphate 5'-thymidine, d(GpT)) in buffered aqueous solution. Experimentally, we unequivocally demonstrate that the Franck-Condon excited states of d(GpT) are significantly delocalised across both nucleobases, and mediate d(G⁺pT^-) exciplex product formation on an ultrafast (<350 fs) timescale. Theoretical studies show that the nature of the vertical excited states is very dependent on the specific geometry of the dinucleotide, and dictate the degree of delocalised, charge-transfer or localised character. Our mechanism for prompt exciplex formation involves a rapid change in electronic structure and includes a diabatic surface crossing very close to the Franck-Condon region mediating fast d(G⁺pT^-) formation. Exciplexes are quickly converted back to neutral ground state molecules on a ∼10 ps timescale with a high quantum yield, ensuring the photostability of the nucleotide sequence.

Class I and II aminoacyl-tRNA synthetase tRNA groove discrimination created the first synthetase-tRNA cognate pairs and was therefore essential to the origin of genetic coding

The genetic code likely arose when a bidirectional gene replicating as a quasi-species began to produce ancestral aminoacyl-tRNA synthetases (aaRS) capable of distinguishing between two distinct sets of amino acids. The synthetase class division therefore necessarily implies a mechanism by which the two ancestral synthetases could also discriminate between two different kinds of tRNA substrates. We used regression methods to uncover the possible patterns of base sequences capable of such discrimination and find that they appear to be related to thermodynamic differences in the relative stabilities of a hairpin necessary for recognition of tRNA substrates by Class I aaRS. The thermodynamic differences appear to be exploited by secondary structural differences between models for the ancestral aaRS called synthetase Urzymes and reinforced by packing of aromatic amino acid side chains against the nonpolar face of the ribose of A76 if and only if the tRNA CCA sequence forms a hairpin. The patterns of bases 1, 2, and 73 and stabilization of the hairpin by structural complementarity with Class I, but not Class II, aaRS Urzymes appear to be necessary and sufficient to have enabled the generation of the first two aaRS-tRNA cognate pairs, and the launch of a rudimentary binary genetic coding related recognizably to contemporary cognate pairs. As a consequence, it seems likely that nonrandom aminoacylation of tRNAs preceded the advent of the tRNA anticodon stem-loop. Consistent with this suggestion, coding rules in the acceptor-stem bases also reveal a palimpsest of the codon-anticodon interaction, as previously proposed. © 2019 IUBMB Life, 2019 © 2019 IUBMB Life, 71(8):1088-1098, 2019.

Charge-dipole interactions in G-quadruplex thrombin-binding aptamer

DNAs form various structures through hydrogen-bonding, base-stacking and electrostatic interactions. Although these noncovalent interactions are known to be cooperative in stabilizing a G-quadruplex (G4) structure of DNA, we find from all-atom molecular dynamics simulations that the electrostatic charge-dipole interaction is competitive with both hydrogen-bonding and base-stacking interactions. For the thrombin-binding aptamer (TBA) forming a chair-type antiparallel G4 structure, we have examined effects of an intercalating metal ion [K+, Sr2+, Mn+: an ion having a charge of n+ (n = 1-4) with the ionic radius of K+] on structural properties and noncovalent interactions. When K+ in the TBA·K+ complex is replaced with Sr2+, guanine dipoles in the two G-tetrads are realigned toward the central metal ion, thereby distorting the planar G4 geometry. Replacing K+ with Sr2+ significantly enhances the charge-dipole interaction but substantially reduces the number of hydrogen bonds in the G-tetrads. In the case of TBA·Mn+ complexes, as the charge n increases, the charge-dipole interaction increases but both of the hydrogen-bonding and base-stacking interactions decrease. These results suggest that the charge-dipole interaction realigning guanine dipoles in the G-tetrads is not cooperative but competitive with both hydrogen-bonding and base-stacking interactions favoring the planar G-tetrad geometry. Obviously, the charge state of an intercalating metal ion is as important as the ionic radius in forming a stable G4 structure. Thus, a delicate balance between these competing noncovalent interactions makes the chair-type antiparallel G4 structure of TBA selective for intercalating metal ions.

