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 () 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 (H) as they undergo the gel to liquid-crystalline phase transition. The T and H-lowering effects of the 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 bond. (ii) For a series of positional isomers of 1-stearoyl-2-cis-octadecenoylphosphatidylcholine, C(18):C(18:1)PC, with a bond at different positions along the sn-2 acyl chain, the T value depends critically on the position of the bond. Specifically, the T value is minimal as the 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 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.

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