Non-conventional Hydrogen Bonds Research Articles

Conformational Preferences at the Glycosidic Linkage of Saccharides in Solution as Deduced from NMR Experiments and MD Simulations: Comparison to Crystal Structures.

Glycans are central to information content and regulation in biological systems. These carbohydrate molecules are active either as oligo- or polysaccharides, often in the form of glycoconjugates. The monosaccharide entities are joined by glycosidic linkages and stereochemical arrangements are of utmost importance in determining conformation and flexibility of saccharides. The conformational preferences and population distributions at the glycosidic torsion angles φ and ψ have been investigated for O-methyl glycosides of three disaccharides where the substitution takes place at a secondary alcohol, viz., in α-l-Fucp-(1→3)-β-d-Glcp-OMe, α-l-Fucp-(1→3)-α-d-Galp-OMe and α-d-Glcp-(1→4)-α-d-Galp-OMe, corresponding to disaccharide structural elements present in bacterial polysaccharides. Stereochemical differences at or adjacent to the glycosidic linkage were explored by solution state NMR spectroscopy using one-dimensional 1 H,1 H-NOESY NMR experiments to obtain transglycosidic proton-proton distances and one- and two-dimensional heteronuclear NMR experiments to obtain 3 JCH transglycosidic coupling constants related to torsion angles φ and ψ. Computed effective proton-proton distances from molecular dynamics (MD) simulations showed excellent agreement to experimentally derived distances for the α-(1→3)-linked disaccharides and revealed that for the bimodal distribution at the ψ torsion angle for the α-(1→4)-linked disaccharide experiment and simulation were at variance with each other, calling for further force field developments. The MD simulations disclosed a highly intricate inter-residue hydrogen bonding pattern for the α-(1→4)-linked disaccharide, including a nonconventional hydrogen bond between H5' in the glucosyl residue and O3 in the galactosyl residue, supported by a large downfield 1 H NMR chemical shift displacement compared to α-d-Glcp-OMe. Comparison of population distributions of the glycosidic torsion angles φ and ψ in the disaccharide entities to those of corresponding crystal structures highlighted the potential importance of solvation on the preferred conformation.

Characteristics of nonconventional hydrogen bonds and stability of dimers of chalcogenoaldehyde derivatives: a noticeable role of oxygen compared to other chalcogens.

In this work, twenty-four stable dimers of RCHZ with R = H, F, Cl, Br, CH3 or NH2 and Z = O, S, Se or Te were determined. It was found that the stability of most dimers is primarily contributed by the electrostatic force, except for the dominant role of the induction term in those involving a Te atom, which has been rarely observed. Both electron-donating and -withdrawing groups in substituted formaldehyde cause an increase in the strength of nonconventional Csp2-H⋯Z hydrogen bonds, as well as the dimers, in which the electron donating effect plays a more crucial role. The strength of nonconventional hydrogen bonds decreases in the following order: Csp2-H⋯O ≫ Csp2-H⋯S > Csp2-H⋯Se > Csp2-H⋯Te. Remarkably, a highly significant role of the O atom compared to S, Se and Te in increasing the Csp2-H stretching frequency and strength of the nonconventional hydrogen bonds and dimers is found. A Csp2-H stretching frequency red-shift is observed in Csp2-H⋯S/Se/Te, while a blue-shift is obtained in Csp2-H⋯O. When Z changes from O to S to Se and to Te, the Csp2-H blue-shift tends to decrease and eventually turns to a red-shift, in agreement with the increasing order of the proton affinity at Z in the isolated monomer. The magnitude of the Csp2-H stretching frequency red-shift is larger for Csp2-H⋯Te than Csp2-H⋯S/Se, consistent with the rising trend of proton affinity at the Z site and the polarity of the Csp2-H bond in the substituted chalcogenoaldehydes. The Csp2-H blue-shifting of the Csp2-H⋯O hydrogen bonds is observed in all dimers regardless of the electron effect of the substituents. Following complexation, the electron-donating derivatives exhibit a stronger Csp2-H blue-shift compared to the electron-withdrawing ones. Notably, the stronger Csp2-H blue-shift turns out to involve a less polarized Csp2-H bond and a decrease in the occupation at the σ*(Csp2-H) antibonding orbital in the isolated monomer.

