Hydration changes upon DNA folding studied by osmotic stress experiments

Vesicle encapsulation stabilizes intermolecular association and structure formation of functional RNA and DNA

During the origin of life, encapsulation of RNA inside vesicles is believed to have been a defining feature of the earliest cells (protocells). The confined biophysical environment provided by membrane encapsulation differs from that of bulk solution and has been shown to increase activity as well as evolutionary rate for functional RNA. However, the structural basis of the effect on RNA has not been clear. Here, we studied how encapsulation of the hairpin ribozyme inside model protocells affects ribozyme kinetics, ribozyme folding into the active conformation, and cleavage and ligation activities. We further examined the effect of encapsulation on the folding of a stem-loop RNA structure and on the formation of a triplex structure in a pH-sensitive DNA switch. The results indicate that encapsulation promotes RNA-RNA association, both intermolecular and intramolecular, and also stabilizes tertiary folding, including the docked conformation characteristic of the active hairpin ribozyme and the triplex structure. The effects of encapsulation were sufficient to rescue the activity of folding-deficient mutants of the hairpin ribozyme. Stabilization of multiple modes of nucleic acid folding and interaction thus enhanced the activity of encapsulated nucleic acids. Increased association between RNA molecules may facilitate the formation of more complex structures and cooperative interactions. These effects could promote the emergence of biological functions in an "RNA world" and may have utility in the construction of minimal synthetic cells.

Apolar chemical environments compact unfolded RNAs and can promote folding

It is well documented that the structure, and thus function, of nucleic acids depends on the chemical environment surrounding them, which often includes potential proteinaceous binding partners. The nonpolar amino acid side chains of these proteins will invariably alter the polarity of the local chemical environment around the nucleic acid. However, we are only beginning to understand how environmental polarity generally influences the structural and energetic properties of RNA folding. Here, we use a series of aqueous-organic cosolvent mixtures to systematically modulate the solvent polarity around two different RNA folding constructs that can form either secondary or tertiary structural elements. Using single-molecule Förster resonance energy transfer spectroscopy to simultaneously monitor the structural and energetic properties of these RNAs, we show that the unfolded conformations of both model RNAs become more compact in apolar environments characterized by dielectric constants less than that of pure water. In the case of tertiary structure formation, this compaction also gives rise to more energetically favorable folding. We propose that these physical changes arise from an enhanced accumulation of counterions in the low dielectric environment surrounding the unfolded RNA.

Nearest-neighbor parameters for predicting DNA duplex stability in diverse molecular crowding conditions

The intracellular environment is crowded and heterogeneous. Although the thermodynamic stability of nucleic acid duplexes is predictable in dilute solutions, methods of predicting such stability under specific intracellular conditions are not yet available. We recently showed that the nearest-neighbor model for self-complementary DNA is valid under molecular crowding condition of 40% polyethylene glycol with an average molecular weight of 200 (PEG 200) in 100 mM NaCl. Here, we determined nearest-neighbor parameters for DNA duplex formation under the same crowding condition to predict the thermodynamics of DNA duplexes in the intracellular environment. Preferential hydration of the nucleotides was found to be the key factor for nearest-neighbor parameters in the crowding condition. The determined parameters were shown to predict the thermodynamic parameters (∆H°, ∆S°, and ∆G°₃₇) and melting temperatures (T _m) of the DNA duplexes in the crowding condition with significant accuracy. Moreover, we proposed a general method for predicting the stability of short DNA duplexes in different cosolutes based on the relationship between duplex stability and the water activity of the cosolute solution. The method described herein would be valuable for investigating biological processes that occur under specific intracellular crowded conditions and for the application of DNA-based biotechnologies in crowded environments.

Stability prediction of canonical and non-canonical structures of nucleic acids in various molecular environments and cells

This review provides the biophysicochemical background and recent advances in stability prediction of canonical and non-canonical structures of nucleic acids in various molecular environments and cells.

