Cooperative Unit Size in the Gel-liquid Crystalline Phase Transition of Dipalmitoyl Phosphatidylcholine-water Multilayers: an Estimate from Raman Spectroscopy

The temperature dependence of the Raman spectral transitions assigned to the acyl chain C-C stretching modes of dipalmitoyl phosphatidylcholine was determined for the gel, phase transition and liquid crystalline states of the lipid multilayers. The van't Hoff enthalpy differences delta HVH between trans and gauche rotational isomers were obtained from the Raman spectral data for the temperature region characteristic of each bilayer state. An average size for the cooperative unit undergoing the chain melting process during the phase transition was estimated from the ratio of the appropriate van't Hoff enthalpy to an adjusted calorimetric enthalpy.