Charge distribution in oxygen[formula omitted]fluorobenzene complex anions [O[formula omitted]C[formula omitted]H[formula omitted]F[formula omitted]][formula omitted] ([formula omitted] 0–6)

Recently, temporary anion states of fluorine substituted benzenes have been probed via photodetachment experiments on oxygen-fluorobenzene anion complexes [O2⋅C6H6−nFn]−, n=0…6. Here, we complement these experiments with a computational characterization. For the ground electronic states, two isomers are identified: The first isomer class shows non-conventional hydrogen bonds, while the second shows carbon-oxygen contacts. For both isomer classes, the electron affinity of the complex is significantly higher than that of either moiety, and the electron affinity increase upon complex formation is studied in detail. Both isomer classes show strong O2−−C6H6−nFn interactions, and the extent of charge donation from O2− to the organic moiety is characterized. Moreover, we characterize charge-transfer excited states of [O2⋅C6H6−nFn]− anion complexes corresponding to neutral O2 bound to C6H6−nFn−. The charge transfer complexes form doublets and quartets, and here we focus on the minimal energy structure of the quartet state and characterize the doublet at the same geometry.

Synthesis of a glycan hairpin

The primary sequence of a biopolymer encodes the essential information for folding, permitting to carry out sophisticated functions. Inspired by natural biopolymers, peptide and nucleic acid sequences have been designed to adopt particular three-dimensional (3D) shapes and programmed to exert specific functions. In contrast, synthetic glycans capable of autonomously folding into defined 3D conformations have so far not been explored owing to their structural complexity and lack of design rules. Here we generate a glycan that adopts a stable secondary structure not present in nature, a glycan hairpin, by combining natural glycan motifs, stabilized by a non-conventional hydrogen bond and hydrophobic interactions. Automated glycan assembly enabled rapid access to synthetic analogues, including site-specific 13C-labelled ones, for nuclear magnetic resonance conformational analysis. Long-range inter-residue nuclear Overhauser effects unequivocally confirmed the folded conformation of the synthetic glycan hairpin. The capacity to control the 3D shape across the pool of available monosaccharides has the potential to afford more foldamer scaffolds with programmable properties and functions.

Contributions of nonconventional intramolecular hydrogen bond on the bioactivity of a series of substituted 6-amino, 4-phenyl, tetrahydroquinoline derivatives

Although intramolecular hydrogen bond formation/disruption is a critical tool in the drug absorption process, it is difficult to be determined due to its group-atom nature. We proposed a proper descriptor to reasonably correlate pIC50 of a series of 19 substituted 6-amino, 4-phenyl, tetrahydroquinoline (6A4PTHQ) derivatives with their non-conventional intramolecular hydrogen bond (nCIHB) using quantum chemical calculations at B3LYP/6-31++G (d,p) level of theory. Effects of the protic and aprotic solvents and water on the nCIHB were studied. Solvents, especially protic solvents, weaken nCIHB in all 6A4PTHQ derivatives. It was found that the nCIHB is preserved within the 6A4PTHQ derivatives when docked in the human follicle-stimulating hormone (FSH) receptor. However, Molecular dynamic simulations showed that nCIHB disruption occurs in the aqueous compartment of the two-layered membrane, which enhances the absorption process via entropy increasing mechanism. Our approach may lead to the clarification of the complex role of nCIHB in drug transport and absorption processes in further research.

Glycan Stability and Flexibility: Thermodynamic and Kinetic Characterization of Nonconventional Hydrogen Bonding in Lewis Antigens.

We provide evidence for CH-based nonconventional hydrogen bonds (H-bonds) for 10 Lewis antigens and two of their rhamnose analogues. We also characterize the thermodynamics and kinetics of the H-bonds in these molecules and present a plausible explanation for the presence of nonconventional H-bonds in Lewis antigens. Using an alternative method to simultaneously fit a series of temperature-dependent fast exchange nuclear magnetic resonance (NMR) spectra, we determined that the H-bonded conformation is favored by ∼1 kcal/mol over the non-H-bonded conformation. Additionally, a comparison of temperature-dependent 13C linewidths in various Lewis antigens and the two rhamnose analogues reveals H-bonds between the carbonyl oxygen of the N-acetyl group of N-acetylglucosamine and the OH2 group of galactose/fucose. The data presented herein provide insight into the contribution of nonconventional H-bonding to molecular structure and could therefore be used for the rational design of therapeutics.

