A Cytidine Phosphoramidite with Protected Nitroxide Spin Label: Synthesis of a Full-Length TAR RNA and Investigation by In-Line Probing and EPR Spectroscopy

Revealing light-induced structural shifts in G-quadruplex-porphyrin complexes: a pulsed dipolar EPR study

The binding of G-quadruplex structures (G4s) with photosensitizers is of considerable importance in medicinal chemistry and drug discovery due to their promising potential in photodynamic therapy applications. G4s can experience structural changes as a result of ligand interactions and light exposure. Understanding these modifications is essential to uncover the fundamental biological roles of the complexes and optimize their therapeutic potential. The structural diversity of G4s makes it challenging to study their complexes with ligands, necessitating the use of various complementary methods to fully understand these interactions. In this study, we introduce, for the first time, the application of laser-induced dipolar EPR as a method to characterize G-quadruplex DNA complexes containing photosensitizers and to investigate light-induced structural modifications in these systems. To demonstrate the feasibility of this approach, we studied complexes of the human telomeric G-quadruplex (HTel-22) with cationic 5,10,15,20-tetrakis(1-methyl-4-pyridinio) porphyrin tetra(p-toluenesulfonate) (TMPyP4). In addition to showcasing a new methodology, we also aimed to provide insights into the mechanisms underlying photoinduced HTel-22/TMPyP4 structural changes, thereby aiding in the advancement of approaches targeting G4s in photodynamic therapy. EPR revealed G-quadruplex unfolding and dimer formation upon light exposure. Our findings demonstrate the potential of EPR spectroscopy for examining G4 complexes with photosensitizers and contribute to a better understanding of G4s' interactions with ligands under light.

A review on recent advances in methods for site-directed spin labeling of long RNAs

RNAs are important biomolecules that play essential roles in various cellular processes and are crucially linked with many human diseases. The key to elucidate the mechanisms underlying their biological functions and develop RNA-based therapeutics is to investigate RNA structure and dynamics and their connections to function in detail using a variety of approaches. Magnetic resonance techniques including paramagnetic nuclear magnetic resonance (NMR) and electron magnetic resonance (EPR) spectroscopies have proved to be powerful tools to gain insights into such properties. The prerequisites for paramagnetic NMR and EPR studies on RNAs are to achieve site-specific spin labeling of the intrinsically diamagnetic RNAs, which however is not trivial, especially for long ones. In this review, we present some covalent labeling strategies that allow site-specific introduction of electron spins to long RNAs. Generally, these strategies include assembly of long RNAs via enzymatic ligation of short oligonucleotides, co- and post-transcriptional site-specific labeling empowered with the unnatural base pair system, and direct enzymatic functionalization of natural RNAs. We introduce a few case studies to discuss the advantages and limitations of each strategy, and to provide a vision for the future development.

Spin-Labeled Riboswitch Synthesized from a Protected TPA Phosphoramidite Building Block

The nitroxide TPA (2,2,5,5-tetramethyl-pyrrolin-1-oxyl-3-acetylene) is an excellent spin label for EPR studies of RNA. Previous synthetic methods, however, are complicated and require special equipment. Herein, we describe a uridine derived phosphoramidite with a photocaged TPA unit attached. The light sensitive 2-nitrobenzyloxymethyl group can be removed in high yield by short irradiation at 365 nm. Based on this approach, a doubly spin-labeled 27mer neomycin sensing riboswitch was synthesized and studied by PELDOR. The overall thermal stability of the fold is not much reduced by TPA. In-line probing nevertheless detected changes in local mobility.

Recent advances in RNA structurome

RNA structures are essential to support RNA functions and regulation in various biological processes. Recently, a range of novel technologies have been developed to decode genome-wide RNA structures and novel modes of functionality across a wide range of species. In this review, we summarize key strategies for probing the RNA structurome and discuss the pros and cons of representative technologies. In particular, these new technologies have been applied to dissect the structural landscape of the SARS-CoV-2 RNA genome. We also summarize the functionalities of RNA structures discovered in different regulatory layers-including RNA processing, transport, localization, and mRNA translation-across viruses, bacteria, animals, and plants. We review many versatile RNA structural elements in the context of different physiological and pathological processes (e.g., cell differentiation, stress response, and viral replication). Finally, we discuss future prospects for RNA structural studies to map the RNA structurome at higher resolution and at the single-molecule and single-cell level, and to decipher novel modes of RNA structures and functions for innovative applications.

