Unusual enthalpy changes which accompany the titration of dimyristoylphosphatidylcholine vesicles with triton X-100

Abstract The enthalpy changes which accompany the titration of 0.1% and 0.25% small unilamellar and multiameller vesicle samples of dimyristoylphosphatidylcholine with 2% Triton X-100 in 0.067 M phosphate buffer (pH 7.4) containing 0.15 M NaCl have been determined by titration calorimetry at 21°C and 28°C. At 28°C, the enthalpy change for both type of vesicles was zero within the limits of experimental error. At 21°C, the multilamellar vesicle samples exhibited an enthalpy change of 1.35 ± 0.48 and 2.47 ± 0.98 kcal/mol dimyristoylphosphatidylcholine which was complete at a molar ratio of dimyristoylphosphatidylcholine to Triton of 3.21 ± 0.84 and 5.77 ± 1.05 for 0.1% and 0.25% dimyristoylphosphatidylcholine solutions, respectively. An exothermic transition of −2.39 ± 0.30 and −2.05 ± 0.69 kcal/mol phospholipid followed by an endothermic transition of 1.37 ± 0.12 and 1.94 ± 0.20 kcal/mol dimyristoylphosphatidylchlorine was observed at 21°C for 0.1% and 0.25% small unilamellar vesicle samples, respectively. In addition the nearly athermal association of the small unilemellar vesicle samples at 21°C was observed, which may be an appropriate model for biological membrane fusion.