Contribution of Hyperconjugation and Inductive Effects to the Pseudo-anomeric Effect in 4-Substituted Methoxycyclohexanes

The importance of electrostatic nonconventional hydrogen bonds (NCHBs) to the pseudo-anomeric effect of 4-substituted methoxycyclohexanes is evaluated using theory [natural bond orbital (NBO)] to deconvolute electrostatic from other contributing effects. There is an interesting interplay between σCH → σCX* hyperconjugation and the electropositive charge on 3,5-axial hydrogens (Hax). In essence, better σCX* (or πCO*) acceptors increase the charge on 3,5-CHax, which in turn strengthens Cδ+Hax···δ-OMe NCHB interactions.

Efficiency of C = Se as hydrogen bond acceptor in controlling regioselective amination of 5,7-dinitroquinazoline-4-selenone: Quantum mechanical, AIM and docking analysis by density functional method

Herein, we explored the theoretical aspect of reaction between 5,7-dinitroquinazoline-4-selenone and methylamine by employing density functional theory (DFT) calculations. The energetic parameters confirmed that the reaction progressed via. concerted mechanism with the single step process instead of two step process via intermediate formation. Further, role of Se = C bond as non-conventional H-bond acceptor is reported in reaction controlling factors that monitors the regioselectivity of the present proposed reaction. Despite of substantial steric hindrance at ortho-position of 5,7-dinitroquinazoline-4-selenones, the regioselective amination resulted into ortho-substituted product (P2) rather than para-substituted product (P1) attributed to non-conventional NH---Se = C intramolecular hydrogen bonding. The topological properties of the electron density distributions have been studied by using quantum theory of atoms in molecules (QTAIM). The presence of non-conventional hydrogen bond acceptor–donor interactions (NH---Se = C) in ortho-positioned amination is achieved by electron density ρ(r) and Laplacian of electron density ∇2ρ(r) in QTAIM analysis and supported the results of DFT studies hybrid with B3LYP/6-31G(d,p) basis set. Due to the lack of intramolecular hydrogen bonding interactions (NH---Se = C), the para-positioned amination on 5,7-dinitroquinazoline-4-selenone is founded to be less feasible. Further, the DFT technique has been used to investigate frontier molecular orbital (FMO) analysis and global reactive parameters using the aforesaid level of theory and basis set. HOMO-LUMO analysis revealed that ortho-substituted product (P2) has a wider energy gap (3.0684 eV) than para-substituted product (P1) (2.6402 eV) suggesting more stability of P2. The presence of hydrogen bonding interactions is further confirmed by the change in electrostatic potential regions in ortho-substituted product (P2) as compared to reactants in molecular electrostatic potential surface (MEPS) studies. Both para-substituted product (P1) and ortho-substituted product (P2) have been docked into the protein 5FCT's active site to predict favored binding location, activity and affinity by using docking studies.

Hydrogen bonds in mixed-solvent Au-(CH3OH)(H2O) complex: A joint experimental and theoretical study

Compared to the conventional hydrogen bonds, nonconventional hydrogen bonds have gained increasing interest in recent years. In this work, we firstly investigate hydrogen bonds in mixed-solvent Au-(CH3OH)(H2O) complex via gas phase anion photoelectron velocity map imaging associated with quantum chemical calculations. The vertical detachment energy of Au-(CH3OH)(H2O) complex is determined as 3.22 ± 0.02 eV. The calculations reveal these mixed-solvent solvated complexes having two stable low-energy isomers: Iso-a and Iso-b. And the experimental spectrum is ascribed to the contribution from Iso-a isomer. Detailed hydrogen bonding patterns have been revealed for both isomers.

Comparison of Conventional and Nonconventional Hydrogen Bond Donors in Au- Complexes.