Paramagnetic Chemical Probes for Studying Biological Macromolecules

Paramagnetic chemical probes have been used in electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopy for more than four decades. Recent years witnessed a great increase in the variety of probes for the study of biological macromolecules (proteins, nucleic acids, and oligosaccharides). This Review aims to provide a comprehensive overview of the existing paramagnetic chemical probes, including chemical synthetic approaches, functional properties, and selected applications. Recent developments have seen, in particular, a rapid expansion of the range of lanthanoid probes with anisotropic magnetic susceptibilities for the generation of structural restraints based on residual dipolar couplings and pseudocontact shifts in solution and solid state NMR spectroscopy, mostly for protein studies. Also many new isotropic paramagnetic probes, suitable for NMR measurements of paramagnetic relaxation enhancements, as well as EPR spectroscopic studies (in particular double resonance techniques) have been developed and employed to investigate biological macromolecules. Notwithstanding the large number of reported probes, only few have found broad application and further development of probes for dedicated applications is foreseen.

Long-range distance determination in fully deuterated RNA with pulsed EPR spectroscopy

Pulsed electron-electron double resonance (PELDOR or DEER) spectroscopy is powerful in structure and dynamics study of biological macromolecules by providing distance distribution information ranging from 1.8 to 6 nm, providing that the biomolecules are site-specifically labeled with paramagnetic tags. However, long distances up to 16 nm have been measured on perdeuterated and spin-labeled proteins in deuterated solvent by PELDOR. Here we demonstrate long-range distance measurement on a large RNA, the 97-nucleotide 3'SL RNA element of the Dengue virus 2 genome, by combining a posttranscriptional site-directed spin labeling method using an unnatural basepair system with RNA perdeuteration by enzymatic synthesis using deuterated nucleotides. The perdeuteration removes the coupling of the electron spins of the nitroxide spin labels from the proton nuclear spin system of the RNA and does extend the observation time windows of PELDOR up to 50 μs. This enables one to determine long distances up to 14 nm for large RNAs and their conformational flexibility.

Preparation of Site-Specifically Spin-Labeled RNA by in Vitro Transcription Using an Expanded Genetic Alphabet

Recent advances in pulsed electron paramagnetic resonance (EPR) spectroscopy enable studying structure and folding of nucleic acids. An efficient introduction of spin labels at specific positions within the oligonucleotide sequence is a prerequisite. We here present a step-by-step guide to synthesize long RNA oligonucleotides bearing spin labels at specific positions within the sequence. RNA preparation is achieved enzymatically via in vitro transcription using an expanded genetic alphabet. Highly structured, several hundred nucleotides long RNAs with two nitroxide spin labels at specific positions can be prepared by this method.

Strategies for Covalent Labeling of Long RNAs

The introduction of chemical modifications into long RNA molecules at specific positions for visualization, biophysical investigations, diagnostic and therapeutic applications still remains challenging. In this review, we present recent approaches for covalent internal labeling of long RNAs. Topics included are the assembly of large modified RNAs via enzymatic ligation of short synthetic oligonucleotides and synthetic biology approaches preparing site-specifically modified RNAs via in vitro transcription using an expanded genetic alphabet. Moreover, recent approaches to employ deoxyribozymes (DNAzymes) and ribozymes for RNA labeling and RNA methyltransferase based labeling strategies are presented. We discuss the potentials and limits of the individual methods, their applicability for RNAs with several hundred to thousands of nucleotides in length and indicate future directions in the field.

Covalent labeling of nucleic acids

Labeling of nucleic acids is required for many studies aiming to elucidate their functions and dynamics in vitro and in cells. Out of the numerous labeling concepts that have been devised, covalent labeling provides the most stable linkage, an unrivaled choice of small and highly fluorescent labels and - thanks to recent advances in click chemistry - an incredible versatility. Depending on the approach, site-, sequence- and cell-specificity can be achieved. DNA and RNA labeling are rapidly developing fields that bring together multiple areas of research: on the one hand, synthetic and biophysical chemists develop new fluorescent labels and isomorphic nucleobases as well as faster and more selective bioorthogonal reactions. On the other hand, the number of enzymes that can be harnessed for post-synthetic and site-specific labeling of nucleic acids has increased significantly. Together with protein engineering and genetic manipulation of cells, intracellular and cell-specific labeling has become possible. In this review, we provide a structured overview of covalent labeling approaches for nucleic acids and highlight notable developments, in particular recent examples. The majority of this review will focus on fluorescent labeling; however, the principles can often be readily applied to other labels. We will start with entirely chemical approaches, followed by chemo-enzymatic strategies and ribozymes, and finish with metabolic labeling of nucleic acids. Each section is subdivided into direct (or one-step) and two-step labeling approaches and will start with DNA before treating RNA.

