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Volume 270,
Number 5,
Issue of February 3, 1995 pp. 2014-2023
©1995 by The American Society for Biochemistry and Molecular Biology, Inc.
Phase Transition
Behavior and Molecular Structures of Monounsaturated
Phosphatidylcholines
CALORIMETRIC STUDIES AND MOLECULAR MECHANICS SIMULATIONS
(Received for publication, October 6,
1994)
Zhao-qing
Wang ,
Hai-nan
Lin,
Shusen
Li ,
Ching-hsien
Huang
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 T value 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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Copyright © 1995 by the American Society for Biochemistry and Molecular Biology.
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