MA'AT Analysis of Aldofuranosyl Rings: Unbiased Modeling of Conformational Equilibria and Dynamics in Solution

Geminal 13C-1H NMR Spin-Coupling Constants in Furanose Rings: New Empirical Correlations with Conformation

Density functional theory (DFT) calculations have been used to develop a new approach to interpreting geminal (two-bond) 2 J CCH NMR spin-coupling constants in saccharides containing aldofuranosyl (five-membered) rings. In the biologically important β-d-ribofuranosyl and 2-deoxy-β-d-ribofuranosyl (2-deoxy-β-d-erythro-pentofuranosyl) rings that were used as models, many of the 2 J CCH values associated with coupling pathways involving an endocyclic C-C bond depend linearly on P/π, a measure of ring conformation. In most cases, the endocyclic C-C bond is present in the coupling pathway. In other cases, the 2 J CCH value depends linearly on either an adjacent C-C bond torsion angle or shows no systematic relationship with any endocyclic C-C bond torsion angle. In the latter case, secondary (remote) structural effects, defined as those that primarily affect C-C or C-H bond lengths in the C-C-H coupling pathway, cause the 2 J CCH value to behave with less predictability. These effects apparently cancel and lead to linearity involving an adjacent C-C bond in some cases. These findings provide a new conceptual framework to understand and exploit the dependencies of geminal 13C-1H NMR spin-couplings on furanose ring conformation. They also reveal the effect of exocyclic C-O bond torsion angles on the magnitudes and signs of 2 J CCH values in saccharides, a complication that remains to be addressed before 2 J CCH values can be used quantitatively in single- and multi-state MA'AT modeling of redundant NMR J-values in furanosyl rings.

Context Effects on Human Milk Oligosaccharide Linkage Conformation and Dynamics Revealed by MA'AT Analysis

An emerging NMR method, MA'AT analysis, has been applied to investigate context effects on the conformational properties of several human milk oligosaccharides (HMOs). The MA'AT model of the β-(1→4) linkage in the disaccharide, methyl β-lactoside (MeL), was compared to those obtained for the same linkage in the HMO trisaccharides, methyl 2'-fucosyllactoside (Me2'FL) and methyl 3-fucosyllactoside (Me3FL), and in the tetrasaccharide, methyl 2',3-difucosyllactoside (Me2',3DFL). MA'AT analysis revealed significant context effects on the mean values and circular standard deviations (CSDs) of the psi (ψ) torsion angles in these linkages. α-Fucosylation at both O2'Gal and O3Glc of MeL to give Me2',3DFL significantly constrained librational motion about ψ (70% reduction in the CSD) and shifted its mean value by ∼18°. α-Fucosylation at the O3Glc of MeL to give Me3FL constrained ψ more than α-fucosylation at the O2Gal to give Me2'FL. These effects can be explained by the expected solution conformation of Me3FL, which closely resembles the Lewisx trisaccharide. Comparisons of MA'AT models of ψ to those obtained by 1 μs aqueous molecular dynamics simulation (GLYCAM06) revealed identical trends, that is, MA'AT analysis was able to recapitulate molecular behavior in solution that was heretofore only available from MD simulation. The results highlight the capabilities of MA'AT analysis to determine probability distributions of molecular torsion angles in solution as well as degrees of librational averaging of these angles.

Methyl α-D-galactopyranosyl-(1→3)-β-D-galactopyranoside and methyl β-D-galactopyranosyl-(1→3)-β-D-galactopyranoside: Glycosidic linkage conformation determined from MA'AT analysis

