Structural calorimetry of main transition of supported DMPC billayers by temperature-controlled AFM

Atomic force microscopy at high temperature resolution (DeltaTless than or similar to0.1 K) provided a quantitative structural calorimetry of the transition from the fluid (L-alpha)- to the gel (P-beta')-phase of supported dimyristoylphosphatidylcholine bilayers. Besides a determination of the main transition temperature (T-0) and the van't Hoff transition enthalpy (DeltaH(vH)), the structural analysis in the nm-scale at T close to T-0 of the ripple phase allowed an experimental estimation of the area of cooperative units from small lipid domains. Thereby, the corresponding transition enthalpy (DeltaH) of single molecules could be determined. The lipid organization and the corresponding parameters T-0 and DeltaH(vH) (DeltaH) were modulated by heptanol or external Ca2+ and compared with physiological findings. The size of the cooperative unit was not significantly affected by the presence of 1 mM heptanol. The observed linear relationship of DeltaH(vH) and T-0 was discussed in terms of a change in heat capacity.