Calculation of the Ramachandran potential of mean force for a disaccharidein aqueous solution

Molecular dynamics simulations employing adaptive umbrella sampling have be en used to calculate the Ramachandran conformational potential of mean forc e in aqueous (TIP3P) solution for the alpha(1-->4)-linked dimer of D-xylopy ranose (4-O-alpha-D-xylopyranosyl-alpha-D-xylopyranose), a pentose analogue of malto se and a useful general model for the effects of solvent structur ing upon biopolymer hydration. The vacuum adiabatic energy map for this mol ecule closely resembles that for maltose, but the solution pmf is quite dif ferent, with one of the principal vacuum minima almost completely disappear ing in solution and with the global minimum-energy conformation being a new minimum which does not occur at all on the vacuum surface. This conformati on is apparently stabilized by a water molecule which hydrogen bonds to a h ydroxyl group on each ring, bridging between the two rings. The new conform ation also places the two hydrophobic methylene groups almost in van der Wa als contact, reducing their exposed surface area. Unfortunately, the result s reaffirm the dependence of hydration effects upon the specific details of each molecule's chemical structure, making the application of simple gener al models for hydration more difficult.