Base-Pairing Energies of Proton-Bound Dimers and Proton Affinities of 1-Methyl-5-Halocytosines: Implications for the Effects of Halogenation on the Stability of the DNA i-Motif

Influence of 2'-Modifications (O-Methylation, Fluorination, and Stereochemical Inversion) on the Base Pairing Energies of Protonated Cytidine Nucleoside Analogue Base Pairs: Implications for the Stabilities of i-Motif Structures

Naturally occurring and chemically engineered modifications are among the most powerful strategies explored for fine-tuning the conformational characteristics and intrinsic stability of nucleic acids topologies. Modifications at the 2'-position of the ribose or 2'-deoxyribose moieties differentiate nucleic acid structures and have a significant impact on their electronic properties and base-pairing interactions. 2'-O-Methylation, a common post-transcriptional modification of tRNA, is directly involved in modulating specific anticodon-codon base-pairing interactions. 2'-Fluorinated and arabino nucleosides possess novel and beneficial medicinal properties and find use as therapeutics for treating viral diseases and cancer. However, the potential to deploy 2'-modified cytidine chemistries for tuning i-motif stability is largely unknown. To address this knowledge gap, the effects of 2'-modifications including O-methylation, fluorination, and stereochemical inversion on the base-pairing interactions of protonated cytidine nucleoside analogue base pairs, the core stabilizing interactions of i-motif structures, are examined using complementary threshold collision-induced dissociation techniques and computational methods. The 2'-modified cytidine nucleoside analogues investigated here include 2'-O-methylcytidine, 2'-fluoro-2'-deoxycytidine, arabinofuranosylcytosine, 2'-fluoro-arabinofuranosylcytosine, and 2',2'-difluoro-2'-deoxycytidine. All five 2'-modifications examined here are found to enhance the base-pairing interactions relative to the canonical DNA and RNA cytidine nucleosides with the greatest enhancements arising from 2'-O-methylation and 2',2'-difluorination, suggesting that these modifications should well be tolerated in the narrow grooves of i-motif conformations.

A Comprehensive Ab Initio Study of Halogenated A···U and G···C Base Pair Geometries and Energies

Unraveling the binding preferences involved in the formation of a supramolecular complex is key to properly understand molecular recognition and aggregation phenomena, which are of pivotal importance to biology. The halogenation of nucleic acids has been routinely carried out for decades to assist in their X-ray diffraction analysis. The incorporation of a halogen atom on a DNA/RNA base not only affected its electronic distribution, but also expanded the noncovalent interactions toolbox beyond the classical hydrogen bond (HB) by incorporating the halogen bond (HalB). In this regard, an inspection of the Protein Data Bank (PDB) revealed 187 structures involving halogenated nucleic acids (either unbound or bound to a protein) where at least 1 base pair (BP) exhibited halogenation. Herein, we were interested in disclosing the strength and binding preferences of halogenated A···U and G···C BPs, which are predominant in halogenated nucleic acids. To achieve that, computations at the RI-MP2/def2-TZVP level of theory together with state of the art theoretical modeling tools (including the computation of molecular electrostatic potential (MEP) surfaces, the quantum theory of "Atoms in Molecules" (QTAIM) and the non-covalent interactions plot (NCIplot) analyses) allowed for the characterization of the HB and HalB complexes studied herein.

Influence of 5-Halogenation on the Base-Pairing Energies of Protonated Cytidine Nucleoside Analogue Base Pairs: Implications for the Stabilities of Synthetic i-Motif Structures for DNA Nanotechnology Applications

