Theory of the Main Lipid Bilayer Phase Transition

Fortunately, the day when it was necessary to argue the biological relevance of fundamental physical and chemical studies on pure syn­ thetic lipid bilayers is past, and space will not be devoted to flogging that dead horse. The lipid bilayer phase transition (to be abbreviated LBPT), often called the gel to liquid crystal phase transition, appears to have direct biological relevance (1-5). Even if it did not, it is neverthe­ less a striking physical event, the study of which leads to a better understanding of the structure of biological membranes. In addition, the theoretical study of phase transitions in lipid bilayers is an interesting chemical physics problem in its own right and not just a problem for routine application of theoretical methods developed for simpler sys­ tems. The emphasis of this review is upon the lipid systems for which the most detailed and quantitative experimental and theoretical studies can be performed; the lecithin (phosphatidylcholine, abbreviated PC) bi­ layers with saturated and homogeneous hydrocarbon chains are the preeminent system, although a number of studies involving variations in the lipid molecules are mentioned. This review is not concerned with the proliferation of phases that occur at low water content (6a) or with the theory of self-assembly of the lipids into a bilayer in contrast to micellar forms (6b). Furthermore, this review is more concerned with equilibrium properties of the LBPT than with the dynamics of molecular motions. Even with these restrictions, the subject is a large one, and the author apologizes for omissions due to lack of space or oversight. Because this is an interdisciplinary area that attracts readers and authors from a variety of backgrounds with different perspectives, there has been confusion in evaluating the advances of different theories and