Solvent isotope effects on the phase-transition properties of lipid bilayers

Highly sensitive differential scanning microcalorimetry (DSC) has been used to investigate the phase transition properties of lipid vesicles prepared from 1,2-distearoyl-L-3-glyceryl-phosphatidylchloine (DSPC) in H2O and D2O The data show that the response of pre-transition properties to D2O --> H2O substitution is stronger than the main transition properties. We find that there is a small increase in the phase transition temperature (DeltaT similar to 0.5 K) and in the co-operative unit in the main transition. The increase in enthalpy (DeltaH congruent to 1 kJ mol(-1)) and in transition temperature (DeltaT congruent to 2 K) observed in the pre-transition is comparable with that observed in quite different processes and systems, i.e. melting of nucleic acids and proteins and gel formation. It is suggested that D2O --> H2O substitution affects the thermal transition in these systems in such a way that the contributions of enthalpy and entropy to structural reorganization of water in these processes is modified. (C) 2001 Elsevier Science B.V. All rights reserved.