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

Abstract

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.