On the quantitative interpretation of biomembrane structure by Raman spectroscopy

The now well-established use of Raman spectroscopy to examine the structure of biomembranes is extended through an examination of the origins of the structure-sensitive features of phospholipid spectra and the development of quantitative order-parameters. One parameter gives a quantitative measure of the fraction of all-trans bonds in the hydrocarbon chains while the other provides a semiquantitative estimate of the lateral crystal-like order between the chains. The parameters are used to study the differences between vesicles and dispersions of dipalmitoyl phosphotidylcholine, dimyristoylcholine and egg lecithin. We find that the vesicles of dipalmitoyl phosphotidylcholine are substantially less ordered than the dispersions in terms of both longitudinal and lateral order which are greatly decreased. A very careful measurement of the order as a function of temperature shows that there is a pre-melting transition in the dispersions of dipalmitoyl phosphotidylcholine which does not exist in the vesicles. Remarkable agreement is obtained between the Raman technique and that previously reported by calorimetric measurements and theoretical calculations.