DNA Hairpin Hybridization under Extreme Pressures: A Single-Molecule FRET Study

Organisms have evolved to live in a variety of complex environments, which clearly has required cellular biology to accommodate to extreme conditions of hydraulic pressure and elevated temperature. In this work, we exploit single-molecule Forster resonance energy transfer (FRET) spectroscopy to probe structural changes in DNA hairpins as a function of pressure and temperature, which allows us to extract detailed thermodynamic information on changes in free energy (ΔG°), free volume (ΔV°), enthalpy (ΔH°), and entropy (ΔS°) associated with DNA loop formation and sequence-dependent stem hybridization. Specifically, time-correlated single-photon counting experiments on freely diffusing 40A DNA hairpin FRET constructs are performed in a 50 μm × 50 μm square quartz capillary cell pressurized from ambient pressure up to 3 kbar. By pressure-dependent van't Hoff analysis of the equilibrium constants, ΔV° for hybridization of the DNA hairpin can be determined as a function of stem length (n_stem = 7-10) with single base-pair resolution, which further motivates a simple linear deconstruction into additive stem (ΔV°_stem = ΔV°_bp x n_stem) and loop (ΔV°_loop) contributions. We find that increasing pressure destabilizes the DNA hairpin stem region [ΔV°_bp = +1.98(16) cm³/(mol bp)], with additional positive free volume changes [ΔV°_loop = +7.0(14) cm³/mol] we ascribe to bending and base stacking disruption of the 40-dA loop. From a van't Hoff temperature-dependent analysis of the DNA 40A hairpin equilibria, the data support a similar additive loop/stem deconstruction of enthalpic (ΔH° = ΔH°_loop + ΔH°_stem) and entropic (ΔS° = ΔS°_loop + ΔS°_stem) contributions, which permits insightful comparison with predictions from nearest-neighbor thermodynamic models for DNA duplex formation. In particular, the stem thermodynamics is consistent with exothermically favored (ΔH°_stem < 0) and entropically penalized (ΔS°_stem < 0) hydrogen bonding but with additional enthalpic (ΔH°_loop > 0) and entropic (ΔS°_loop > 0) contributions due to loop bending effects consistent with distortion of dA base stacking in the 40-dA linker.

Validation of the nearest-neighbor model for Watson-Crick self-complementary DNA duplexes in molecular crowding condition

Recent advancement in nucleic acid techniques inside cells demands the knowledge of the stability of nucleic acid structures in molecular crowding. The nearest-neighbor model has been successfully used to predict thermodynamic parameters for the formation of nucleic acid duplexes, with significant accuracy in a dilute solution. However, knowledge about the applicability of the model in molecular crowding is still limited. To determine and predict the stabilities of DNA duplexes in a cell-like crowded environment, we systematically investigated the validity of the nearest-neighbor model for Watson–Crick self-complementary DNA duplexes in molecular crowding. The thermodynamic parameters for the duplex formation were measured in the presence of 40 wt% poly(ethylene glycol)200 for different self-complementary DNA oligonucleotides consisting of identical nearest-neighbors in a physiological buffer containing 0.1 M NaCl. The thermodynamic parameters as well as the melting temperatures (T_m) obtained from the UV melting studies revealed similar values for the oligonucleotides having identical nearest-neighbors, suggesting the validity of the nearest-neighbor model in the crowding condition. Linear relationships between the measured ΔG°₃₇ and T_m in crowding condition and those predicted in dilute solutions allowed us to predict ΔG°₃₇, T_m and nearest-neighbor parameters in molecular crowding using existing parameters in the dilute condition, which provides useful information about the thermostability of the self-complementary DNA duplexes in molecular crowding.

Thermodynamic characterization and nearest neighbor parameters for RNA duplexes under molecular crowding conditions

It is essential to study RNA under molecular crowding conditions to better predict secondary structures of RNAs in vivo. No systematic study has been completed to determine the effects of molecular crowding on RNA duplexes of varying lengths and sequence composition. Here, optical melting, circular dichroism, and osmometry data were collected for RNA duplexes in a 20% polyethylene glycol (with an average molecular weight of 200 g/mol) solution (PEG 200), and nearest neighbor parameters were derived using this data. RNA duplexes are destabilized, on average, 1.02 kcal/mol in the presence of 20% PEG 200. The ΔG°₃₇ values predicted by the nearest neighbor parameters for RNA duplexes in 20% PEG 200 were ∼0.65 kcal/mol closer to experimental ΔG°₃₇ values than those predicted by the standard nearest neighbor model. For one DNA sequence in solution with small crowders, the ΔG°₃₇ values predicted by the 20% PEG 200 RNA nearest neighbor parameters were closer to the experimental values than ΔG°₃₇ values predicted by either the RNA or DNA standard nearest neighbor models. This indicates that the nearest neighbor parameters for RNA duplexes in 20% PEG 200 may be generalizable to RNA and DNA duplexes in solutions with small crowding agents.