From in vitro towards in situ: structure-based investigation of ABC exporters by electron paramagnetic resonance spectroscopy

ATP-binding cassette (ABC) exporters have been studied now for more than four decades, and recent structural investigation has produced a large number of protein database entries. Yet, important questions about how ABC exporters function at the molecular level remain debated, such as which are the molecular recognition hotspots and the allosteric couplings dynamically regulating the communication between the catalytic cycle and the export of substrates. This conundrum mainly arises from technical limitations confining all research to in vitro analysis of ABC transporters in detergent solutions or embedded in membrane-mimicking environments. Therefore, a largely unanswered question is how ABC exporters operate in situ, namely in the native membrane context of a metabolically active cell. This review focuses on novel mechanistic insights into type I ABC exporters gained through a unique combination of structure determination, biochemical characterization, generation of conformation-specific nanobodies/sybodies and double electron-electron resonance.

Thiocoumarin Caged Nucleotides: Synthetic Access and Their Photophysical Properties

Photocages have been successfully applied in cellular signaling studies for the controlled release of metabolites with high spatio-temporal resolution. Commonly, coumarin photocages are activated by UV light and the quantum yields of uncaging are relatively low, which can limit their applications in vivo. Here, syntheses, the determination of the photophysical properties, and quantum chemical calculations of 7-diethylamino-4-hydroxymethyl-thiocoumarin (thio-DEACM) and caged adenine nucleotides are reported and compared to the widely used 7-diethylamino-4-hydroxymethyl-coumarin (DEACM) caging group. In this comparison, thio-DEACM stands out as a phosphate cage with improved photophysical properties, such as red-shifted absorption and significantly faster photolysis kinetics.

Do the P1 and P2 hairpins of the Guanidine-II riboswitch interact?

Riboswitches regulate genes by adopting different structures in responds to metabolite binding. The guanidine-II riboswitch is the smallest representative of the ykkC class with the mechanism of its function being centred on the idea that its two stem loops P1 and P2 form a kissing hairpin interaction upon binding of guanidinium (Gdm⁺). This mechanism is based on in-line probing experiments with the full-length riboswitch and crystal structures of the truncated stem loops P1 and P2. However, the crystal structures reveal only the formation of the homodimers P1 | P1 and P2 | P2 but not of the proposed heterodimer P1 | P2. Here, site-directed spin labeling (SDSL) in combination with Pulsed Electron–Electron Double Resonance (PELDOR or DEER) is used to study their structures in solution and how they change upon binding of Gdm⁺. It is found that both hairpins adopt different structures in solution and that binding of Gdm⁺ does indeed lead to the formation of the heterodimer but alongside the homodimers in a statistical 1:2:1 fashion. These results do thus support the proposed switching mechanism.

Investigation of Strand-Selective Interaction of SNA-Modified siRNA with AGO2-MID

Small interfering RNA (siRNA) has been recognized as a powerful gene-silencing tool. For therapeutic application, chemical modification is often required to improve the properties of siRNA, including its nuclease resistance, activity, off-target effects, and tissue distribution. Careful siRNA guide strand selection in the RNA-induced silencing complex (RISC) is important to increase the RNA interference (RNAi) activity as well as to reduce off-target effects. The passenger strand-mediated off-target activity was previously reduced and on-target activity was enhanced by substitution with acyclic artificial nucleic acid, namely serinol nucleic acid (SNA). In the present study, the reduction of off-target activity caused by the passenger strand was investigated by modifying siRNAs with SNA. The interactions of SNA-substituted mononucleotides, dinucleotides, and (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO)-labeled double-stranded RNA (dsRNA) with the MID domain of the Argonaute 2 (AGO2) protein, which plays a pivotal role in strand selection by accommodation of the 5’-terminus of siRNA, were comprehensively analyzed. The obtained nuclear magnetic resonance (NMR) data revealed that AGO2-MID selectively bound to the guide strand of siRNA due to the inhibitory effect of the SNA backbone located at the 5’ end of the passenger strand.

EPR Distance Measurements on Long Non-coding RNAs Empowered by Genetic Alphabet Expansion Transcription

We present herein a novel nitroxide spin label-containing RNA triphosphate TPT3NO and its application for site-specific spin-labeling of RNA through in vitro transcription using an expanded genetic alphabet. Our strategy allows the facile preparation of spin-labeled RNAs with sizes ranging from short RNA oligonucleotides to large, complex RNA molecules with over 370 nucleotides by standard in vitro transcription. As a proof of concept, inter-spin distance distributions are measured by pulsed electron paramagnetic resonance (EPR) spectroscopy in short self-complementary RNA sequences and in a well-studied 185 nucleotide non-coding RNA, the B. subtilis glmS ribozyme. The approach is then applied to probe for the first time the folding of the 377 nucleotide A-region of the long non-coding RNA Xist, by PELDOR.

