Acyl chain length effects related to glycosphingolipid crypticity in phospholipid membranes: probed by 2H-NMR

Abstract

Wideline 2H-NMR was used to consider the relationships amongst glycosphingolipid and phospholipid fatty acid chain length and glycosphingolipid receptor function, in a system classically associated with crypticity. Galactosyl ceramide (GalCer), having 18−or 24−carbon fatty acid, was deuterium labelled at the conformationally-restricted fatty acid α-carbon (C−2). 2H-NMR spectra of N-[2,2− 2 H 2] stearoyl and N-[2,2− 2 H 2] lignoceroyl GalCer (GalCer with 18−vs. 24−carbon selectively deuterated fatty acid) were then compared over a range of temperatures in phosphatidylcholine/cholesterol membranes in which the host phospholipid had dimyristoyl, dipalmitoyl, or distearoyl fatty acid composition. Findings were evaluated in the light of known sensitivity of antibody interaction with GalCer to temperature and to both glycolipid fatty acid chain length and host matrix fatty acid chain length. Under the conditions of experimentation, spectra were not obtainable for glycolipids having rigid body motions that were slow on the NMR timescale (10 −4-10 −5s)- i.e.. motions typical of non-fluid (gel phase) membranes. The systems, DPPC/cholesterol and DSPC/cholesterol, in which the original observation was made of increased antibody binding to GalCer with long fatty acid, proved to be characterised by receptor motions that were in this slow timescale for both 18:0 and 24:0 GalCer at 22–24δC. Under conditions for which spectra could be obtained, those for GalCer with [2,2− 2H 2]lignoceroyl (24–carbon α-deuterated) fatty acid were qualitatively similar to those of its 18-carbon analogue in all (fluid) membranes examined. However, spectral splittings differed quantitatively between deuterated 18:0 and 24:0 GalCer at a given temperature, dependent upon host matrix. These differences were most marked at lower temperatures and in the longer chain (more ordered) matrices, DPPC/cholesterol and DSPC/cholesterol. This suggests that maximum effects of glycolipid chain length on glycolipid receptor function may be expected to occur in spatially and motionally constrained lipid environments. There was little effect of temperature on spectral splittings seen for a given sample containing deuterated 18:0 GalCer. The small differences seen could be adequately accounted for by relatively minor alterations in glycolipid order and backbone conformation. In contrast, 24:0 GalCer in DPPC/cholesterol and DSPC/cholesterol displayed significant variation in its spectral splittings as the temperature was reduced; and these proved to be the source of the quantitative differences between 18:0 and 24:0 GalCer referred to above. For 18:0 GalCer, the only spectral feature seen to be notably sensitive to temperature and to choice of host membrane phospholipid in the range, 22–65°C, was whole-body motion; which could be abruptly ‘frozen out’ with decreasing membrane fluidity. The 24:0 GalCer analogue was seen to undergo the same loss of motional freedom, in the same temperature range, as 18:0 GalCer. We suggest therefore that several previous observations of GalCer crypticity in phospholipid/cholesterol systems having longer vs. shorter chain phospholipids, may derive most importantly from reduced motional freedom of the glycolipid recognition sites.