Microscopic picture of water-ethylene glycol interaction near a model DNA by computer simulation: Concentration dependence, structure, and localized thermodynamics

It is known that crowded molecular environment affects the structure, thermodynamics, and dynamics of macromolecules. Most of the previous works on molecular crowding have majorly focused on the behavior of the macromolecule with less emphasis on the behavior of the crowder and water molecules. In the current study, we have precisely focused on the behavior of the crowder, (ethylene glycol (EG)), salt ions, and water in the presence of a DNA with the increase of the EG concentration. We have probed the behavior of water and crowder using molecular dynamics (MD) simulation and by calculating localized thermodynamic properties. Our results show an interesting competition between EG and water molecules to make hydrogen bonds (H-bond) with DNA. Although the total number of H-bonds involving DNA with both EG and water remains essentially same irrespective of the increase in EG concentration, there is a proportional change in the H-bonding pattern between water-water, EG-EG, and EG-water near DNA and in bulk. At low concentrations of EG, the displacement of water molecules near DNA is relatively easy. However, the displacement of water becomes more difficult as the concentration of EG increases. The density of Na⁺ (Cl^-) near DNA increases (decreases) as the concentration of EG is increased. The density of Cl^- near Na⁺ increases with the increase in EG concentration. It was also found that the average free energy per water in the first solvation shell increases with the increase in EG concentration. Putting all these together, a microscopic picture of EG, water, salt interaction in the presence of DNA, as a function of EG concentration, has emerged.

Linking Temperature, Cation Concentration and Water Activity for the B to Z Conformational Transition in DNA

High concentrations of Na⁺ or [Co(NH₃)₆]³⁺ can induce the B to Z conformational transition in alternating (dC-dG) oligo and polynucleotides. The use of short DNA oligomers (dC-dG)₄ and (dm⁵C-dG)₄ as models can allow a thermodynamic characterization of the transition. Both form right handed double helical structures (B-DNA) in standard phosphate buffer with 115 mM Na⁺ at 25 °C. However, at 2.0 M Na⁺ or 200 μM [Co(NH₃)₆]³⁺, (dm⁵C-dG)₄ assumes a left handed double helical structure (Z-DNA) while the unmethylated (dC-dG)₄ analogue remains right handed under those conditions. We have previously demonstrated that the enthalpy of the transition at 25 °C for either inducer can be determined using isothermal titration calorimetry (ITC). Here, ITC is used to investigate the linkages between temperature, water activity and DNA conformation. We found that the determined enthalpy for each titration varied linearly with temperature allowing determination of the heat capacity change (ΔC_p) between the initial and final states. As expected, the ΔC_p values were dependent upon the cation (i.e., Na⁺ vs. [Co(NH₃)₆]³⁺) as well as the sequence of the DNA oligomer (i.e., methylated vs. unmethylated). Osmotic stress experiments were carried out to determine the gain or loss of water by the oligomer induced by the titration. The results are discussed in terms of solvent accessible surface areas, electrostatic interactions and the role of water.

Effect of Cavity Size of Mesoporous Silica on Short DNA Duplex Stability

We studied the stabilities of short (4- and 3-bp) DNA duplexes within silica mesopores modified with a positively charged trimethyl aminopropyl (TMAP) monolayer (BJH pore diameter 1.6-7.4 nm). The DNA fragments with fluorescent dye were introduced into the pores, and their fluorescence resonance energy transfer (FRET) response was measured to estimate the structuring energies of the short DNA duplexes under cryogenic conditions (temperature 233-323 K). The results confirmed the enthalpic stability gain of the duplex within size-matched pores (1.6 and 2.3 nm). The hybridization equilibrium constants found for the size-matched pores were 2 orders of magnitude larger than those for large pores (≥3.5 nm), and this size-matching effect for the enhanced duplex stability was explained by a tight electrostatic interaction between the duplex and the surface TMAP groups. These results indicate the requirement of the precise regulation of mesopore size to ensure the stabilization of hydrogen-bonded supramolecular assemblies.