DNA nanotechnology has been employed to develop devices based on i-motif structures. The protonated cytosine-cytosine base pairs that stabilize i-motif conformations are favored under slightly acidic conditions. This unique property has enabled development of the first DNA molecular motor driven by pH changes. The ability to alter the stability and pH transition range of such DNA molecular motors is desirable. Understanding how i-motif structures are influenced by modifications, and which modifications enhance stability and/or affect the pH characteristics, are therefore of great interest. Here, the influence of 5-halogenation of the cytosine nucleobases on the base pairing of protonated cytidine nucleoside analogue base pairs is examined using complementary threshold collision-induced dissociation techniques and computational methods. The nucleoside analogues examined here include the 5-halogenated forms of the canonical DNA and RNA cytidine nucleosides. Comparisons among these systems and to the analogous canonical base pairs previously examined enable the influence of 5-halogenation and the 2'-hydroxy substituent on the base pairing to be elucidated. 5-Halogenation of the cytosine nucleobases is found to enhance the strength of base pairing of DNA base pairs and generally weakens the base pairing for RNA base pairs. Trends in the strength of base pairing indicate that both inductive and polarizability effects influence the strength of base pairing. Overall, the present results suggest that 5-halogenation, and in particular, 5-fluorination and 5-iodination, provide effective means of stabilizing DNA i-motif conformations for applications in nanotechnology, whereas only 5-iodination is effective for stabilizing RNA i-motif conformations but the enhancement in stability is less significant.

Influence of 5-Methylation and the 2'- and 3'-Hydroxy Substituents on the Base Pairing Energies of Protonated Cytidine Nucleoside Analogue Base Pairs: Implications for the Stabilities of i-Motif Structures

Repetitive nucleic acid sequences, which occur in abundance throughout the mammalian genome, are of enormous research interest due to their potential to adopt fascinating and unusual molecular structures such as the i-motif. In remarkable contrast to the DNA double helix, i-motif conformations are stabilized by protonated cytosine base pairs, (Cyt)H⁺(Cyt), that are centrally located in the core of the i-motif and intercalated vertically in an antiparallel fashion. An in-depth understanding of how modifications influence the stability of i-motif conformations is a prerequisite to understanding their biological functions and the development of effective means of tuning their stability for specific medical and technological applications. Here, the influence of the 2'- and 3'-hydroxy substituents of the sugar moieties and 5-methylation of the cytosine nucleobases on the base-pairing interactions of protonated cytidine nucleoside analogue base pairs, (xCyd)H⁺(xCyd), are examined by complementary threshold collision-induced dissociation techniques and computational methods. The xCyd nucleosides examined include the canonical DNA and RNA cytidine nucleosides, 2'-deoxycytidine (dCyd) and cytidine (Cyd), as well as several modified cytidine nucleoside analogues, 2',3'-dideoxycytidine (ddCyd), 5-methyl-2'-deoxycytidine (m⁵dCyd), and 5-methylcytidine (m⁵Cyd). Comparisons among these model base pairs indicate that the 2'- and 3'-hydroxy substituents of the sugar moieties have very little influence on the strength of the base-pairing interactions, whereas 5-methylation of the cytosine nucleobases is found to enhance the strength of the base-pairing interactions. The increase in stability resulting from 5-methylation is only modest but is more than twice as large for the DNA than RNA protonated cytidine base pair. Overall, present results suggest that canonical DNA i-motif conformations should be more stable than analogous RNA i-motif conformations and that 5-methylation of cytosine residues, a significant epigenetic marker, provides greater stabilization to DNA than RNA i-motif conformations.

A Combined Experimental and Computational Study of Halogen and Hydrogen Bonding in Molecular Salts of 5-Bromocytosine

Although natural or artificial modified pyrimidine nucleobases represent important molecules with valuable properties as constituents of DNA and RNA, no systematic analyses of the structural aspects of bromo derivatives of cytosine have appeared so far in the literature. In view of the biochemical and pharmaceutical relevance of these compounds, six different crystals containing proton-transfer derivatives of 5-bromocytosine are prepared and analyzed in the solid-state by single crystal X-ray diffraction. All six compounds are organic salts, with proton transfer occurring to the N_imino atom of the pyridine ring. Experimental results are then complemented with Hirshfeld surface analysis to quantitively evaluate the contribution of different intermolecular interactions in the crystal packing. Furthermore, theoretical calculations, based on different arrangements of molecules extracted from the crystal structure determinations, are carried out to analyze the formation mechanism of halogen bonds (XBs) in these compounds and provide insights into the nature and strength of the observed interactions. The results show that the supramolecular architectures of the six molecular salts involve extensive classical intermolecular hydrogen bonds. However, in all but one proton-transfer adducts, weak to moderate XBs are revealed by C-Br^…O short contacts between the bromine atom in the fifth position, which acts as XB donor (electron acceptor). Moreover, the lone pair electrons of the oxygen atom of adjacent pyrimidine nucleobases and/or counterions or water molecules, which acts as XB acceptor (electron donor).