Although gold has become a well-known nonconventional hydrogen bond acceptor, interactions with nonconventional hydrogen bond donors have been largely overlooked. In order to provide a better understanding of these interactions, two conventional hydrogen bonding molecules (3-hydroxytetrahydrofuran and alaninol) and two nonconventional hydrogen bonding molecules (fenchone and menthone) were selected to form gas-phase complexes with Au–. The Au–[M] complexes were investigated using anion photoelectron spectroscopy and density functional theory. Au–[fenchone], Au–[menthone], Au–[3-hydroxyTHF], and Au–[alaninol] were found to have vertical detachment energies of 2.71 ± 0.05, 2.76 ± 0.05, 3.01 ± 0.03, and 3.02 ± 0.03 eV, respectively, which agree well with theory. The photoelectron spectra of the complexes resemble the spectrum of Au– but are blueshifted due to the electron transfer from Au– to M. With density functional theory, natural bond orbital analysis, and atoms-in-molecules analysis, we were able to extend our comparison of conventional and nonconventional hydrogen bonding to include geometric and electronic similarities. In Au–[3-hydroxyTHF] and Au–[alaninol], the hydrogen bonding comprised of Au–···HO as a strong, primary hydrogen bond, with secondary stabilization by weaker Au–···HN or Au–···HC hydrogen bonds. Interestingly, the Au–···HC bonds in Au–[fenchone] and Au–[menthone] can be characterized as hydrogen bonds, despite their classification as nonconventional hydrogen bond donors.

Role of O-H⋯O/S conventional hydrogen bonds in considerable Csp2 -H blue-shift in the binary systems of acetaldehyde and thioacetaldehyde with substituted carboxylic and thiocarboxylic acids.

Stable binary complexes of RCZOH⋯CH3CHZ (R = CH3, H, F; Z = O, S) are due to contributions from the O-H⋯O/S and Csp2 -H⋯O/S hydrogen bonds. The strength of Csp2 /O-H⋯O is 1.5 to 2 times greater than that of the Csp2 /O-H⋯S bond. The substitution of H(Csp2 ) of HCZOH by CH3 causes a decrease in complex stability, while the opposite trend occurs for the F atom. A very large red shift of the O-H stretching frequency in O-H⋯O/S bonds was observed. A surprising Csp2 -H blue shift up to 104.5 cm-1 was observed for the first time. It is found that the presence of O-H⋯O/S hydrogen bonds and a decisive role of intramolecular hyperconjugation interactions in the complex cause a significant blue shift of the Csp2 -H covalent bonds. A striking role of O compared to the S atom in determining the blue shift of Csp2 -H stretching vibration and stability of binary complexes is proposed. The obtained results show that the ratio of deprotonation enthalpy and proton affinity could be considered as an index for the classification of the non-conventional hydrogen bond. SAPT2+ results show that the strength of RCSOH⋯CH3CHS complexes is dominated by electrostatic and induction energies, while a larger contribution to the stability of remaining complexes is detected for the electrostatic component.

Theoretical Aspects of Nonconventional Hydrogen Bonds in the Complexes of Aldehydes and Hydrogen Chalcogenides.

Hydrogen bonds (H-bonds) in the complexes between aldehydes and hydrogen chalcogenides, XCHO...nH2Z with X = H, F, Cl, Br, and CH3, Z = O, S, Se, and Te, and n = 1,2, were investigated using high-level ab initio calculations. The Csp2-H...O H-bonds are found to be about twice as strong as the Csp2-H...S/Se/Te counterparts. Remarkably, the S/Se/Te-H...S/Se/Te H-bonds are 4.5 times as weak as the O-H...O ones. The addition of the second H2Z molecule into binary systems induces stronger complexes and causes a positive cooperative effect in ternary complexes. The blue shift of Csp2-H stretching frequency involving the Csp2-H...Z H-bond sharply increases when replacing one H atom in HCHO by a CH3 group. In contrast, when one H atom in HCHO is substituted with a halogen, the magnitude of blue-shifting of the Csp2-H...Z H-bond becomes smaller. The largest blue shift up to 92 cm-1 of Csp2-H stretching frequency in Csp2-H...O H-bond in CH3CHO...2H2O has rarely been observed and is much greater than that in the cases of the Csp2-H...S/Se/Te ones. The Csp2-H blue shift of Csp2-H...Z bonds in the halogenated aldehydes is converted into a red shift when H2O is replaced by a heavier analogue, such as H2S, H2Se, or H2Te. The stability and classification of nonconventional H-bonds including Csp2-H...Se/Te, Te-H...Te, and Se/Te-H...O have been established for the first time.