2'-Alkynyl spin-labelling is a minimally perturbing tool for DNA structural analysis

The determination of distances between specific points in nucleic acids is essential to understanding their behaviour at the molecular level. The ability to measure distances of 2-10 nm is particularly important: deformations arising from protein binding commonly fall within this range, but the reliable measurement of such distances for a conformational ensemble remains a significant challenge. Using several techniques, we show that electron paramagnetic resonance (EPR) spectroscopy of oligonucleotides spin-labelled with triazole-appended nitroxides at the 2' position offers a robust and minimally perturbing tool for obtaining such measurements. For two nitroxides, we present results from EPR spectroscopy, X-ray crystal structures of B-form spin-labelled DNA duplexes, molecular dynamics simulations and nuclear magnetic resonance spectroscopy. These four methods are mutually supportive, and pinpoint the locations of the spin labels on the duplexes. In doing so, this work establishes 2'-alkynyl nitroxide spin-labelling as a minimally perturbing method for probing DNA conformation.

Spacer-Mediated Control of Coumarin Uncaging for Photocaged Thymidine

Despite its importance in the design of photocaged molecules, less attention is focused on linker chemistry than the cage itself. Here, we describe unique uncaging properties displayed by two coumarin-caged thymidine compounds, each conjugated with (2) or without (1) an extended, self-immolative spacer. Photolysis of 1 using long-wavelength UVA (365 nm) or visible (420, 455 nm) light led to the release of free thymidine along with the competitive generation of a thymidine-bearing recombination product. The occurrence of this undesired side reaction, which is previously unreported, was not present with the photolysis of 2, which released thymidine exclusively with higher quantum efficiency. We propose that the spatial separation between the cage and the substrate molecule conferred by the extended linker can play a critical role in circumventing this unproductive reaction. This report reinforces the importance of linker selection in the design of coumarin-caged oligonucleosides and other conjugates.

Site-Directed Spin Labeling of RNA with a Gem-Diethylisoindoline Spin Label: PELDOR, Relaxation, and Reduction Stability

Ribonucleic acid function is governed by its structure, dynamics, and interaction with other biomolecules and influenced by the local environment. Thus, methods are needed that enable one to study RNA under conditions as natural as possible, possibly within cells. Site-directed spin-labeling of RNA with nitroxides in combination with, for example, pulsed electron-electron double resonance (PELDOR or DEER) spectroscopy has been shown to provide such information. However, for in-cell measurements, the usually used gem-dimethyl nitroxides are less suited, because they are quickly reduced under in-cell conditions. In contrast, gem-diethyl nitroxides turned out to be more stable, but labeling protocols for binding these to RNA have been sparsely reported. Therefore, we describe here the bioconjugation of an azide functionalized gem-diethyl isoindoline nitroxide to RNA using a copper (I)-catalyzed azide-alkyne cycloaddition ("click"-chemistry). The labeling protocol provides high yields and site selectivity. The analysis of the orientation selective PELDOR data show that the gem-diethyl and gem-dimethyl labels adopt similar conformations. Interestingly, in deuterated buffer, both labels attached to RNA yield TM relaxation times that are considerably longer than observed for the same type of label attached to proteins, enabling PELDOR time windows of up to 20 microseconds. Together with the increased stability in reducing environments, this label is very promising for in-cell Electron Paramagnetic Resonance (EPR) studies.

Protein Spin Labeling with a Photocaged Nitroxide Using Diels-Alder Chemistry

EPR spectroscopy of diamagnetic bio-macromolecules is based on site-directed spin labeling (SDSL). Herein, a novel labeling strategy for proteins is presented. A nitroxide-based spin label has been developed and synthesized that can be ligated to proteins by an inverse-electron-demand Diels-Alder (DAinv ) cycloaddition to genetically encoded noncanonical amino acids. The nitroxide moiety is shielded by a photoremovable protecting group with an attached tetra(ethylene glycol) unit to achieve water solubility. SDSL is demonstrated on two model proteins with the photoactivatable nitroxide for DAinv reaction (PaNDA) label. The strategy features high reaction rates, combined with high selectivity, and the possibility to deprotect the nitroxide in Escherichia coli lysate.