MA'AT analysis has been applied to two biologically-important O-glycosidic linkages in two disaccharides, α-D-Galp-(1→3)-β-D-GalpOMe (3) and β-D-Galp-(1→3)-β-D-GalpOMe (4). Using density functional theory (DFT) to obtain parameterized equations relating a group of trans-O-glycosidic NMR spin-couplings to either phi (ϕ') or psi (ψ'), and experimental 3JCOCH, 2JCOC, and 3JCOCC spin-couplings measured in aqueous solution in 13C-labeled isotopomers, probability distributions of ϕ' and ψ' in each linkage were determined and compared to those determined by aqueous 1-μs molecular dynamics (MD) simulation. Good agreement was found between the MA'AT and single-state MD conformational models of these linkages for the most part, with modest (approximately <15°) differences in the mean values of ϕ' and ψ', although the envelope of allowed angles (encoded in circular standard deviations or CSDs) is consistently larger for ϕ' determined from MA'AT analysis than from MD for both linkages. The MA'AT model of the α-Galp-(1→3)-β-Galp linkage agrees well with those determined previously using conventional NMR methods (3JCOCH values and/or 1H-1H NOEs), but some discrepancy was observed for the β-Galp-(1→3)-β-Galp linkage, which may arise from errors in the conventions used to describe the linkage torsion angles. Statistical analyses of X-ray crystal structures show ranges of ϕ' and ψ' for both linkages that include the mean angles determined from MA'AT analyses, although both angles adopt a wide range of values in the crystalline state, with ϕ' in β-Galp-(1→3)-β-Galp linkages showing greater-than-expected conformational variability.

MA'AT Analysis: Unbiased Multi-State Conformational Modeling of Exocyclic Hydroxymethyl Group Conformation in Methyl Aldohexopyranosides

MA'AT analysis (J. Chem. Inf. Model. 2022, 62, 3135-3141) has been applied to model exocyclic hydroxymethyl group conformation in methyl β-D-glucopyranoside (βGlcOMe), methyl β-D-galactopyranoside (βGalOMe), and methyl β-D-mannopyranoside (βManOMe) in an unbiased manner. Using up to eight NMR J-couplings sensitive to rotation about the C5-C6 bond (torsion angle ω), two-state models of ω were obtained that are qualitatively consistent with the relative populations of the gg, gt, and tg rotamers reported previously. MA'AT analysis gave consistent unbiased gt ⇌ tg models of ω in βGalOMe, with gt more populated than tg and mean values of ω for each population similar to those obtained from aqueous 1-μs MD simulation. Using different combinations of J-couplings had little effect on the βGalOMe model in terms of the mean values of ω and circular standard deviations (CSDs). In contrast, MA'AT analysis of ω in βGlcOMe and βManOMe produced more than one two-state model independent of the ensemble of J-values used in the analyses. These models were characterized by gg ⇌ gt conformer exchange as expected, but the mean values of ω in both conformers varied significantly in the different fits, especially for the gg rotamer. Constrained (biased) MA'AT analyses in which only staggered geometries about ω were allowed gave RMSDs slightly larger than those obtained from the unbiased fits, precluding an assignment of an unbiased model. It is unclear why MA'AT analysis gives consistent and predictable unbiased models of ω in βGalOMe but not in βGlcOMe and βManOMe. One possibility is that the distribution of ω in one or both of the gg and gt conformers in the latter does not conform to a von Mises function (i.e., is not Gaussian-like), but rather to a broad and/or flat distribution that cannot be fit by the current version of MA'AT. Nevertheless, the results of this study provide new evidence of the ability of MA'AT analysis to treat multi-state conformational exchange using only experimental NMR data, extending recent MA'AT applications to furanosyl ring pseudorotation (Biochemistry 2022, 61, 239-251).

Conformational preferences of guanine-containing threose nucleic acid building blocks in B3LYP studies

