Effects of chain length and chain length mismatch on orientational order in fluid state lipid bilayer membranes

Lu, Dalian (1995) Effects of chain length and chain length mismatch on orientational order in fluid state lipid bilayer membranes. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (17MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

Deuterium NMR spectroscopy was used to study the effects of chain length and chain length mismatch on orientational order in fluid state lipid bilayer model membranes. Samples like diacyl phospholipids, glycolipids, binary mixtures of phospholipids, binary mixtures of glycolipids with phospholipids, and ternary mixtures containing cholesterol were used. Special attention was paid to examining how the line shape of the orientational order parameter profile and the temperature dependence of first moment near the gel/fluid phase transition are determined by a limited set of factors. -- The shape of the smoothed order parameter profile has been found to be determined largely by the mean order parameter for the diacyl phospholipid bilayers if position along the chain is normalized. Even in the case of binary mixtures, the observed shapes of the smoothed order parameter profiles with the same mean order parameter were largely independent of the constituent's concentration within the membrane. These observations suggest that the mean order parameter is one of a few parameters which determine the shape of smoothed orientational order parameter profiles. It was also observed that with decreasing chain length, the amplitude of the discontinuity in M₁ decreases and the slope of M₁ versus T increase for the diacyl phospholipid bilayers at the transition temperature. It was found that the dependence of M₁ on T could be accounted for by a Landau theory and that the observed behaviour was sensitive to the chain length dependent differences between the spinodal temperature and the transition temperature. -- In chain-length mismatched bilayers, the orientational order parameter profiles for very long chains were found to have a second plateau near the methyl end of the chain. The existence of such a second plateau in the order parameter profile suggests that the mismatched part of the long chains and the methyl ends of the short chains from the opposite monolayers are mingled with each other, forming a very fluid central layer in the middle of the bilayer. This fluid central layer may link the two sides of the bilayer in the biological membranes. The existence of cholesterol in these chain mismatched systems has large influence on order parameter profiles of its neighboring chains but has a little influence on the second plateau for the very long chain in the bilayer mixture. -- The phase diagrams for binary mixtures with either component deuterated can be different. It was found that the ²H-NMR spectra which represent different labeled components at the same temperature simultaneously represent the states of the two components in a given mixture, whereas this is not the case for the profiles selected to have the same plateau value. This discovery may be helpful for interpreting ²H-NMR experimental results for binary lipid bilayer mixtures.

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