Vibrational and calorimetric study on the effect of di-n-propylsulfoxide (DPSO) on DMPC, DPPC and DMPE liposomes

In this paper, the effect of increasing amounts of DPSO on the behaviour of lecithins (DMPC, DPPC) and cephalin (DMPE) liposomes was investigated by means of Raman, FT-IR and DSC techniques. The results show that DPSO strongly affect, but in a different way, both lecithins and cephalin liposomes. In presence of low DPSO amounts, the main effect was a small decrease followed by an increase in the transition temperatures of all lipids; on the contrary, marked differences between the two classes were observed at higher sulfoxide content and the thermograms are strongly affected by memory effects. The partial dehydration of the lipid surface and the modification induced on the water structure might explain the effects observed in presence of low DPSO content, whereas, at higher DPSO concentrations, the main role is played by the direct interactions between the sulfoxide and the liposome surface. Spectroscopic data exhibit that the DPSO interact with CO, NH3+ and PO groups in cephalin and only with N(CH3)3+ groups in lecithins.The extension of the study to the concentrated DPSO containing systems presents a relevant biological significance; as a consequence of the affinity to biomembranes, the sulfoxide concentration near the lipid surface increases noticeably.