Quantum-chemical and empirical calculations on phospholipids: V. Conformational calculations on model headgroups with varying ammonium group methylation

Using empirical 0-1-6-12 atom-atom potential functions and the PCILO method the conformational properties of anhydrous and hydrated model headgroups with varying ammonium group methylation were investigated. With a phosphoryl gauche^(?)-gauche^(?) conformation the torsion angle α₄ lies in the region of 180°–300° for all compounds. Torsion angles α₄ = 300° ? 100° are forbidden due to intramolecular sterical hindrance. The torsion angles α₅ and α₆ are influenced by the stage of ammonium group methylation and bound water molecules. The minima of energy with respect to α₅ were found at an ± syn-clinal (and for PC and DMPE + H₂O at an anti-periplanar) conformation.