Proton and carbon-13 nuclear magnetic resonance studies of the polar lipids of Halobacterium halobium

Abstract

The thermotropic behavior and the dynamic properties of the polar lipids of Halobacterium halobium were studied by 1H and 13C NMR. The studies were performed on three different preparations: micellar solutions in deuterated chloroform, multilamellar dispersions in 2H2O, and unilamellar vesicles obtained from the latter by ultrasonication. Due to the methyl side groups in the alkyl chains of these lipids, the 13C spectra are highly resolved in particular in the micellar solution, allowing peak assignment of the alkyl chain carbons. Proton and 13C T1 relaxation and line width measurements were made for all three preparations. In both lamellar dispersions, sharp discontinuities in these parameters were observed around 35 degrees C. This behavior is attributed to an abrupt change in the dynamics of the lipids within the membrane. The variation of the specific relaxation rate 1/NT1 of the 13C nuclei along the hydrocarbon chains of the H. halobium lipids exhibits maxima at the tertiary carbons to which the methyl side groups are attached. These maxima are particularly pronounced in the lamellar phases and suggest that the segmental motion ("kink" formation) at the tertiary carbons is hindered by the presence of the methyl groups.