Thermal stability and conformation of DNA and proteins under the confined condition in the matrix of hydrogels

Spatially confined environments are seen in biological systems and in the fields of biotechnology and nanotechnology. The confinement restricts the conformational space of polymeric molecules and increasing the degree of molecular crowding. Here, we developed preparation methods for agarose and polyacrylamide gels applicable to UV spectroscopy that can evaluate the confinement effects on DNA and protein structures. Measurements of UV absorbance and CD spectra showed no significant effect of the confinement in the porous media of agarose gels on the base-pair stability of DNA polynucleotides [poly(dA)/poly(dT)] and oligonucleotides (hairpin, duplex, and triplex structures). On the other hand, a highly confined environment created by polyacrylamide gels at high concentrations increased the stability of polynucleotides while leaving that of oligonucleotides unaffected. The changes in the base-pair stability of the polynucleotides were accompanied by the perturbation of the helical conformation. The polyacrylamide gels prepared in this study were also used for the studies on proteins (lysozyme, bovine serum albumin, and myoglobin). The effects on the proteins were different from the effects on DNA structures, suggesting different nature of interactions within the gel. The experimental methods and results are useful to understand the physical properties of nucleic acids and proteins under confined conditions.

Impact of macromolecular crowding on RNA/spermine complex coacervation and oligonucleotide compartmentalization

We report the effect of neutral macromolecular crowders poly(ethylene glycol) (PEG) (8 kDa) and Ficoll (70 kDa) on liquid-liquid phase separation in a polyuridylic acid (polyU)/spermine complex coacervate system. The addition of PEG decreased both the amount of spermine required for phase separation and the coacervation temperature (T_C). We interpret these effects on phase behavior as arising due to excluded volume and preferential interactions on both the secondary structure/condensation of spermine-associated polyU molecules and on the association of soluble polyU/spermine polyelectrolyte complexes to form coacervate droplets. Examination of coacervates formed in the presence of fluorescently-labeled PEG or Ficoll crowders indicated that Ficoll is accumulated while PEG is excluded from the coacervate phase, which provides further insight into the differences in phase behavior. Crowding agents impact distribution of a biomolecular solute: partitioning of a fluorescently-labeled U15 RNA oligomer into the polyU/spermine coacervates was increased approximately two-fold by 20 wt% Ficoll 70 kDa and by more than two orders of magnitude by 20 wt% PEG 8 kDa. The volume of the coacervate phase decreased in the presence of crowder relative to a dilute buffer solution. These findings indicate that potential impacts of macromolecular crowding on phase behavior and solute partitioning should be considered in model systems for intracellular membraneless organelles.

Applications of PNA-Based Artificial Restriction DNA Cutters

More than ten years ago, artificial restriction DNA cutters were developed by combining two pseudo-complementary peptide nucleic acid (pcPNA) strands with either Ce(IV)/EDTA or S1 nuclease. They have remarkably high site-specificity and can cut only one predetermined site in the human genome. In this article, recent progress of these man-made tools have been reviewed. By cutting the human genome site-selectively, desired fragments can be clipped from either the termini of chromosomes (telomeres) or from the middle of genome. These fragments should provide important information on the biological functions of complicated genome system. DNA/RNA hybrid duplexes, which are formed in living cells, are also site-selectively hydrolyzed by these cutters. In order to further facilitate the applications of the artificial DNA cutters, various chemical modifications have been attempted. One of the most important successes is preparation of PNA derivatives which can form double-duplex invasion complex even under high salt conditions. This is important for in vivo applications, since the inside of living cells is abundant of metal ions. Furthermore, site-selective DNA cutters which require only one PNA strand, in place of a pair of pcPNA strands, are developed. This progress has opened a way to new fields of PNA-based biochemistry and biotechnology.

