Monovalent cation dependent phase behaviour of dipalmitoylphosphatidylglycerol

Abstract

The phase behaviour of dipalmitoylphosphatidylglycerol (DPPG) was studied as a function of [NaCl] using differential scanning calorimetry and the effects of monovalent cations, Na +, K +, Li + and Cs + were compared at 300 mM cation concentration. Main phase transition, pretransition, and subtransition temperatures and enthalpies were found to depend on the concentration of NaCl and the monovalent cation species. Compared to the main phase transition both pretransition and subtransition were more sensitive and also exerted greater specificity with respect to the different cations. Considerable hysteresis ( δT m ) in the main transition temperature ( T m ) upon heating and cooling was evident in the absence of salt. Upon increasing [NaCl], a decrease in δT m was observed. Subtransition and pretransition temperatures and enthalpies increased upon increasing [NaCl] from 50 mM to 1 M. The appearance of subtransition was considerably faster in the presence of high NaCl concentrations. No pretransition was observed in the absence of salt or in the presence of 300 mM LiCl. Pretransition temperatures of DPPG in the presence of 300 mM NaCl, KCl, and CsCl were 34.0, 34.3 and 30.4°C, respectively. The subtransition temperatures after 5 days of incubation at 6°C with 300 mM NaCl, KCl, CsCl were 29.3, 22.6 and 23.6°C, respectively. The highest subtransition temperature of 32.6°C was observed in the presence of 300 mM LiCl but compared to other cations the appearance of subtransition in the presence of Li + required considerably longer incubation times.