Conformational analyses of native and permethylated disaccharides
Effects of permethylation on disaccharide conformation were studied with relaxed-residue φ,ψ maps for eight disaccharides of glucose and their permethylated derivatives. Many orientations of the methyl groups were generated with the conformation generator in the Chem-X software program and filtered with a rules-based method. For native molecules, clockwise and counterclockwise rings of hydrogen bonds were also used. MM3(96) energies were calculated with a dielectric constant (ε) of 1.5. The native disaccharide maps were also calculated at ε = 3.5 and 7.5 to reduce hydrogen-bonding strength. Maps for native and permethylated structures were generally similar. The permethylated structures, which do not make hydrogen bonds, gave maps that were most similar to the native maps that had reduced hydrogen-bonding strength. All φψ values for the global minima of the permethylated models fell within the 1-kcal/mol contours on the ε = 7.5 maps. Flexibility values based on a partition function were substantially less for permethylated α,α-trehalose and laminarabiose, compared to their native counterparts at any ε. On the other hand, strong interresidue hydrogen bonding at ε = 1.5 for the global minimum structures of cellobiose and maltose caused those models to be more rigid than their permethylated counterparts. All permethylated models were less flexible than their native counterparts at ε = 7.5 and their backbone analogues based on tetrahydropyran calculated at ε = 1.5.