Model studies of the effects of intracellular crowding on nucleic acid interactions

Molecular interactions and reactions in living cells occur with high concentrations of background molecules and ions. Many research studies have shown that intracellular molecules have characteristics different from those obtained using simple aqueous solutions. To better understand the behavior of biomolecules in intracellular environments, biophysical experiments were conducted under cell-mimicking conditions in a test tube. It has been shown that the molecular environments at the physiological level of macromolecular crowding, spatial confinement, water activity and dielectric constant, have significant effects on the interactions of DNA and RNA for hybridization, higher-order folding, and catalytic activity. The experimental approaches using in vitro model systems are useful to reveal the origin of the environmental effects and to bridge the gap between the behaviors of nucleic acids in vitro and in vivo. This paper highlights the model experiments used to evaluate the influences of intracellular environment on nucleic acid interactions.

Thermal Stability of RNA Structures with Bulky Cations in Mixed Aqueous Solutions

Bulky cations are used to develop nucleic-acid-based technologies for medical and technological applications in which nucleic acids function under nonaqueous conditions. In this study, the thermal stability of RNA structures was measured in the presence of various bulky cations in aqueous mixtures with organic solvents or polymer additives. The stability of oligonucleotide, transfer RNA, and polynucleotide structures was decreased in the presence of salts of tetrabutylammonium and tetrapentylammonium ions, and the stability and salt concentration dependences were dependent on cation sizes. The degree to which stability was dependent on salt concentration was correlated with reciprocals of the dielectric constants of mixed solutions, regardless of interactions between the cosolutes and RNA. Our results show that organic solvents affect the strength of electrostatic interactions between RNA and cations. Analysis of ion binding to RNA indicated greater enhancement of cation binding to RNA single strands than to duplexes in media with low dielectric constants. Furthermore, background bulky ions changed the dependence of RNA duplex stability on the concentration of metal ion salts. These unique properties of large tetraalkylammonium ions are useful for controlling the stability of RNA structures and its sensitivity to metal ion salts.

The Effect of Small Cosolutes that Mimic Molecular Crowding Conditions on the Stability of Triplexes Involving Duplex DNA

Triplex stability is studied in crowding conditions using small cosolutes (ethanol, acetonitrile and dimethylsulfoxide) by ultraviolet (UV), circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopies. The results indicate that the triplex is formed preferentially when the triplex forming oligonucleotide (TFO) is RNA. In addition, DNA triplexes (D:D·D) are clearly less stable in cosolute solutions while the stability of the RNA triplexes (R:D·D) is only slightly decreased. The kinetic of triplex formation with RNA-TFO is slower than with DNA-TFO and the thermal stability of the triplex is increased with the salt concentration in EtOH-water solutions. Accordingly, RNA could be considered a potential molecule to form a stable triplex for regulatory purposes in molecular crowding conditions.

RNA Hairpin Folding in the Crowded Cell

Precise secondary and tertiary structure formation is critically important for the cellular functionality of ribonucleic acids (RNAs). RNA folding studies were mainly conducted in vitro, without the possibility of validating these experiments inside cells. Here, we directly resolve the folding stability of a hairpin‐structured RNA inside live mammalian cells. We find that the stability inside the cell is comparable to that in dilute physiological buffer. On the contrary, the addition of in vitro artificial crowding agents, with the exception of high‐molecular‐weight PEG, leads to a destabilization of the hairpin structure through surface interactions and reduction in water activity. We further show that RNA stability is highly variable within cell populations as well as within subcellular regions of the cytosol and nucleus. We conclude that inside cells the RNA is subject to (localized) stabilizing and destabilizing effects that lead to an on average only marginal modulation compared to diluted buffer.

The structural stability and catalytic activity of DNA and RNA oligonucleotides in the presence of organic solvents

Organic solvents and apolar media are used in the studies of nucleic acids to modify the conformation and function of nucleic acids, to improve solubility of hydrophobic ligands, to construct molecular scaffolds for organic synthesis, and to study molecular crowding effects. Understanding how organic solvents affect nucleic acid interactions and identifying the factors that dominate solvent effects are important for the creation of oligonucleotide-based technologies. This review describes the structural and catalytic properties of DNA and RNA oligonucleotides in organic solutions and in aqueous solutions with organic cosolvents. There are several possible mechanisms underlying the effects of organic solvents on nucleic acid interactions. The reported results emphasize the significance of the osmotic pressure effect and the dielectric constant effect in addition to specific interactions with nucleic acid strands. This review will serve as a guide for the selection of solvent systems based on the purpose of the nucleic acid-based experiments.

