EFFECTS OF CA2+ AND MG2+ ON DYNAMICS OF THE POLAR HEAD GROUP OF PHOSPHATIDYLSERINE BILAYERS

The effects of Ca2+ and Mg2+ on the molecular motion of the polar head group in phosphatidylserine (PS) bilayers were measured by the time-r esolved fluorescence depolarization method probed by 1,2-dihexadecanoy l-sn-glycero-3-phospho- [N-(4-nitrobenzo-2-oxa-1,3-diazole)]ethanolami ne (NBD-PE). By this method, the rate and width of the molecular motio n at the fluorescent moieties in the probe molecules could be evaluate d as the wobbling diffusion rate (D-w, s(-1)) and the half cone angle of the wobbling cone (theta(c), degree). The values of D-w and theta(c ) measured for NBD-PE embedded in bovine brain phosphatidylserine bila yers were 3.7 X 10(7) s(-1) and 46 degrees in the absence of divalent cations at 25 degrees C. When 3 mM of Ca2+ was added, both D-w and the ta(c) distinctly dropped to 1.7 X 10(7) s(-1) and 38 degrees, respecti vely. By the addition of 3 mM of Mg2+, however, only D-w decreased to 2.7 X 10(7) s(-1) and theta(c) remained unchanged. These results show that both Ca2+ and Mg2+ decrease the rate of motion at the head part i n PS molecules, but only Ca2+ narrows the distance between the neighbo ring head groups. Since Mg2+ does not promote vesicle fusion, it appea rs that the deformation at the head group region in the bilayer struct ure induced by Ca2+ is an important step in the membrane fusion proces s.