In this paper, detailed and systematic gas-phase B3LYP conformational studies of four monomers of threose nucleic acid (TNA) with guanine attached at the C1' atom and bearing different substituents (OH, OP(=O)OH2 and OCH3) in the C2' and C3' positions of the α-l-threofuranose moiety are presented. All exocyclic single-bond (χ, ε and γ) rotations, as well as the ν0-ν4 endocyclic torsion angles, were taken into consideration. Three (threoguanosines TG1-TG3) or two (TG4) energy minima were found for the rotation about the χ torsion angle. The syn orientation (the A rotamer family) is strongly privileged in geometries TG1 and TG2, whereas the anti orientation (the C rotamer family) and the syn orientation are observed to be in equilibrium (with populations of 56% and 44%, respectively) for TG3. In the case of TG4, the high-anti orientation (the B rotamer family) turned out to be by far the most favourable, with the contribution exceeding 90% in equilibrium. Such a preference can be attributed to the inability of H-bonding between sugar and nucleobase and possibly because of the steric strains. The low-energy conformers of TG1-TG4 occupy the northeastern (P ∼ 40°) and/or southern (P ∼ 210°) parts of the pseudorotational wheel, which fits the A- and B-type DNA helices quite well. Additionally, in the case of TG4, some relatively stable geometries have the furanoid ring in conformation lying on the northwestern part of the pseudorotational wheel (P ∼ 288°).

MA'AT Analysis: Probability Distributions of Molecular Torsion Angles in Solution from NMR Spectroscopy

ConspectusMonosaccharides adopt multiple conformations in solution, and this structural complexity increases significantly when they are assembled into oligosaccharides and polysaccharides. Characterization of the conformational properties of saccharides in solution by NMR spectroscopy has been hampered by several complicating factors, including difficulty interpreting spectra because of significant signal overlap, population averaging of NMR parameters, and unique properties of the spectra that make accurate measurements of NMR parameters prone to error (e.g., non-first-order effects on J-couplings). Current conformational assignments rely heavily on theoretical calculations, especially molecular dynamics (MD) simulations, to interpret the experimental NMR parameters. While these studies assert that the available experimental data fit the calculated models well, a lack of independent experimental validation of the force fields from which MD models are derived and an inability to test all possible models that might be compatible with the experimental data in an unbiased manner make the approach less than ideal.NMR spin couplings or J-couplings have been used as structure constraints in organic and other types of molecules for more than six decades. The dihedral angle dependence of vicinal (three-bond) 1H-1H spin couplings (3JHH) first described by Karplus led to an explosion of applications for a wide range of conformational problems. Other vicinal J-couplings (e.g., 3JCCOP, 3JHCOP, and 3JCOCH) have been found to exhibit similar dihedral angle dependencies. 3J values have been used to assign the preferred conformation in molecules that are conformationally homogeneous. However, many molecules, particularly those in biological systems, are conformationally flexible, which complicates structural interpretations of J values in solution. Three-state staggered models are often assumed in order to deconvolute the conformationally averaged J values into conformer populations. While widely applied, this approach assumes highly idealized models of molecular torsion angles that are likely to be poor representations of those found in solution. In addition, this treatment often gives negative populations and neglects the presence of librational averaging of molecular torsion angles.Recent work in this research group has focused on the development of a hybrid experimental-computational method, MA'AT analysis, that provides probability distributions of molecular torsion angles in solution that can be superimposed on those obtained by MD. Ensembles of redundant NMR spin couplings, including 3J (vicinal), 2J (geminal), and sometimes 1J (direct) values, are used in conjunction with circular statistics to provide single- and multistate models of these angles. MA'AT analysis provides accurate mean torsion angles and circular standard deviations (CSDs) of each mean angle that describe the librational motion about the angle. Both conformational equilibria and dynamics are revealed by the method. In this Account, the salient features of MA'AT analysis are discussed, including some applications to conformational problems involving saccharides and peptides.

MA'AT: A Web-Based Application to Determine Rotamer Population Distributions in Solution from Nuclear Magnetic Resonance Spin-Coupling Constants

A hybrid experimental-computational method to determine conformational equilibria of molecules in solution has been developed based on the use of redundant nuclear magnetic resonance (NMR) spin-spin coupling constants (spin-couplings; J-couplings), density functional theory (DFT) calculations, and circular statistics. The mathematics that underpins the method, known as MA'AT analysis, is presented, and key components of a computer program that applies this algorithm are discussed. The method was tested using single-state and multi-state models to identify the factors required to obtain reliable results, to establish the limitations of the method, and to highlight techniques to evaluate the uniqueness of solution.