Thermodynamic properties of water molecules in the presence of cosolute depend on DNA structure: a study using grid inhomogeneous solvation theory

In conditions that mimic those of the living cell, where various biomolecules and other components are present, DNA strands can adopt many structures in addition to the canonical B-form duplex. Previous studies in the presence of cosolutes that induce molecular crowding showed that thermal stabilities of DNA structures are associated with the properties of the water molecules around the DNAs. To understand how cosolutes, such as ethylene glycol, affect the thermal stability of DNA structures, we investigated the thermodynamic properties of water molecules around a hairpin duplex and a G-quadruplex using grid inhomogeneous solvation theory (GIST) with or without cosolutes. Our analysis indicated that (i) cosolutes increased the free energy of water molecules around DNA by disrupting water–water interactions, (ii) ethylene glycol more effectively disrupted water–water interactions around Watson–Crick base pairs than those around G-quartets or non-paired bases, (iii) due to the negative electrostatic potential there was a thicker hydration shell around G-quartets than around Watson–Crick-paired bases. Our findings suggest that the thermal stability of the hydration shell around DNAs is one factor that affects the thermal stabilities of DNA structures under the crowding conditions.

Chemical modifications of artificial restriction DNA cutter (ARCUT) to promote its in vivo and in vitro applications

Recently, completely chemistry-based tools for site-selective scission of DNA (ARCUT) have been prepared by combining 2 strands of pseudo-complementary PNA (pcPNA: site-selective activator) and a Ce(IV)-EDTA complex (molecular scissors). Its site-specificity is sufficient to cut the whole human genome at one predetermined site. In this first-generation ARCUT, however, there still remain several problems to be solved for wider applications. This review presents recent approaches to solve these problems. They are divided into (i) covalent modification of pcPNA with other functional groups and (ii) new strategies using conventional PNA, in place of pcPNA, as site-selective activator. Among various chemical modifications, conjugation with positively-charged nuclear localization signal peptide is especially effective. Furthermore, unimolecular activators, a single strand of which successfully activates the target site in DNA for site-selective scission, have been also developed. As the result of these modifications, the site-selective scission by Ce(IV)-EDTA was achieved promptly even under high salt conditions which are otherwise unfavourable for double-duplex invasion. Furthermore, it has been shown that "molecular crowding effect," which characterizes the inside of living cells, enormously promotes the invasion, and thus the invasion seems to proceed effectively and spontaneously in the cells. Strong potential of pcPNA for further applications in vivo and in vitro has been confirmed.

Structural foundation for DNA behavior in hydrated ionic liquid: An NMR study

A well known rule of high thermal stability of GC-rich DNA helices can be reversed with the use of certain ions, rendering AT-rich duplexes more stable. We have sought to elucidate the structural basis of this phenomenon for choline dihydrogen phosphate, an ionic liquid known for extension of long-term chemical stability of biomolecules. NMR experiments complemented with CD spectroscopy revealed subtle changes of GC and AT-rich double helix structures in choline dihydrogen phosphate compared to NaCl solution. Chemical shift changes observed for different environments were used as a guide to determine choline ions' localization hotspots. For d(5'-AAATATATTT-3') choline ions are localized in the central part, especially in the minor groove near sugar protons of thymidine and H2 protons of adenine residues. In agreement with NMR data, thermodynamic analysis points to the involvement of choline ions in the hydration network as a crucial part of thermal stabilization of AT-rich helices. Analysis for GC-rich d(5'-GGGCGCGCCC-3') oligonucleotide showed preference of choline ions for major groove with less clearly defined localizations spots than in the case of its AT-rich counterpart.

Dangling ends perturb the stability of RNA duplexes responsive to surrounding conditions

Unpaired terminal nucleotides (dangling ends) occur in various biologically important RNA structures. We studied the thermal stability of RNA duplexes with dangling ends under conditions that mimic those in cells. Dangling ends of one or two nucleotides stabilized a duplex up to approximately 2.7 kcal mol(-1) in the absence of cosolutes. RNA duplexes with dangling purine nucleotides were more stable than those with pyrimidine nucleotides. Interestingly, in the presence of various cosolutes, RNA duplexes with purine dangling ends were significantly destabilized, although those with pyrimidine dangling ends were destabilized slightly. For example, in 30 wt % poly(ethylene glycol), stabilization resulting from adenine dangling ends was reduced by 1.4 kcal mol(-1) . Our quantitative analyses also showed that the number of water molecules bound to the dangling ends in an aqueous solution was independent of the nucleotide type but dependent on the stability of the dangling-end region. It has been considered that dangling ends stabilize helices; however, our results suggest that the stabilization is responsive to the surrounding conditions.