Synthesis, X-ray molecular structure and QTAIM and NCI-RDG theoretic studies of a new cadmium (II) (4′4 diaminodiphenylmethane) (meso-arylporphyrin) coordination compound

In this study, a new five-coordinated cadmium porphyrin complex axially ligated with 4,4′-diaminodiphenylmethane (4,4′-mda) with the formula [Cd(TClPP)(4,4′-mda)]•CHCl3•(4,4′-mda)•2H2O (complex (1)) where TClPP is the meso-tetra(para-chlorophenyl)porphyrinate was prepared and characterized by FT-IR,1H NMR, and UV/Vis spectroscopy. The crystal structure was determined using single crystal X-ray diffraction. This complex crystallizes in the monoclinic crystal system with the space group P-1. The coordination sphere around the cadmium center is shown as a distorted square pyramidal coordination environment and the porphyrin core presents a dome-type deformation due to the large size of the central cadmium(II) ion. The crystal lattice of (1) is mainly stabilized by classic and non-conventional hydrogen bonds as indicated by the experimental data explored by the PLATON program. To obtain an insight into the physical nature of these intermolecular bonds, we extensively used Bader's quantum theory of atoms-in-molecules (QTAIM). This analysis proved that the coordinated 4,4′-mda (ligand I) stabilizes the structure of (1) by the formation of Van der Waals (VdW) type interactions. Additionally, the non-covalent interaction via the reduced density gradient (NCI-RDG) analysis established nicely the presence of such intermolecular interactions type hydrogen bonding interactions between ligands I and II, from one side, and (iii) hydrogen bonding and Van der Waal interactions, from the other side, with involving [Cd(TClPP)(4,4′-mda)], ligand I and II and the neighboring [Cd(TClPP)(4,4′-mda)] molecules.

Non-conventional interactions of N3 inhibitor with the main protease of SARS-CoV and SARS-CoV-2.

The extensive spread of COVID-19 in every continent shows that SARS-CoV-2 virus has a higher transmission rate than SARS-CoV virus which emerged in 2002. This results in a global pandemic that is difficult to control. In this investigation, we analyze the interaction of N3 inhibitor and the main protease of SARS-CoV and SARS-CoV-2 by quantum chemistry calculations. Non-covalent interactions involved in these systems were studied using a model of 469 atoms. Density Functional Theory and Quantum Theory of Atoms in Molecules calculations lead us to the conclusion that non-conventional hydrogen bonds are important to describe attractive interactions in these complexes. The energy of these non-conventional hydrogen bonds represents more than a half of the estimated interaction energy for non-covalent contacts. This means that hydrogen bonds are crucial to correctly describe the bonds between inhibitors and the main proteases. These results could be useful for the design of new drugs, since non-covalent interactions are related to possible mechanisms of action of molecules used against these viruses.

Conventional and Nonconventional Hydrogen Bonds Origin of the Different Behavior

Conventional and Nonconventional Hydrogen Bonds Origin of the Different Behavior

A MEDT computational study of the mechanism, reactivity and selectivity of non-polar [3+2] cycloaddition between quinazoline-3-oxide and methyl 3-methoxyacrylate.

The Molecular Electron Density Theory (MEDT) was used for the study of the mechanism and the selectivity of the [3+2] cycloaddition reaction between quinazoline-3-oxide and methyl 3-methoxyacrylate, using the B3LYP/6-31G(d,p) DFT method. In gas phase, this [3+2] cycloaddition reaction is characterized by a completely ortho regioselectivity and a moderate exo stereoselectivity. Dichloroethane solvent did not modify the selectivities obtained in gas phase but increase the activation energies and decrease the exothermic character. Analysis of thermodynamic characters indicates that by the inclusion of the experimental conditions, the reaction becomes endergonic and thereby under thermodynamic control favouring the formation of the most stable product as observed experimentally, explaining the exo stereoselectivity. The analysis of the global electron density transfer (GEDT) at the transition states and bond order (BO) show that this reaction takes place via a very slightly synchronous and non-polar one-step mechanism. Conceptual DFT reactivity indices analysis accounts for the electrophilic character of the reagents, explaining the high obtained free activation energies, while local Parr functions analysis allows us to explain the ortho regioselectivity observed experimentally. ELF topological analysis of the most favoured reactive pathways indicates that mechanism of this 32CA reaction is one stage, one step, synchronous and non-concerted. The stability of the favourable cycloadduct is attributed to the presence of different non-conventional hydrogen bonds interactions as indicated by NCI and QTAIM analyses. Graphical Abstract.

