A DSC and Raman study of the interaction between tricresyl phosphates (TCP) and phospholipid liposomes

This paper reports on the DSC and Raman measurements of hydrated multilamellar dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylethanolamine (DMPE) liposomes in the presence of raw tricresyl phosphates (TCP) and pure tri-o-cresyl phosphate (TOCP). The results on TCP/DMPC and TOCP/DMPC liposomes showed no significant differences. Both the T-m decrease and Delta T-1/2 increase, obtained by increasing the relative amount of TCP with respect to liposomes, suggested that the hydrophobic core is strongly affected by the presence of TCP molecules, whose deep penetration into the bilayer is prevented by the polar interactions between the P=O group of the TCP molecules and the polar head on DMPC. On the contrary, the TCP/DMPE and TOCP/DMPE systems showed a phase segregation that takes place in the presence of a very small amount of TCP, and occurs somewhat more easily in the presence of TOCP than of raw TCP. Both TCP and TOCP molecules interact mainly with the outer face of the bilayer and only secondly affect the hydrophobic core of membranes.