AB-INITIO AND SEMIEMPIRICAL CONFORMATION POTENTIALS FOR PHOSPHOLIPID HEAD GROUPS

The conformational potential of the dimethyl phosphate (DMP) anion and the 2-ammonioethanol (AME) cation, which are substructures of the pho sphoethanolamine head group of phospholipids, has been investigated at the Hartree-Fock (HF) level using the 3-21G, 3-21G(), 6-31G*, and 6- 31+G basis set. For this purpose, both the DMP anion and the AME cati on were considered as geminal double rotors with the two rotor groups OCH3 in the case of DMP and OH and CH2NH3+ in the case;of AME. Extensi ve scans (17 points including eight stationary points for DMP and 30 p oints including seven stationary points for AME) of the conformational energy surface were carried out by complete geometry optimizations at the HF/3-21G and HF/3-21G() levels, respectively, and subsequent sin gle point calculations with larger basis sets. The most stable DMP for m has the two OCH3 groups in syn-clinal (+sc) positions (both dihedral angles alpha(2) and alpha(3) = 75.3 degrees) while the AME cation pre fers an anti-periplanar (ap) (alpha(4) = 173.3 degrees), syn-clinal (sc) (alpha(5) = 48.5 degrees) conformation with regard to the OH and C H2NH3+ groups. The DMP anion is a rather flexible rotor that can under go various flip-flop rotations (barriers 1 and 6 kcal/mol) that indica te strong coupling between the rotor groups. The AME cation, on the ot her hand, is conformationally not flexible, which has to do with;the f act that the two rotor groups OH and CH2NH3+ are electronically very d ifferent. The preferred rotational processes of the AME cation involve inwardly or outwardly directed rotations at the CC bond (barriers of 5.1 and 9.3 kcal/mol) with the OH group kept essentially in an ap posi tion. Calculations reveal that semiempirical methods such as PM3 are n ot able to describe the conformational tendencies of either DMP anion or AME cation correctly.