Nucleic acid enzymes based on functionalized nucleosides

Nucleic acid-based enzymes have recently joined their proteinaceous counterparts as important biocatalysts. While RNA enzymes (ribozymes) are found in nature, deoxyribozymes or DNAzymes are man-made entities. Numerous ribozymes and DNAzymes have been identified by Darwinian selection methods to catalyze a broad array of chemical transformations. Despite these important advances, practical applications involving nucleic acid enzymes are often plagued by relatively poor pharmacokinetic properties and cellular uptake, rapid degradation by nucleases and/or by the limited chemical arsenal carried by natural DNA and RNA. In this review, the two main chemical approaches for the modification of nucleic acid-based catalysts, particularly DNAzymes, are described. These methods aim at improving the functional properties of nucleic acid enzymes by mitigating some of these shortcomings. In this context, recent developments in the post-SELEX processing of existing nucleic acid catalysts as well as efforts for the selection of DNAzymes and ribozymes with modified nucleoside triphosphates are summarized.

Site-Specific Cleavage of RNAs Derived from the PIM1 3'-UTR by a Metal-Free Artificial Ribonuclease

Oligonucleotide conjugates of tris(2-aminobenzimidazole) have been reported previously to cleave complementary RNA strands with high levels of sequence and site specificity. The RNA substrates used in these studies were oligonucleotides not longer than 29-mers. Here we show that ~150⁻400-mer model transcripts derived from the 3'-untranslated region of the PIM1 mRNA reacted with rates and specificities comparable to those of short oligonucleotide substrates. The replacement of DNA by DNA/LNA mixmers further increased the cleavage rate. Tris(2-aminobenzimidazoles) were designed to interact with phosphates and phosphate esters. A cell, however, contains large amounts of phosphorylated species that may cause competitive inhibition of RNA cleavage. It is thus important to note that no loss in reaction rates was observed in phosphate buffer. This opens the way to in-cell applications for this type of artificial nuclease. Furthermore, we disclose a new synthetic method giving access to tris(2-aminobenzimidazoles) in multigram amounts.

Posttranscriptional spin labeling of RNA by tetrazine-based cycloaddition

Spin labeling of in vitro transcribed RNA by iEDDA click chemistry is demonstrated. This allows the determination of distance distributions between two nitroxide spin labels by PELDOR in a self-complementary RNA duplex.

Influence of Mg2+ on the conformational flexibility of a tetracycline aptamer

The tetracycline-binding RNA aptamer (TC-aptamer) is a synthetic riboswitch that binds the antibiotic tetracycline (TC) with exceptionally high affinity. Although a crystal structure exists of the TC-bound state, little is known about the conformational dynamics and changes upon ligand binding. In this study, pulsed electron paramagnetic resonance techniques for measuring distances (PELDOR) in combination with rigid nitroxide spin labels (Çm spin label) were used to investigate the conformational flexibility of the TC-aptamer in the presence and absence of TC at different Mg²⁺ concentrations. TC was found to be the essential factor for stabilizing the tertiary structure at intermediate Mg²⁺ concentrations. At higher Mg²⁺ concentrations, Mg²⁺ alone is sufficient to stabilize the tertiary structure. In addition, the orientation of the two spin-labeled RNA helices with respect to each other was analyzed with orientation-selective PELDOR and compared to the crystal structure. These results demonstrate for the first time the unique value of the Çm spin label in combination with PELDOR to provide information about conformational flexibilities and orientations of secondary structure elements of biologically relevant RNAs.

Phosphoramidite building blocks with protected nitroxides for the synthesis of spin-labeled DNA and RNA

TEMPO spin labels protected with 2-nitrobenzyloxymethyl groups were attached to the amino residues of three different nucleosides: deoxycytidine, deoxyadenosine, and adenosine. The corresponding phosphoramidites could be incorporated by unmodified standard procedures into four different self-complementary DNA and two RNA oligonucleotides. After photochemical removal of the protective group, elimination of formic aldehyde and spontaneous air oxidation, the nitroxide radicals were regenerated in high yield. The resulting spin-labeled palindromic duplexes could be directly investigated by PELDOR spectroscopy without further purification steps. Spin–spin distances measured by PELDOR correspond well to the values obtained from molecular models.

Advances in the cellular structural biology of nucleic acids

Conventional biophysical and chemical biology approaches for delineating relationships between the structure and biological function of nucleic acids (NAs) abstract NAs from their native biological context. However, cumulative experimental observations have revealed that the structure, dynamics and interactions of NAs might be strongly influenced by a broad spectrum of specific and nonspecific physical-chemical environmental factors. This consideration has recently sparked interest in the development of novel tools for structural characterization of NAs in the native cellular context. Here, we review the individual methods currently being employed for structural characterization of NA structure in a native cellular environment with a focus on recent advances and developments in the emerging fields of in-cell NMR and electron paramagnetic resonance spectroscopy and in-cell single-molecule FRET of NAs.