PHASE-TRANSITION BEHAVIOR AND MOLECULAR-STRUCTURES OF MONOUNSATURATED PHOSPHATIDYLCHOLINES - CALORIMETRIC STUDIES AND MOLECULAR MECHANICS SIMULATIONS

High resolution differential scanning calorimetric studies were performed to investigate the thermotropic phase behavior of 26 molecular species of sn-l 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 (Delta) 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-m) and transition enthalpy (Delta H) as they undergo the gel to liquid-crystalline phase transition, The T-m and Delta H-lowering effects of the Delta 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-l and sn-2 acyl chains, and a local perturbation of the chain chain van der Waals interaction in the vicinity of the Delta bond, (ii) For a series of positional isomers of 1-stearoyl-2-cis-octadecenoylphosphatidylcholine, C(18):C(18:1 Delta(n))PC, with a Delta bond at different positions along the sn-2 acyl chain, the T-m value depends critically on the position of the Delta bond, Specifically, the T-m value is minimal as the Delta bond is located at the geometric center of the linear segment of the sn-2 acyl chain, and the T-m value is progressively increased as the Delta 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-m values of most lipids in each group can be correlated in an identical manner with their structural parameters, yielding a common T-m-structure relationship.