INFRARED SPECTROSCOPIC STUDY OF THE INTERACTION OF DIACYLGLYCEROL WITH PHOSPHATIDYLSERINE IN THE PRESENCE OF CALCIUM

The interaction of 1,2-dipalmitoylglycerol (DPG) with dipalmitoylphosp hatidylserine (DPPS) has been studied in aqueous dispersion in the pre sence and in the absence of Ca2+ by using Fourier transform infrared s pectroscopy (FT-IR) and 45 Ca2+-binding. FT-IR showed that DPG increas ed the phase transition of DPPS and induced a rigidification of the DP PS/DPG-Ca2+ complex. In the absence of Ca2+, the incorporation of DPG produced an increase in the proportion of dehydrated carbonyl groups i n the mixture of DPPS plus DPG whereas, in the presence of Ca2+, DPG s uppressed the solid-solid phase transition of phosphatidylserine-Ca2complexes. The phosphate band of DPPS was analyzed using a multivariat e statistical analysis, indicating that DPG induced a higher dehydrati on of the PO2- group in the presence of subsaturating Ca2+ concentrati ons. Even very low concentrations of DPG, such as 2 mol%, already prod uced a significant effect. In the presence of both DPG and Ca2+, dehyd ration of DPPS increased, so that full dehydration was reached at a DP PS/Ca2+ molar ratio of 2.94 instead of 2.04 as observed for pure DPPS. However, the stoichiometry of the binding of Ca2+ to DPPS was not sig nificantly altered by the inclusion of DPG as revealed by (Ca2+)-Ca-45 -binding experiments, indicating that, in this situation, full dehydra tion of the PO2- groups of DPPS was reached when approx. 2 out of ever y 3 molecules of DPPS were binding Ca2+. The effects reported here for the interaction of DPG with DPPS may be significant for a number of b iological situations where Ca2+, phosphatidylserine and diacylglycerol s are involved, such as fusion of membranes or the activation of prote in kinase C, where the dehydration effect produced by diacylglycerols may explain, at least in part, their effects.