Hammerhead ribozyme activity and oligonucleotide duplex stability in mixed solutions of water and organic compounds

Nucleic acids are useful for biomedical targeting and sensing applications in which the molecular environment is different from that of a dilute aqueous solution. In this study, the influence of various types of mixed solutions of water and water-soluble organic compounds on RNA was investigated by measuring the catalytic activity of the hammerhead ribozyme and the thermodynamic stability of an oligonucleotide duplex. The compounds with a net neutral charge, such as poly(ethylene glycol), small primary alcohols, amide compounds, and aprotic solvent molecules, added at high concentrations changed the ribozyme-catalyzed RNA cleavage rate, with the magnitude of the effect dependent on the NaCl concentration. These compounds also changed the thermodynamic stability of RNA base pairs of an oligonucleotide duplex and its dependence on the NaCl concentration. Specific interactions with RNA molecules and reduced water activity could account for the inhibiting effects on the ribozyme catalysis and destabilizing effects on the duplex stability. The salt concentration dependence data correlated with the dielectric constant, but not with water activity, viscosity, and the size of organic compounds. This observation suggests the significance of the dielectric constant effects on the RNA reactions under molecular crowding conditions created by organic compounds.

Addition of bases to the 5'-end of human telomeric DNA: influences on thermal stability and energetics of unfolding

Telomeric DNA has been intensely investigated for its role in chromosome protection, aging, cell death, and disease. In humans the telomeric tandem repeat (TTAGGG)_n is found at the ends of chromosomes and provides a novel target for the development of new drugs in the treatment of age related diseases such as cancer. These telomeric sequences show slight sequence variations from species to species; however, each contains repeats of 3 to 4 guanines allowing the G-rich strands to fold into compact and stable nuclease resistant conformations referred to as G-quadruplexes. The focus of this manuscript is to examine the effects of 5'-nucleotides flanking the human telomeric core sequence 5'-AGGG(TTAGGG) ₃-3' (h-Tel22). Our studies reveal that the addition of the 5'-flanking nucleotides (5'-T, and 5'-TT) results in significant changes to the thermodynamic stability of the G-quadruplex structure. Our data indicate that the observed changes in stability are associated with changes in the number of bound waters resulting from the addition of 5'-flanking nucleotides to the h-Tel22 sequence as well as possible intermolecular interactions of the 5' overhang with the core structure.

The human telomere sequence, (TTAGGG)4, in the absence and presence of cosolutes: a spectroscopic investigation

Historically, biophysical studies of nucleic acids have been carried out under near ideal conditions, i.e., low buffer concentration (e.g., 10 mM phosphate), pH 7, low ionic strength (e.g., 100 mM) and, for optical studies, low concentrations of DNA (e.g., 1×10⁻⁶ M). Although valuable structural and thermodynamic data have come out of these studies, the conditions, for the most, part, are inadequate to simulate realistic cellular conditions. The increasing interest in studying biomolecules under more cellular-like conditions prompted us to investigate the effect of osmotic stress on the structural and thermodynamic properties of DNA oligomers containing the human telomere sequence (TTAGGG). Here, we report the characterization of (TTAGGG)4 in potassium phosphate buffer with increasing percent PEG (polyethylene glycol) or acetonitrile. In general, the presence of these cosolutes induces a conformational change from a unimolecular hybrid structure to a multimolecular parallel stranded structure. Hence, the structural change is accompanied with a change in the molecularity of quadruplex formation.

