Minor influence of sialic acid on conformation of a membrane-bound oligosaccharide recognition site.

Wideline 2H NMR spectroscopy was used to assess the conformational and orientational effects of N-acetylneuraminic acid (NeuAc) (sialic acid) as a component of a particular oligosaccharide chain at a bilayer membrane surface. For this purpose, three glycosphingolipids, sharing a neutral core tetrasaccharide and differing only in the number of sialic acid residues, were compared. The starting compound was GD1A, which has terminal sialic acid attached to the second and fourth sugars of its neutral tetrasaccharide core. GD1A was probe-labeled in a non-perturbing fashion on both of these sialic acid residues and on its single GalNAc residue by replacement of -COCH3 with -COCD3 giving [(d3NeuAc)2,d3-GalNAc]GA1a. This represents the most complex glycolipid to have been studied by 2H NMR spectroscopy at a bilayer membrane surface. The sialic acid residue on the fourth sugar from the membrane was subsequently removed to produce the glycolipid [d3NeuAc,d3GalNAc]GM1, deuterated at the two remaining amino sugars. The neutral glycolipid [d3GalNAc]asialo-GM1 was then generated by removal of the second sialic acid residue, leaving an uncharged species deuterated at one (internal) oligosaccharide chain site (GalNAc). The effect of sialic acid was futher examined by selective deuteration of GM1 and asialo-GM1 at C6 of the terminal Gal residue, giving [d2Gal]GM1 and [d2Gal]asialo-GM1. Spectra of the three glycosphingolipids were compared at 7.7 mol % in unsoncicated fluid bilayers of 1-palmitoyl-2-oleoylphosphatidylcholine containing 23 mol % cholesterol. For liposomes suspend in buffered salt solutions with 2 mM Ca2+, 2H NMR spectra demonstrated the presence of well defined average conformation for each oligosaccharide chain. This preferred average conformation persisted over a wide temperature range, consistent with there being a single major oligosaccharide conformer in each case. Spectral features arising from both deuterated amino sugar (GalNAc) of asialo-GM1 could be identified, little changed, in spectra of GM1 and GD1A. Similarly, deuterons in the terminal Gal residue of asialo-Gm1 produced the same spectrum seen for this residue in GM1. Our findings indicate that certain major conformational and orientational features of this complex oligosaccharide recognition site are preserved, within maximum angular deviation + or -5 degrees or less upon addition or removal of a sialic acid residue.