Mass spectrometry and computational study of collision-induced dissociation of 9-methylguanine–1-methylcytosine base-pair radical cation: intra-base-pair proton transfer and hydrogen transfer, non-statistical dissociation, and reaction with a water ligand

A combined experimental and theoretical study is presented on the collision-induced dissociation of 9-methylguanine–1-methylcytosine base-pair radical cation ([9MG·1MC]˙⁺) and its monohydrate ([9MG·1MC]˙⁺·H₂O) with Xe and Ar gases.

Structures and Relative Glycosidic Bond Stabilities of Protonated 2'-Fluoro-Substituted Purine Nucleosides

The 2'-substituent is the primary distinguishing feature between DNA and RNA nucleosides. Modifications to this critical position, both naturally occurring and synthetic, can produce biologically valuable nucleoside analogues. The unique properties of fluorine make it particularly interesting and medically useful as a synthetic nucleoside modification. In this work, the effects of 2'-fluoro modification on the protonated gas-phase purine nucleosides are examined using complementary tandem mass spectrometry and computational methods. Direct comparisons are made with previous studies on related nucleosides. Infrared multiple photon dissociation action spectroscopy performed in both the fingerprint and hydrogen-stretching regions allows for the determination of the experimentally populated conformations. The populated conformers of protonated 2'-fluoro-2'-deoxyadenosine, [Adofl+H]⁺, and 2'-fluoro-2'-deoxyguanosine, [Guofl+H]⁺, are highly parallel to their respective canonical DNA and RNA counterparts. Both N3 and N1 protonation sites are accessed by [Adofl+H]⁺, stabilizing syn and anti nucleobase orientations, respectively. N7 protonation and anti nucleobase orientation dominates in [Guofl+H]⁺. Spectroscopically observable intramolecular hydrogen-bonding interactions with fluorine allow more definitive sugar puckering determinations than possible for the canonical systems. [Adofl+H]⁺ adopts C2'-endo sugar puckering, whereas [Guofl+H]⁺ adopts both C2'-endo and C3'-endo sugar puckering. Energy-resolved collision-induced dissociation experiments with survival yield analyses provide relative glycosidic bond stabilities. The N-glycosidic bond stabilities of the protonated 2'-fluoro-substituted purine nucleosides are found to exceed those of their canonical analogues. Further, the N-glycosidic bond stability is found to increase with increasing electronegativity of the 2'-substituent, i.e., H < OH < F. The N-glycosidic bond stability is also greater for the adenine nucleoside analogues than the guanine nucleoside analogues.

Alternated Branching Ratios by Anomaly in Collision-Induced Dissociation of Proton-Bound Hoogsteen Base Pairs of 1-Methylcytosine with 1-Methylguanine and 9-Methylguanine