Noncanonical structures and their thermodynamics of DNA and RNA under molecular crowding: beyond the Watson-Crick double helix

How does molecular crowding affect the stability of nucleic acid structures inside cells? Water is the major solvent component in living cells, and the properties of water in the highly crowded media inside cells differ from that in buffered solution. As it is difficult to measure the thermodynamic behavior of nucleic acids in cells directly and quantitatively, we recently developed a cell-mimicking system using cosolutes as crowding reagents. The influences of molecular crowding on the structures and thermodynamics of various nucleic acid sequences have been reported. In this chapter, we discuss how the structures and thermodynamic properties of nucleic acids differ under various conditions such as highly crowded environments, compartment environments, and in the presence of ionic liquids, and the major determinants of the crowding effects on nucleic acids are discussed. The effects of molecular crowding on the activities of ribozymes and riboswitches on noncanonical structures of DNA- and RNA-like quadruplexes that play important roles in transcription and translation are also described.

Control of stability and structure of nucleic acids using cosolutes

The stabilities, structures, and functions of nucleic acids are responsive to surrounding conditions. Living cells contain biomolecules, including nucleic acids, proteins, polysaccharides, and other soluble and insoluble low-molecular weight components, that occupy a significant fraction of the cellular volume (up to 40%), resulting in a highly crowded intracellular environment. We have proven that conditions that mimic features of this intra-cellular environment alter the physical properties affect the stability, structure, and function of nucleic acids. The ability to control structure of nucleic acids by mimicking intra-cellular conditions will be useful in nanotechnology applications of nucleic acids. This paper describes methods that can be used to analyze quantitatively the intra-cellular environment effects caused by cosolutes on nucleic acid structures and to regulate properties of nucleic acids using cosolutes.

Quantitative analyses of nucleic Acid stability under the molecular crowding condition induced by cosolutes

A variety of biomolecules, including nucleic acids, proteins, polysaccharides, and other soluble and insoluble low-molecular weight components, are present in living cells. These molecules occupy a significant fraction of the cellular volume (up to 40%), resulting in a highly crowded intracellular environment. This situation is referred to as molecular crowding. Although the thermodynamic stabilities of DNA structures are known to be altered in a crowded environment, less is known about the behavior of nucleic acids and their interactions with cations and water molecules under such conditions. This unit describes methods that can be used to quantitatively analyze the molecular crowding effects caused by cosolutes on the thermodynamic stability, hydration, and cation binding of nucleic acid structures.

Base-pairing selectivity of a ureido-linked phenyl-2'-deoxycytidine derivative

Incorporation of modified nucleotides into nucleic acid strands often produces conformational constraints and steric hindrances that may change the property of base pairing. In this study, we investigated a 2'-deoxycytidine derivative that tethers a phenyl moiety to the exocyclic amino group of cytosine linked through a ureido group. This derivative compound is structurally similar to the carbamoylated DNA base lesions produced in cells. The thermodynamic and structural studies showed that the modified dC formed the base pair with dG in the complementary strand, but the base-pairing selectivity toward dG was decreased under poly(ethylene glycol)-mediated osmotic stress. The phenyl group and the ureido linker attached to dC provided selectivity for the formation of base pairing exclusively with dG in a wide range of pH conditions, whereas unmodified dC stabilized the pairings with dA or dC in acidic solutions. Moreover, this modified base could not form self-pairing through intermolecular hydrogen bonds. We suggest that formation of weak pairing and protonation of the cytosine base are hindered due to the base modification. These data provide insights into the pairing selectivity of carbamoylated cytosine lesions produced in cells, and suggest applications of the 2'-deoxycytidine derivatives in medical technologies, molecular biology experiments, and synthesis of a supramolecular network of DNA strands.

Crowding effect on DNA melting: a molecular thermodynamic model with explicit solvent

A molecular thermodynamic model is developed to examine crowding effect on DNA melting. Each pair of nucleotides in double-stranded DNA and each nucleotide in single-stranded DNA are represented by two types of charged Lennard-Jones segments, respectively. Water molecules are mimicked explicitly as spherical particles, embedded in a dielectric continuum. Crowders with varying concentration, size, interaction strength, and chain length are considered. For DNA with a sequence of A(20), the melting temperature is predicted to increase by 1 K in the presence of Ficoll70 and by 7.5 K in the presence of Ficoll70-polyvinyl pyrrolidone360 mixture. The predictions agree well with experimental data. Furthermore, the melting temperature is found to increase with increasing crowder size, but reduce with increasing interaction strength and crowder length. The predicted changes of Gibbs energy, entropy and enthalpy are consistent with experimentally measured values. The study reveals that DNA melting in a crowded environment is influenced by both entropic and enthalpic effects.