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Eighteen empirical force fields and the semi-empirical quantum method PM3CARB-1 were compared for studying beta-cellobiose, alpha-maltose, and alpha-galabiose [alpha-D-Galp-(1-->4)-alpha-D-Galp]. For each disaccharide, the energies of 54 conformers with differing hydroxymethyl, hydroxyl, and glycosidic linkage orientations were minimized by the different methods, some at two dielectric constants. By comparing these results and the available crystal structure data and/or higher level density functional theory results, it was concluded that the newer parameterizations for force fields (GROMOS, GLYCAM06, OPLS-2005 and CSFF) give results that are reasonably similar to each other, whereas the older parameterizations for Amber, CHARMM or OPLS were more divergent. However, MM3, an older force field, gave energy and geometry values comparable to those of the newer parameterizations, but with less sensitivity to dielectric constant values. These systems worked better than MM2 variants, which were still acceptable. PM3CARB-1 also gave adequate results in terms of linkage and exocyclic torsion angles. GROMOS, GLYCAM06, and MM3 appear to be the best choices, closely followed by MM4, CSFF, and OPLS-2005. With GLYCAM06 and to a lesser extent, CSFF, and OPLS-2005, a number of the conformers that were stable with MM3 changed to other forms.

Abstract: Adiabatic ϕ/ψ maps for cellobiose were prepared with B3LYP density functional theory. A mixed basis set was used for minimization, followed with 6-31+G(d) single-point calculations, with and without SMD continuum solvation. Different arrangements of the exocyclic groups (38 starting geometries) were considered for each ϕ/ψ point. The vacuum calculations agreed with earlier computational and experimental results on the preferred gas phase conformation (anti-ϕH, syn-ψH), and the results from the solvated calculations were consistent with the (syn ϕH/ψH conformations from condensed phases (crystals or solutions). Results from related studies were compared, and there is substantial dependence on the solvation model as well as arrangements of exocyclic groups. New stabilizing interactions were revealed by Atoms-In-Molecules theory. [Copyright &y& Elsevier]

Abstract: To better understand computational predictions of disaccharide conformations, ϕ,ψ maps were constructed for two analogs in which all hydroxyl groups were replaced with fluorine atoms (F-cellobiose and F-maltose). These molecules do not permit hydrogen bonding but should give better steric representation than analogs in which hydrogen atoms replaced exo-cyclic groups. Hartree Fock and B3LYP density functional quantum mechanics (QM) theory were used. The preferred ring shape for fluorinated glucose depends on the level of QM theory, but over the limited ϕ,ψ space that was studied, the rings remained in the 4C1 form. Also, fluorine atoms are remote enough that they do not affect the torsional energies for the glycosidic bonds. F-Cellobiose maps were predictive of the conformations in crystals, but F-maltose maps were less so. The QM F-cellobiose map and an MM4::QM hybrid map for cellobiose itself were similar. However, the hybrid maltose map had many more experimental conformations within its 2-kcal/mol contour than did the QM F-maltose map. The apparent mean strength of an intra-molecular, inter-residue hydrogen bond is about 3kcal/mol, based on the energy for many of the hydrogen bonded maltose structures on the F-maltose map. The F-maltose map was similar to a new QM map for an analog of maltose in which all hydroxyl groups were replaced with hydrogen atoms. [Copyright &y& Elsevier]