A comparative study on the proton-bound complexes of 1-methylcytosine (1-mC) with 1-methylguanine (1-mG) and 9-methylguanine (9-mG), [1-mC:1-mG:H]⁺ and [1-mC:9-mG:H]⁺, respectively, was carried out using energy-resolved collision-induced dissociation (ER-CID) experiments in combination with quantum chemical calculations. In ER-CID experiments, the measured survival yields indicated an essentially identical stability for the two proton-bound complexes. In comparison with the lowest-energy structures and base-pairing energetics predicted at the B3LYP/6-311+G(2d,2p) theory level, both complexes produced in this study were suggested to be proton-bound Hoogsteen base pairs. Curiously, despite the similarity in structures, binding energetics, and potential energy surfaces predicted by the B3LYP theory, the fragment branching ratios exhibited an intriguing alternation between the two proton-bound Hoogsteen base pairs. The CID of [1-mC:1-mG:H]⁺ produced protonated cytosines, [1-mC:H]⁺, more abundantly than [1-mG:H]⁺, whereas that of [1-mC:9-mG:H]⁺ gave rise to a more pronounced production of protonated guanines, [9-mG:H]⁺. However, using the proton affinities of moieties predicted by the high-accuracy methods, including CBS-QB3 and the Guassian-4 theory, the anomaly known for [Cytosine:Guanine:H]⁺ (J. Am. Soc. Mass Spectrom. 29, 2368-2379 (2018)) successfully accounted for the alternated branching ratios. Thereby, the anomaly, more specifically, the production of proton-transferred fragments of O-protonated cytosines in the CID of proton-bound Hoogsteen base pairs, is indeed real, which is disclosed as the alternated branching ratios in the CID spectra of [1-mC:1-mG:H]⁺ and [1-mC:9-mG:H]⁺ in this study. Graphical Abstract .

Proton Transfer Accounting for Anomalous Collision-Induced Dissociation of Proton-Bound Hoogsteen Base Pair of Cytosine and Guanine

To understand the anomalous collision-induced dissociation (CID) behavior of the proton-bound Hoogsteen base pair of cytosine (C) and guanine (G), C:H⁺∙∙∙G, we investigated CID of a homologue series of proton-bound heterodimers of C, 1-methylcytosine, and 5-methylcytosine with G as a common base partner. The CID experiments were performed in an energy-resolved way (ER-CID) under both multiple and near-single collision conditions. The relative stabilities of the protonated complexes examined by ER-CID suggested that the proton-bound complexes produced by electrospray ionization in this study are proton-bound Hoogsteen base pairs. On the other hand, in contrast to the other base pairs, CID of C:H⁺∙∙∙G exhibited more abundant productions of C:H⁺, the fragment protonated on the moiety with a smaller proton affinity, than that of G:H⁺. This appeared to contradict general prediction based on the kinetic method. However, further theoretical exploration of potential energy surfaces found that there can be facile proton transfers in the proton-bound Hoogsteen base pairs during the CID process, which makes the process accessible to an additional product state of O-protonated C for C:H⁺ fragments. The presence of an additional dissociation channel, which in other words corresponds to twofold degeneracy in the transition state leading to C:H⁺ fragments, effectively doubles the apparent reaction rate for production of C:H⁺. In this way, the process gives rise to the anomaly, the observed pronounced formation of C:H⁺ in the CID of the proton-bound Hoogsteen base pair, C:H⁺∙∙∙G. Graphical Abstract ᅟ.

Understanding proton capture and cation-induced dimerization of [Ag29(BDT)12]3−clusters by ion mobility mass spectrometry

We present a unique reaction of [Ag₂₉(BDT)₁₂]³⁻cluster with protons and dimerization of the cluster induced by alkali metal ions.

N3 and O2 Protonated Conformers of the Cytosine Mononucleotides Coexist in the Gas Phase

