Fourier transform infrared study of fully hydrated dimyristoylphosphatidylglycerol. Effects of Na+ on the sn-1' and sn-3' headgroup stereoisomers.

Molecular packing and the thermotropic phase behavior of fully hydrated ammonium salts of 1,2-dimyristoyl-sn-glycero-3-phosphatidyl-sn-1'-glycerol (1'-DMPG) and the corresponding 3' stereoisomer (3'-DMPG) as well as the effects of 300 mM NaCl on these lipids were studied by Fourier transform infrared (FTIR) spectroscopy. The ammonium salts of both stereoisomer show similar thermotropic phase behavior and have an order-disorder phase transition at approximately 21 degrees C. While complexing with Na+, however, an incubation of liposomes at +6 degrees C for 3 days results in significant structural differences between liposomes of 1'-DMPG and 3'-DMPG. In the presence of 300 mM NaCl the infrared spectra for 3'-DMPG reveal the appearance of a more solidified lipid nominated here as the highly crystalline phase with a transition into the liquid-crystalline state at a significantly higher temperature (approximately at 33 degrees C) than that for 1'-DMPG (approximately at 23 degrees C). Crystal field splitting resulting from interchain vibrational coupling is observed in the CH2 scissoring mode of the 3'-DMPG(Na+) complex in the highly crystalline phase (T less than 33 degrees C); i.e., the acyl chains are packed in a rigid orthorhombic- or monoclinic-like crystal lattice. At temperatures above the transition at 33 degrees C the acyl chains of 3'-DMPG(Na+) give rise to infrared spectra indicative of hexagonal packing. The latter type of hydrocarbon chain packing is also found for the ammonium salts of 1'-DMPG and 3'-DMPG without Na+ as well as for 1'-DMPG with Na+. In addition, the binding of Na+ to 3'-DMPG causes narrowing of the bands associated with the interfacial and polar headgroup regions of 3'-DMPG and thus reveals reduced motional freedom. This demonstrates that Na+ binds tightly to 3'-DMPG, leading to the immobilization of the entire phospholipid polar headgroup. Such effects by Na+ are not observed for 1'-DMPG.