Glycosphingolipid interdigitation in phospholipid bilayers examined by deuterium NMR and EPR

Glycosphingolipid fatty acids commonly have up to eight methylene carbons more than do their surrounding phospholipid-attached counterparts. The resultant 'extra' segment may very well modulate glycosphingolipid function as receptor and structural element. As part of an investigation of this phenomenon, galactosylceramide was prepared with a deuterated 18-carbon fatty acid chain. Deuterium-labelled galactosylceramide was assembled at 10 mol% into unsonicated phosphatidylcholine bilayers having all 14-carbon or all 18-carbon saturated fatty acid chains (DMPC and DSPC, respectively). The systems were studied by 2H-NMR spectroscopy above and below the phase transition temperatures, Tm, of the host matrices. At comparable reduced temperatures in fluid membranes the degree of motional order exhibited by the glycolipid fatty acid was significantly higher in the phospholipid host matrix that was four carbons shorter. The fatty acid chain segment least affected by the change from long to short chain host matrix was the terminal (deutero)methyl group (an increase of 8% in quadrupolar splitting for the terminal methyl vs. 16% for deuterons at C17 and 23-28% for the remainder of the chain). Order parameter profiles for galactosylceramide were qualitatively very similar in the two host membranes, arguing against any major conformational difference between the arrangement of the 18-carbon glycolipid fatty acid in the 18-carbon vs. 14-carbon host matrices. Similarly a nitroxide spin probe covalently attached to carbon-12 of the galactosylceramide fatty acid gave clear indication of greater order in the fluid 14-carbon fatty acid phospholipid bilayer. These results are consistent with 'tethering' of the extra length of fatty acid via interdigitation into the opposing monolayer. There was no spectroscopic evidence of any intrinsic difference in glycolipid behaviour in the two fluid host matrices. 2H-NMR spectra of galactosylceramide at comparable reduced temperatures below Tm of the phospholipid bilayer were very different for 14-carbon vs. 18-carbon host matrices. The glycolipid fatty acid showed evidence of relatively reduced mobility in the shorter chain matrix.