  Differential microcalorimetric measurements have been performed in aqueous
dispersions of dimyristoylphosphatidylcholine (DMPC) multilamellar vesicles in
a wide range of temperatures covering the whole transition between the lamellar
gel and lamellar fluid states (the chain-melting/ordering transition). The
shape of calorimetric curves was analyzed in a temperature range some distance
away but close to the chain-ordering transition point where small nuclei of gel
(solid state) are formed. In this range, where the fraction of the "new-state"
lipid is small, the nucleation process can be considered independent of the
interlayer interactions and determined mainly by the lateral interactions. The
small-scale nucleation was analyzed in terms of a two-dimensional Ising-like
lattice model. The gel-fluid contact energy related to the critical temperature
(in terms of Ising model) was estimated for DMPC studied here and
dipalmitoylphosphatidylcholine studied earlier. The contact energy was found to
be not high enough to provide the discontinuous, first-order character of
transition. Therefore, the signs of first-order character observed in the
vesicles made of these lipids are not due to lateral but rather interlayer
3D-interactions. The extrapolation to longer lipids shows that the transition
discontinuity inherent to a lipid layer (i.e., determined by the lateral
interactions themselves) is expected for the saturated phosphatidylcholines
having more than 16-18 C-atoms per chain. Interestingly, it is the saturated
chains of this length that are the most abundant in biological membranes.
Probably, the biological membrane "prefers" to be near the critical state where
the system is the most responsive to physical actions.
