Phopholipids are amphiphilic with a hydrophilic (water-loving) phosphate head part and lipophilic (fat-loving or hydrophobic) fatty acid tails. They are flexible long molecules so as to show a variety of conformations in water. We developed FUJI force field based on general AMBER atom types and original AMBER van der Waals parameters in order to describe arbitrary organic molecules in a unified manner including proteins and nucleic acids. In order to refine the force field we use the first principle theoretical method in contrast to the common empirical fitting to experimental data. The dihedral torsion parameters of protein backbone were determined to agree with the torsion energy profiles calculated by high-level quantum mechanical theories such as local coupled-cluster singles and doubles (LCCSD) for the model systems of protein backbone. Under the same philosophy we examined several kinds of phospholipids (DLPC, DMPC, DOPC, POPC, SOPC, POPE, DOPE, etc). The lipid flexibility mainly depends on point charges and dihedral torsion energy profiles of lipid backbone. To explore the variety of conformations we generated 400 conformations of a lipid and then performed HF/6-31G* calculations in vacuum for structure optimization. We selected 256 lower energy conformations from the 400 structures. The lipid point charges were constructed by AMBER RESP program from 256 electric-static potential (ESP) per a lipid. As the lipid is amphiphilic (combination of different characters), careful arrangements are necessary for the RESP calculation. Torsion energy profiles of lipid backbones were calculated by LCCSD(T) with aug-cc-pVTZ after structure optimization by LMP2/aug-cc-pVTZ. We observed significant differences from the AMBER LIPID14 torsion profiles of alkane backbone, a capped lauroyl tail and phosphatidylcholine backbone. We reports some basic characters of phospholipid bilayers obtained with new FUJI lipid force field.

Refined AMBER force field (FUJI) force field for phospholipids

253rd National Meeting (2017)

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