Order and dynamics in mixtures of membrane glucolipids from Acholeplasma laidlawii studied by 2H NMR

The two dominant glucolipids in Acholeplasma laidlawii, viz., 1,2-diacyl-3-O-(α-D-glucopyranosyl)-sn-glycerol (MGlcDG) and 1,2-diacyl-3-O-[α-D-glucopyranosyl-(1→2)-O-α-D-glucopyranosyl]-sn-glycerol (DGlcDG), have markedly different phase behavior. MGlcDG has an ability to form nonlamellar phases, whereas DGlcDG only forms lamellar phases. For maintenance of a stable lipid bilayer, the polar headgroup composition in A. laidlawii is metabolically regulated in vivo, in response to changes in the growth conditions. To investigate the mechanism behind the lipid regulation the authors have here studied bilayers of mixtures of unsaturated MGlcDG and DGlcDG, containing a small fraction of biosynthetically incorporated per-deuterated palmitic acid, with 2H NMR. The order-parameter profile of the acyl chains and an apparent transverse spin relaxation rate (R2) were determined from dePaked quadrupole-echo spectra. The variation of order with lipid composition is rationalized from simple packing constraints. The relaxation data indicate the presence of slow reorientational motions, such as collective bilayer fluctuations and/or lipid lateral diffusion over a curved bilayer surface. The variation of acyl-chain order and bilayer curvature and/or fluctuations with sample composition are discussed in relation to the tendency of MGlcDG to form nonlamellar phases in vitro and in relation to the lipid regulation in vivo