We present a CHARMM Carbohydrate Solution Force Field (CSFF) suitable for nanosecond molecular dynamics computer simulations. The force field was derived from a recently published sugar parameter set.1 Dihedral angle parameters for the primary alcohol as well as the secondary hydroxyl groups were adjusted. Free energy profiles of the hydroxymethyl group for two monosaccharides (beta-D-glucose and beta-D-galactose) were calculated using the new parameter set and compared with similar force fields. Equilibrium rotamer populations obtained from the CSFF are in excellent agreement with NMR data (glucose gg:gt:tg approximately 66:33:1 and galactose gg:gt:tg approximately 4:75:21). In addition, the primary alcohol rotational frequency is on the nanosecond time scale, which conforms to experimental observations. Equilibrium population distributions of the primary alcohol conformers for glucose and galactose are reached within 10 nanoseconds of molecular dynamics simulations. In addition, gas phase vibrational frequencies computed for beta-D-glucose using this force field compare well with experimental frequencies. Carbohydrate parameter sets that produce both conformational energies and rotational frequencies for the pyranose primary alcohol group that are consistent with experimental observations should allow for increased accuracy in modeling the flexibility of biologically important (1-6)-linked saccharides in solution.

Several blood-feeding organisms, including the malaria parasite detoxify haem released from host haemoglobin by conversion to the insoluble crystalline ferriprotoporphyrin IX dimer known as haemozoin. To date the mechanism of haemozoin formation has remained unknown, although lipids or proteins have been suggested to catalyse its formation. We have found that beta-haematin (synthetic haemozoin) forms rapidly under physiologically realistic conditions near octanol/water, pentanol/water and lipid/water interfaces. Molecular dynamics simulations show that a precursor of the haemozoin dimer forms spontaneously in the absence of the competing hydrogen bonds of water, demonstrating that this substance probably self-assembles near a lipid/water interface in vivo.

The puckered conformations of furanose and pyranose carbohydrate rings are central to analyzing the action of enzymes on carbohydrates. Enzyme reaction mechanisms are generally inaccessible to experiments and so have become the focus of QM(semiempirical)/MM simulations. We show that the complete free energy of puckering is required to evaluate the accuracy of semiempirical methods used to study reactions involving carbohydrates. Interestingly, we find that reducing the free energy space to lower dimensions results in near meaningless minimum energy pathways. We analyze the furanose and pyranose free energy pucker surfaces and volumes using AM1, PM3, PM3CARB-1, and SCC-DFTB. A comparison with DFT optimized structures and a HF free energy surface reveals that SCC-DFTB provides the best semiempirical description of five- and six-membered carbohydrate ring deformation.

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