Thermodynamics of steroid partitioning in dimyristoylphosphatidylcholine liposomes.

The thermodynamics of partitioning of some steroids into dimyristoylphosphatidylcholine (DMPC) liposomes determined above and below the phase-transition temperature (Tc) revealed that: (1) delta Gw leads to 1 for all steroids studied is negative. (2) The process is entropy dominated. (3) delta Hw leads to 1 and delta Sw leads to 1 is more positive below Tc than above it for C-21 steroids. (4) Partitioning occurs into areas slightly more hydrophilic than n-octanol. (5) Ketones substituted on the 11 position of a 21-OH steroid have greater hydrogen-bonding capability than 11-OH compounds. (6) Hydroxyl groups at different positions on the steroid nucleus are non-equivalent, and (7) The group contribution for 21-ester methylenes is greater than that reported for other solutes in DMPC liposome systems.