Phase Transition Behavior and Molecular Structures of Monounsaturated Phosphatidylcholines CALORIMETRIC STUDIES AND MOLECULAR MECHANICS SIMULATIONS

Abstract High resolution differential scanning calorimetric studies were performed to investigate the thermotropic phase behavior of 26 molecular species of sn-1 saturated/sn-2 monounsaturated phosphatidylcholines. In parallel with calorimetric studies, the energy-minimized structures and steric energies of the diglyceride moieties of these monoenoic lipids were determined using a molecular mechanics approach. The combined calorimetric and computational studies led to the following results and conclusions. (i) When a single cis-carbon-carbon double bond (Δ) is incorporated into a saturated diacylphosphatidylcholine molecule at any position within the central segment of the long sn-2 acyl chain, the resulting monoenoic lipid molecules will, in excess water, exhibit reduced phase transition temperature (T) and transition enthalpy (ΔH) as they undergo the gel to liquid-crystalline phase transition. The T and ΔH-lowering effects of the Δ bond can be attributed to a decrease in the chain length of the sn-2 acyl chain, a change in the chain length difference between the sn-1 and sn-2 acyl chains, and a local perturbation of the chain-chain van der Waals interaction in the vicinity of the Δ bond. (ii) For a series of positional isomers of 1-stearoyl-2-cis-octadecenoylphosphatidylcholine, C(18):C(18:1Δ)PC, with a Δ bond at different positions along the sn-2 acyl chain, the T value depends critically on the position of the Δ bond. Specifically, the T value is minimal as the Δ bond is located at the geometric center of the linear segment of the sn-2 acyl chain, and the Tvalue is progressively increased as the Δ bond migrates toward either end of the sn-2 acyl chain. (iii) The various monoenoic phosphatidylcholines under study can be divided into two groups. The T values of most lipids in each group can be correlated in an identical manner with their structural parameters, yielding a commonT-structure relationship.