Probing the Hydrogen Bonding in Microsolvated Clusters of Au1,2-(Solv)n (Solv = C2H5OH, n-C3H7OH; n = 1-3 for Au-; n =1 for Au2-).

The microsolvation of gold anions in different alcohol solvents is demonstrated by the combination of anion photoelectron spectroscopy and quantum chemical calculations on the Au1,2-(Solv)n (Solv = C2H5OH, n-C3H7OH; n = 1-3 for Au-; n = 1 for Au2-). The microsolvation structures of these clusters and their corresponding neutrals are assigned by comparing calculations with experiments. In terms of overall regularity, the increasing solvation number (n) and carbon chain extension both can increase the stability of the anion. When n ≥ 2, these clusters have low-energy isomers, where conventional hydrogen bonds (HBs) compete with nonconventional HBs (NHBs). NHBs are dominant when n ≤ 2 and when n is increased, vice versa. Interestingly, a variety of theoretical calculations show that after the hydroxy H atom of the ethanol molecule forms a weak ionic HB with Au-, there are two lowest conformations of ethanol, trans and gauche, which could be coexisting in the molecular beams. Some theoretical methods also suggest that the gauche isomer is more stable than the trans one, which indicates that Au- may exist as a gold gauche effect similar to fluorine.

Classical/non-classical hydrogen bonds in m-xylene - ethanol solution: FTIR and theoretical approach

FTIR spectroscopic study is performed on pure m-xylene (MXY), ethanol (ETH) and their binary solutions at various mole fractions (0.8 MXY + 0.2 ETH, 0.6 + 0.4, 0.4 + 0.6, 0.2 + 0.8) and density functional theory (DFT) calculations are performed for MXY (monomer and dimer), ETH (monomer, dimer, trimer, open and closed tetramer, pentamer) and various MXY:ETH complex structures. It was found that MXY is a mixture of monomer and dimer, pure ETH exists as trimer, open and closed tetramers and pentamer structures. 1:3, two 1:4 (scheme 1 and 2) and two 1:5 (scheme 1 and 2) structures exist in MXY - ETH solutions of which 1:5 complex is the most stable structure. The NBO analysis has revealed the existence of both conventional and non-conventional hydrogen bonds in MXY:ETH complex structures.

Structural and electronic properties of M-MOF-74 (M = Mg, Co or Mn)

The Metal-Organic Frameworks M-MOF-74 (M = Mg, Co or Mn) were investigated through Density Functional Theory calculations. Structural parameters and band gap energies were determined in agreement with experimental data, with errors under 2%. The methods Electron Localization Function and Quantum Theory of Atoms in Molecules were applied to the analyses of the electronic density topology of the three solids. These methodologies indicated that the bonds between the metallic cations and the oxygen atoms are predominantly ionic while the other ones are predominantly covalent. Furthermore, non-conventional hydrogen bonds were identified to Mg-MOF-74 and Co-MOF-74, which were not observed to Mn-MOF-74.

Nonconventional Hydrogen Bonds between Silver Anion and Nucleobases: Size-Selected Anion Photoelectron Spectroscopy and Density Functional Calculations.

We conducted combined gas-phase anion photoelectron spectroscopy and density functional theory studies on nucleobase-silver complexes. The most probable structures of the nucleobase-Ag- complexes were determined by comparing the theoretical calculations with the experimental measurements. The vertical detachment energies (VDEs) of uracil-Ag-, thymine-Ag-, cytosine-Ag-, and guanine-Ag- were estimated to be 2.18 ± 0.08, 2.11 ± 0.08, 2.04 ± 0.08, and 2.20 ± 0.08 eV, respectively, based on their photoelectron spectra. Adenine-Ag- has two isomers coexisting in the experiment; the experimental VDEs of the two isomers are 2.18 and 2.53 eV, respectively. In the most probable isomers of nucleobases-Ag-, uracil, thymine, and cytosine interact with Ag- anion via N-H···Ag and C-H···Ag hydrogen bonds, while adenine and guanine interact with Ag- anion through two N-H···Ag hydrogen bonds. The N-H···Ag hydrogen bonds can be characterized as medium or strong hydrogen bonds. It is found that binding sites of the Ag anion to the nucleobases are affected by the deprotonation energies and the steric effects of two adjacent X-H groups.