Kinetic and thermodynamic studies of the fusion of small unilamellar phospholipid vesicles.

Small phospholipid vesicles, prepared so as to minimize impurities, fuse relatively slowly resulting in the time-dependent development of a characteristic endotherm in differential scanning calorimetry and corresponding changes in the Raman spectrum. The stability of small vesicles towards fusion increases with increasing acyl chain length for the series C-14 through 18. Within the protocols of these experiments, the fusion rate remains unchanged whether the vesicles are held at 10 degrees C below Tm or at Tm itself. We have determined enthalpies of transition for small vesicles and fusion product for C-14 through C-18. In each case delta H for small vesicles is lower than that of the corresponding multilamellar vesicles, while the fusion product delta H is intermediate between small and multilamellar vesicles. The apparent lack of concensus in the literature as to the nature of the fusion process is ascribed to the variety of protocols used as well as the presence or absence of fusion-inducing impurities.