Hydrated/dehydrated lipid phase transitions measured using nanocalorimetry.

The phase transition evolution with hydration of a model system, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), was investigated with a fast nanocalorimetry system. Using nanocalorimetry, it is possible to measure the gel to liquid phase transitions that occur on millisecond to second time scales and quantify the time to recover the hydrated state. The results show the phase transition occurring in a few milliseconds and the relaxation or recovery time from the dehydrated state back to original hydrated state occurring with times dependent on the local humidity. With relative humidity (RH) of 43% or higher, the recovery time can be less than a few seconds. With RH of 11% or lower, the recovery time is extended to greater than a minute. The recovery process is controlled by mechanisms that depend on the lipid molecular repacking and water transport from the environment. Nanocalorimetry provides a powerful method to investigate the kinetics of such transformations in lipids and other biological and pharmaceutical moieties.