Molecular Dynamics Simulations of 2-Aminopurine-Labeled Dinucleoside Monophosphates Reveal Multiscale Stacking Kinetics

Molecular dynamics (MD) simulations of 2-aminopurine (2Ap)-labeled DNA dinucleoside monophosphates (DNMPs) were performed to investigate the hypothesis that base stacking dynamics occur on timescales sufficiently rapid to influence the emission signals measured in time-resolved fluorescence experiments. Analysis of multiple microsecond-length trajectories shows that the DNMPs sample all four coplanar stacking motifs. In addition, three metastable unstacked conformations are detected. A hidden Markov-state model (HMSM) was applied to the simulations to estimate transition rates between the stacked and unstacked states. Transitions between different stacked states generally occur at higher rates when the number of nucleobase faces requiring desolvation is minimized. Time constants for structural relaxation range between 1.6 and 25 ns, suggesting that emission from photoexcited 2Ap, which has an excited-state lifetime of 10 ns, is sensitive to base stacking kinetics. A master equation model for the excited-state population of 2Ap predicts multiexponential emission decays that reproduce the sub-10 ns emission decay lifetimes and amplitudes seen in experiments. Combining MD simulations with HMSM analysis is a powerful way to understand the dynamics that influence 2Ap excited-state relaxation and represents an important step toward using observed emission signals to validate MD simulations.

"End-to-end" stacking of small dsRNA

PELDOR (pulsed electron-electron double resonance) is an established method to study intramolecular distances and can give evidence for conformational changes and flexibilities. However, it can also be used to study intermolecular interactions as for example oligerimization. Here, we used PELDOR to study the "end-to-end" stacking of small double-stranded (ds) RNAs. For this study, the dsRNA molecules were only singly labeled with the spin label TPA to avoid multispin effects and to measure only the intermolecular stacking interactions. It can be shown that small dsRNAs tend to assemble to rod-like structures due to π-π interactions between the base pairs at the end of the strands. On the one hand, these interactions can influence or complicate measurements aimed at the determining of the structure and dynamics of the dsRNA molecule itself. On the other hand, it can be interesting to study such intermolecular stacking interactions in more detail, as for example their dependence on ion concentration. We quantitatively determined the stacking probability as a function of the monovalent NaCl salt and the dsRNA concentration. From these data, the dissociation constant K_d was deduced and found to depend on the ratio between the NaCl salt and dsRNA concentrations. Additionally, the distances and distance distributions obtained predict a model for the stacking geometry of dsRNAs. Introducing a nucleotide overhangs at one end of the dsRNA molecule restricts the stacking to the other end, leading only to dimer formations. Introducing such an overhang at both ends of the dsRNA molecule fully suppresses stacking, as we demonstrate by PELDOR experiments quantitatively.

Chemically Accurate Relative Folding Stability of RNA Hairpins from Molecular Simulations

To benchmark RNA force fields, we compared the folding stabilities of three 12-nucleotide hairpin stem loops estimated by simulation to stabilities determined by experiment. We used umbrella sampling and a reaction coordinate of end-to-end (5' to 3' hydroxyl oxygen) distance to estimate the free energy change of the transition from the native conformation to a fully extended conformation with no hydrogen bonds between non-neighboring bases. Each simulation was performed four times using the AMBER FF99+bsc0+χ_OL3 force field, and each window, spaced at 1 Å intervals, was sampled for 1 μs, for a total of 552 μs of simulation. We compared differences in the simulated free energy changes to analogous differences in free energies from optical melting experiments using thermodynamic cycles where the free energy change between stretched and random coil sequences is assumed to be sequence-independent. The differences between experimental and simulated ΔΔ G° are, on average, 0.98 ± 0.66 kcal/mol, which is chemically accurate and suggests that analogous simulations could be used predictively. We also report a novel method to identify where replica free energies diverge along a reaction coordinate, thus indicating where additional sampling would most improve convergence. We conclude by discussing methods to more economically perform these simulations.

Conformations of an RNA Helix-Junction-Helix Construct Revealed by SAXS Refinement of MD Simulations

RNA is involved in a broad range of biological processes that extend far beyond translation. Many of RNA's recently discovered functions rely on folding to a specific conformation or transitioning between conformations. The RNA structure contains rigid, short basepaired regions connected by more flexible linkers. Studies of model constructs such as small helix-junction-helix (HJH) motifs are useful in understanding how these elements work together to determine RNA conformation. Here, we reveal the full ensemble of solution structures assumed by a model RNA HJH. We apply small-angle x-ray scattering and an ensemble optimization method to selectively refine models generated by all-atom molecular dynamics simulations. The expectation of a broad distribution of helix orientations, at and above physiological ionic strength, is not met. Instead, this analysis shows that the HJH structures are dominated by two distinct conformations at moderate to high ionic strength. Atomic structures, selected from the molecular dynamics simulations, reveal strong base-base interactions in the junction that critically constrain the conformational space available to the HJH molecule and lead to a surprising re-extension at high salt. These results are corroborated by comparison with previous single-molecule fluorescence resonance energy transfer experiments on the same constructs.

