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

Abstract

Metrics & MoreArticle Recommendations CONSPECTUS: Monosaccharides 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) 1 H− 1 H spin couplings ( 3 J HH ) first described by Karplus led to an explosion of applications for a wide range of conformational problems. Other vicinal J-couplings (e.g., 3J CCOP , 3 J HCOP , and 3 J COCH ) have been found to exhibit similar dihedral angle dependencies. 3 J 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 3 J (vicinal), 2 J (geminal), and sometimes 1 J (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 re...

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

“…MA’AT analysis has been applied successfully to treat single-state conformational behaviors involving O -glycosidic linkages, and N - and O -acetyl side-chains, in mono-, di-, and oligosaccharides. − Pertinent to the present work, the MA’AT method has been tested recently for its ability to treat multi-state conformational equilibria through investigations of the pseudorotational properties of aldopentofuranosyl rings . This work showed that MA’AT analysis provides reliable unbiased models of N / S ring conformational equilibria and revealed discrepancies between MA’AT -derived models of solution behavior and those predicted by MD simulation.…”

“…The present work describes a radical departure from prior NMR treatments of this problem in those ensembles of NMR J -couplings that show strong dependencies on ω are used collectively to obtain continuous rotameric models of ω independent of assumptions about the nature of the rotameric distribution and independent of energetic bias. NMR data alone are used to calculate probability distributions of ω using MA’AT analysis. , No theoretical (calculational) input is required, although theoretical calculations play a central role in J -coupling equation parametrization whose reliability is key to the success of the method. Fortunately, DFT-calculated J -values can be validated experimentally, thus eliminating or at least reducing the errors and uncertainties associated with these calculations.…”

Does Inter-Residue Hydrogen Bonding in β-(1→4)-Linked Disaccharides Influence Linkage Conformation in Aqueous Solution?

“… − MA’AT analysis was then applied to these J -couplings in 3 to obtain mean values of the ϕ and ψ that comprise its linkage (Scheme ) and their circular standard deviations (CSDs) to gain insight into their librational averaging. The advantage of MA’AT analysis for this comparison is the ability of the method to provide experiment-based probability distributions of molecular torsion angles in solution and to give mean angles with high precision and accuracy. − Evidence of significant differences in the linkage conformations in 1 and 3 would support the argument that inter-residue hydrogen bonding plays a role in dictating preferred linkage conformation in aqueous solution, whereas identical (or near identical) conformations would suggest that this hydrogen bonding, if persistent in solution, is not a major determinant of linkage conformation. The results of these studies are described in this report.…”

Glycan Shape, Motions, and Interactions Explored by NMR Spectroscopy

“…Conformationally dependent 1 J CH , 1 J CC , 2 J CH , 2 J CC , and n J CC coupling constants can be used as a complement and with a set of redundant NMR couplings the conformation or conformational equilibria of oligosaccharides in solution can be investigated using single-state and multistate models to determine rotamer population distributions (MA'AT analysis). 39,40 An alternative, still relying on that only certain nuclei are 13 Clabeled is to use uniform [UL- 13 C 6 ]-labeling of one or more specific sugar residue(s) in an oligosaccharide, a labeling scheme that may give access to a large number of spin−spin coupling constants but also to that other NMR parameters can be measured. The power of automated glycan synthesis was shown for a series of a compounds, viz., a β-(1 → 6)-linked hexaglucoside, 13 C isotopologues thereof and 19 F analogues.…”

“…in small oligosaccharides), will give the complete set of three-bond coupling constants across the glycosidic linkage. Conformationally dependent 1 J CH , 1 J CC , 2 J CH , 2 J CC , and n J CC coupling constants can be used as a complement and with a set of redundant NMR couplings the conformation or conformational equilibria of oligosaccharides in solution can be investigated using single-state and multistate models to determine rotamer population distributions ( MA’AT analysis). , …”