POLYMORPHISM OF LIPID-WATER SYSTEMS - EPITAXIAL RELATIONSHIPS, AREA-PER-VOLUME RATIOS, POLAR APOLAR PARTITION

The original purpose of this work was to seek an explanation of the em pirical observation that pairs of phases in thermodynamic equilibrium often display an epitaxial relationship. Considering that the polar/ap olar interfacial interactions appear to play a predominant role among all the forces that stabilize the phases, there is ground for the prop osition that the two phases involved in any particular phase transitio n consist of structure elements whose area/volume ratio is invariant. Volume and area of the structure elements can be expressed as function s of the water content and the cell parameters of the two coexisting p hases: their values can thus be determined experimentally. The volume ratio (structure elements)/(lipid molecules) is equivalent to a partit ion coefficient. These ideas were applied to a large variety of data a vailable in the literature. The partition coefficient was found to dis play wide variations, remarkably correlated with the chemical and the physical parameters of the system, suggesting that the segregation of the hydrocarbon chains away from the polar headgroups is not as sharp as it is commonly assumed. The notion of a variable polar/apolar parti tion is a novelty in the field; moreover, this partition coefficient m ay well turn into an interesting thermodynamic parameter. As to the si gnificance of the epitaxial relationships, a search through the litera ture shows that its very existence has many exceptions. In order to ex plain these observations the conjecture is put forward that the epitax ial coincidences have a kinetic effect on the phase transitions. In pa rticular, it is suggested that any transition involving epitaxially re lated phases is unlikely to display metastable states. The possibility is also evoked that a selective advantage (be it technological, biolo gical or experimental) may be associated with the existence of epitaxi al relationships. This conjecture is illustrated by several examples d rawn from the literature.