Applications Of Fourier Transform Raman Spectroscopy To Highly Unsaturated Phosphatidylcholine Multilamellar Membrane Assemblies.

Abstract

Although the use of visible laser excitation in dispersive Raman spectroscopy is well documented in studies characterizing model membrane preparations composed of lipids with saturated acyl chains, applications of this technique to bilayer assemblies containing highly unsaturated acyl chains are limited due to the tendency of the latter lipids to undergo peroxidation, with the production of potentially fluorescent decomposition products. These lipids represent important constituents of mammalian cell membranes, as demonstrated, for example, in the retinal rod outer segment disc membrane which contains approximately 50% of its phospholipid acyl chains as docosahexaenoic acid (22:6). A method for avoiding the pernicious fluorescence associated with these types of materials involves a relatively new approach coupling the multiplex, throughput and precise frequency measurement advantages of a Michelson interferometer with a near-infrared Nd:YAG laser source and a high sensitivity detector. In this study we use FT-Raman spectroscopy to examine the packing and dynamic properties of multilamellar lipid assemblies composed of various unsaturated lipids containing acyl chains with one, four and six double bonds, respectively, in an attempt to relate the inter- and intramolecular properties of the lipid matrix to the degree of chain unsaturation. The results suggest that increased unsaturation in the sn2 acyl chain of the phospholipid molecules with a completely saturated sni chain leads to increased bilayer order with a concomitant increase in the degree of rotational freedom for the sn2 chain.