How the Conformations of an Internal Junction Contribute to Fold an RNA Domain

Like proteins, some RNAs fold to compact structures. We can model functional RNAs as a series of short, rigid, base-paired elements, connected by non-base-paired nucleotides that serve as junctions. These connecting regions bend and twist, facilitating the formation of tertiary contacts that stabilize compact states. Here, we explore the roles of salt and junction sequence in determining the structures of a ubiquitous connector: an asymmetric internal loop. We focus on the J5/5a junction from the widely studied P4-P6 domain of the Tetrahymena ribozyme. Following the addition of magnesium ions to fold P4-P6, this junction bends dramatically, bringing the two halves of the RNA domain together for tertiary contact engagement. Using single-molecule fluorescence resonance energy transfer (smFRET), we examine the role of sequence and salt on model RNA constructs that contain these junction regions. We explore the wild-type J5/5a junction as well as two sequence variants. These junctions display distinct, salt-dependent conformations. Small-angle X-ray scattering (SAXS) measurements verify that these effects persist in the full-length P4-P6 domain. These measurements underscore the importance of junction sequence and interactions with ions in facilitating RNA folding.

Single-Stranded DNA Oligonucleotides Retain Rise Coordinates Characteristic of Double Helices

The structures of single-stranded DNA oligonucleotides from dimeric to hexameric sequences have been thoroughly investigated. Computations performed at the density functional level of theory including dispersion forces and solvation show that single-stranded helices adopt conformations very close to crystallographic B-DNA, with rise coordinates amounting up to 3.3 Å. Previous results, suggesting that single strands should be shorter than double helices, largely originated from the incompleteness of the adopted basis set. Although sensible deviations with respect to standard B-DNA are predicted, computations indicate that sequences rich in stacked adenines are the most ordered ones, favoring the B-DNA pattern and inducing regular arrangements also on flanking nucleobases. Several structural properties of double helices rich in adenine are indeed already reflected by the corresponding single strands.

Sequence-Dependent Three Interaction Site Model for Single- and Double-Stranded DNA

We develop a robust coarse-grained model for single- and double-stranded DNA by representing each nucleotide by three interaction sites (TIS) located at the centers of mass of sugar, phosphate, and base. The resulting TIS model includes base-stacking, hydrogen bond, and electrostatic interactions as well as bond-stretching and bond angle potentials that account for the polymeric nature of DNA. The choices of force constants for stretching and the bending potentials were guided by a Boltzmann inversion procedure using a large representative set of DNA structures extracted from the Protein Data Bank. Some of the parameters in the stacking interactions were calculated using a learning procedure, which ensured that the experimentally measured melting temperatures of dimers are faithfully reproduced. Without any further adjustments, the calculations based on the TIS model reproduce the experimentally measured salt and sequence-dependence of the size of single-stranded DNA (ssDNA), as well as the persistence lengths of poly(dA) and poly(dT) chains. Interestingly, upon application of mechanical force, the extension of poly(dA) exhibits a plateau, which we trace to the formation of stacked helical domains. In contrast, the force-extension curve (FEC) of poly(dT) is entropic in origin and could be described by a standard polymer model. We also show that the persistence length of double-stranded DNA, formed from two complementary ssDNAs, is consistent with the prediction based on the worm-like chain. The persistence length, which decreases with increasing salt concentration, is in accord with the Odijk-Skolnick-Fixman theory intended for stiff polyelectrolyte chains near the rod limit. Our model predicts the melting temperatures of DNA hairpins with excellent accuracy, and we are able to recover the experimentally known sequence-specific trends. The range of applications, which did not require adjusting any parameter after the initial construction based solely on PDB structures and melting profiles of dimers, attests to the transferability and robustness of the TIS model for ssDNA and dsDNA.

Mechanism of premature translation termination on a sense codon

Accurate translation termination by release factors (RFs) is critical for the integrity of cellular proteomes. Premature termination on sense codons, for example, results in truncated proteins, whose accumulation could be detrimental to the cell. Nevertheless, some sense codons are prone to triggering premature termination, but the structural basis for this is unclear. To investigate premature termination, we determined a cryo-EM structure of the Escherichia coli 70S ribosome bound with RF1 in response to a UAU (Tyr) sense codon. The structure reveals that RF1 recognizes a UAU codon similarly to a UAG stop codon, suggesting that sense codons induce premature termination because they structurally mimic a stop codon. Hydrophobic interaction between the nucleobase of U3 (the third position of the UAU codon) and conserved Ile-196 in RF1 is important for misreading the UAU codon. Analyses of RNA binding in ribonucleoprotein complexes or by amino acids reveal that Ile-U packing is a frequent protein-RNA-binding motif with key functional implications. We discuss parallels with eukaryotic translation termination by the release factor eRF1.

