Cooperative domains in lipid membranes: Size determination by calorimetry

Phase transitions of multibilayer structures of hydrated L - α -dipalmitoyl- and L - α -dimyristoylphosphatidylcholine (DPPC and DMPC) were studied by differential scanning calorimetry. For the main phase transition, thermodynamic parameters (temperature, enthalpy, half-width and cooperative domain size, CN) were determined in a broad range of scanning rates x, from 0.1 to 40.0 K min −1 . Marked dependence on scanning rate was obtained for CN, while for other parameters it was much weaker. A hyperbolic dependence CN ( x ) = c + b /( x + a ) was established ( a , b , c are constants). The lowering of the scanning rate causes an abrupt CN increase at x → 0, which is one of the reasons of the large scatter (by 1–2 orders) in the literature data on CN values. Based on the data obtained, a criterion is proposed for the choice of optimum scanning rate that would minimize the error in CN determination. It is argued that reliable CN values can be obtained if the measured peak half-width is much larger than the corresponding experimental error for the given system.