ABINITIO STUDIES OF LIPID MODEL SPECIES .1. DIMETHYL-PHOSPHATE AND METHYL PROPYL PHOSPHATE ANIONS

With the aim of developing force fields for lipid/membrane simulations, extensive ab initio calculations have been carried out on lipid model compounds. This contribution reports results from searching the potential energy surfaces of two simple model species, the dimethyl phosphate and methyl propyl phosphate anions. It is shown that most of the critical structural features of phospholipids found experimentally in crystals, in the gel and liquid crystal phases, and in solution are also seen from quantum mechanical calculations on these simple systems in the gas phase. Thus, a gauche-gauche conformation about phosphate is favored for the dimethyl phosphate anion, as is also seen experimentally for phospholipids (and nucleotides). Also, the CCOP torsional angle in the methyl propyl phosphate anion tends to be anti-periplanar or trans, as is the case in most of the structures of phospholipids in crystals and in solution. The vibrational spectra of various conformers of these species have been analyzed in detail. The results reveal some diagnostic frequencies for experimental determination of conformational preferences in structurally related phosphates.