The gas-phase conformations of the protonated forms of the DNA and RNA cytosine mononucleotides, [pdCyd+H]⁺ and [pCyd+H]⁺, are examined by infrared multiple photon dissociation (IRMPD) action spectroscopy over the IR fingerprint and hydrogen-stretching regions complemented by electronic structure calculations. The low-energy conformations of [pdCyd+H]⁺ and [pCyd+H]⁺ and their relative stabilities are computed at the B3LYP/6-311+G(2d,2p)//B3LYP/6-311+G(d,p) and MP2(full)/6-311+G(2d,2p)//B3LYP/6-311+G(d,p) levels of theory. Comparisons of the measured IRMPD action spectra and B3LYP/6-311+G(d,p) linear IR spectra computed for the low-energy conformers allow the conformers present in the experiments to be determined. Similar to that found in previous IRMPD action spectroscopy studies of the protonated forms of the cytosine nucleosides, [dCyd+H]⁺ and [Cyd+H]⁺, both N3 and O2 protonated cytosine mononucleotides exhibiting an anti orientation of cytosine are found to coexist in the experimental population. The 2'-hydroxyl substituent does not significantly influence the most stable conformations of [pCyd+H]⁺ versus those of [pdCyd+H]⁺, as the IRMPD spectral profiles of [pdCyd+H]⁺ and [pCyd+H]⁺ are similar. However, the presence of the 2'-hydroxyl substituent does influence the relative intensities of the measured IRMPD bands. Comparisons to IRMPD spectroscopy studies of the deprotonated forms of the cytosine mononucleotides, [pdCyd-H]^- and [pCyd-H]^-, provide insight into the effects of protonation versus deprotonation on the conformational features of the nucleobase and sugar moieties. Likewise, comparisons to results of IRMPD spectroscopy studies of the protonated cytosine nucleosides provide insight into the influence of the phosphate moiety on structure. Comparison with previous ion mobility results shows the superiority of IRMPD spectroscopy for distinguishing various protonation sites. Graphical Abstract ᅟ.

Tautomerization lowers the activation barriers for N-glycosidic bond cleavage of protonated uridine and 2'-deoxyuridine

The gas-phase conformations of protonated uridine, [Urd+H](+), and its 2'-deoxy form, protonated 2'-deoxyuridine, [dUrd+H](+), have been examined in detail previously by infrared multiple photon dissociation action spectroscopy techniques. Both 2,4-dihydroxy tautomers and O4 protonated conformers of [Urd+H](+) and [dUrd+H](+) were found to coexist in the experiments with the 2,4-dihydroxy tautomers dominating the population. In the present study, the kinetic energy dependence of the collision-induced dissociation behavior of [Urd+H](+) and [dUrd+H](+) are examined using a guided ion beam tandem mass spectrometer to probe the mechanisms and energetics for activated dissociation of these protonated nucleosides. The primary dissociation pathways observed involve N-glycosidic bond cleavage leading to competitive elimination of protonated or neutral uracil. The potential energy surfaces (PESs) for these N-glycosidic bond cleavage pathways are mapped out via electronic structure calculations for the mixture of 2,4-dihydroxy tautomers and O4 protonated conformers of [Urd+H](+) and [dUrd+H](+) populated in the experiments. The calculated activation energies (AEs) and heats of reaction (ΔHrxns) for N-glycosidic bond cleavage at both the B3LYP and MP2(full) levels of theory are compared to the measured values. The agreement between experiment and theory indicates that B3LYP provides better estimates of the energetics of the species along the PESs for N-glycosidic bond cleavage than MP2, and that the 2,4-dihydroxy tautomers, which are stabilized by strong hydrogen-bonding interactions, predominantly influence the observed threshold dissociation behavior of [Urd+H](+) and [dUrd+H](+).

Cationic Noncovalent Interactions: Energetics and Periodic Trends

In this review, noncovalent interactions of ions with neutral molecules are discussed. After defining the scope of the article, which excludes anionic and most protonated systems, methods associated with measuring thermodynamic information for such systems are briefly recounted. An extensive set of tables detailing available thermodynamic information for the noncovalent interactions of metal cations with a host of ligands is provided. Ligands include small molecules (H2, NH3, CO, CS, H2O, CH3CN, and others), organic ligands (O- and N-donors, crown ethers and related molecules, MALDI matrix molecules), π-ligands (alkenes, alkynes, benzene, and substituted benzenes), miscellaneous inorganic ligands, and biological systems (amino acids, peptides, sugars, nucleobases, nucleosides, and nucleotides). Hydration of metalated biological systems is also included along with selected proton-based systems: 18-crown-6 polyether with protonated peptides and base-pairing energies of nucleobases. In all cases, the literature thermochemistry is evaluated and, in many cases, reanchored or adjusted to 0 K bond dissociation energies. Trends in these values are discussed and related to a variety of simple molecular concepts.