Cooperative Properties of Lipid Bilayers: Collective Director Fluctuations and the Effects of Hydrophobic Mismatch in Protein/Lipid Membrane Systems

• Main Author: Watnick, Paula Ivonne
• Format: Thesis
• Language: English
• Published: 1989
• Subjects: DML
• Online Access: https://thesis.library.caltech.edu/8065/; https://thesis.library.caltech.edu/8065/1/Watnick%201989.pdf; https://resolver.caltech.edu/CaltechTHESIS:02072014-135012645

Cooperative director fluctuations in lipid bilayers have been postulated for many years. ²H-NMR T₁⁻¹, T₁ ρ ⁻¹, and T₂⁻¹; measurements have been used identify these motions and to determine the origin of increased slow bilayer motion upon addition of unlike lipids or proteins to a pure lipid bilayer. The contribution of cooperative director fluctuations to NMR relaxation in lipid bilayers has been expressed mathematically using the approach of Doane et al.¹ and Pace and Chan.² The T₂⁻¹'s of pure dimyristoyllecithin (DML) bilayers deuterated at the 2, 9 and 10, and all positions on both lipid hydrocarbon chains have been measured. Several characteristics of these measurements indicate the presence of cooperative director fluctuations. First of all, T₂⁻¹ exhibits a linear dependence on S² CD . Secondly, T₂⁻¹ varies across the ²H-NMR powder pattern as sin² (2β), where , β is the angle between the average bilayer director and the external magnetic field. Furthermore, these fluctuations are restricted near the lecithin head group suggesting that the head group does not participate in these motions but, rather, anchors the hydrocarbon chains in the bilayer. T₂⁻¹ has been measured for selectively deuterated liquid crystalline DML hilayers to which a host of other lipids and proteins have been added. The T₂⁻¹ of the DML bilayer is found to increase drastically when chlorophyll a (chl a ) and Gramicidin A' (GA') are added to the bilayer. Both these molecules interfere with the lecithin head group spacing in the bilayer. Molecules such as myristic acid, distearoyllecithin (DSL), phytol, and cholesterol, whose hydrocarbon regions are quite different from DML but which have small,neutral polar head groups, leave cooperative fluctuations in the DML bilayer unchanged. The effect of chl a on cooperative fluctuations in the DML bilayer has been examined in detail using ²H-NMR T₁⁻¹, T₁ ρ ⁻¹, and T₂⁻¹; measurements. Cooperative fluctuations have been modelled using the continuum theory of the nematic state of liquid crystals. Chl ...