DNA triplex structure, thermodynamics, and destabilisation: insight from molecular simulations

Molecular dynamics simulations are used to elucidate the structure and thermodynamics of DNA triplexes associated with the neurodegenerative disease Friedreich's ataxia (FRDA), as well as complexes of these triplexes with the small molecule netropsin, which is known to destabilise triplexes. The ability of molecular simulations in explicit solvent to accurately capture triplex thermodynamics is verified for the first time, with the free energy to dissociate a 15-base antiparallel purine triplex-forming oligomer (TFO) from the duplex found to be slightly higher than reported experimentally. The presence of netropsin in the minor groove destabilises the triplex as expected, reducing the dissociation free energy by approximately 50%. Netropsin binding is associated with localised narrowing of the minor groove near netropsin, an effect that has previously been under contention. This leads to localised widening of the major groove, weakening hydrogen bonds between the TFO and duplex. Consequently, destabilisation is found to be highly localised, occurring only when netropsin is bound directly opposite the TFO. The simulations also suggest that near saturation of the minor groove with ligand is required for complete triplex dissociation. A structural analysis of the DNA triplexes that can form with the FRDA-related duplex sequence indicates that the triplex with a parallel homopyrimidine TFO is likely to be more stable than the antiparallel homopurine-TFO triplex, which may have implications for disease onset and treatment.

RNA Structural Dynamics As Captured by Molecular Simulations: A Comprehensive Overview

With both catalytic and genetic functions, ribonucleic acid (RNA) is perhaps the most pluripotent chemical species in molecular biology, and its functions are intimately linked to its structure and dynamics. Computer simulations, and in particular atomistic molecular dynamics (MD), allow structural dynamics of biomolecular systems to be investigated with unprecedented temporal and spatial resolution. We here provide a comprehensive overview of the fast-developing field of MD simulations of RNA molecules. We begin with an in-depth, evaluatory coverage of the most fundamental methodological challenges that set the basis for the future development of the field, in particular, the current developments and inherent physical limitations of the atomistic force fields and the recent advances in a broad spectrum of enhanced sampling methods. We also survey the closely related field of coarse-grained modeling of RNA systems. After dealing with the methodological aspects, we provide an exhaustive overview of the available RNA simulation literature, ranging from studies of the smallest RNA oligonucleotides to investigations of the entire ribosome. Our review encompasses tetranucleotides, tetraloops, a number of small RNA motifs, A-helix RNA, kissing-loop complexes, the TAR RNA element, the decoding center and other important regions of the ribosome, as well as assorted others systems. Extended sections are devoted to RNA-ion interactions, ribozymes, riboswitches, and protein/RNA complexes. Our overview is written for as broad of an audience as possible, aiming to provide a much-needed interdisciplinary bridge between computation and experiment, together with a perspective on the future of the field.

New tricks for old dogs: improving the accuracy of biomolecular force fields by pair-specific corrections to non-bonded interactions

In contrast to ordinary polymers, the vast majority of biological macromolecules adopt highly ordered three-dimensional structures that define their functions. The key to folding of a biopolymer into a unique 3D structure or to an assembly of several biopolymers into a functional unit is a delicate balance between the attractive and repulsive forces that also makes such self-assembly reversible under physiological conditions. The all-atom molecular dynamics (MD) method has emerged as a powerful tool for studies of individual biomolecules and their functional assemblies, encompassing systems of ever increasing complexity. However, advances in parallel computing technology have outpaced the development of the underlying theoretical models-the molecular force fields, pushing the MD method into an untested territory. Recent tests of the MD method have found the most commonly used molecular force fields to be out of balance, overestimating attractive interactions between charged and hydrophobic groups, which can promote artificial aggregation in MD simulations of multi-component protein, nucleic acid, and lipid systems. One route towards improving the force fields is through the NBFIX corrections method, in which the intermolecular forces are calibrated against experimentally measured quantities such as osmotic pressure by making atom pair-specific adjustments to the non-bonded interactions. In this article, we review development of the NBFIX (Non-Bonded FIX) corrections to the AMBER and CHARMM force fields and discuss their implications for MD simulations of electrolyte solutions, dense DNA systems, Holliday junctions, protein folding, and lipid bilayer membranes.