5 NANOSECOND MOLECULAR-DYNAMICS AND NMR-STUDY OF CONFORMATIONAL TRANSITIONS IN THE SIALYL-LEWIS-X ANTIGEN

The range of internal motions of the sialyl Lewis-X (SLe(x)) tetrasacc haride (NeuNAc alpha 2-->3Gal beta 1-->4(Fuc alpha 1-->3)GlcNAc) [wher e Fuc is L-fucopyranose, Gal is D-galactopyranose, GlcNAc is 2-acetami do-2-deoxy-D-glucopyranose and NeuNAc is D-neuraminic acid (sialic aci d)] was studied by restrained simulated annealing and restrained molec ular dynamics (MD) calculations. Transitions between predominantly two conformational states were observed for the NeuNAc alpha (2-->3)Gal l inkage, consistent with previous observations for this linkage in sial yl-N-acetyllactosamine. The Mn trajectory was simulated for 5 ns of re al time, in order to observe a statistically significant number of the se relatively low-frequency transitions. The Fuc alpha(1-->3)GlcNAc an d Gal beta (1-->4)GlcNAc linkages, however, showed more restricted fle xibility within a single energy well (RMS differences for the time-ave raged glycosidic torsion angles, <phi> and <psi>, were 50% lower than for the NeuNAc alpha(2-->3)Gal linkage), and approximate to a rigid co nformation. MMR parameters [relative rotating-frame Overhauser enhance ment (r.O.e.) and inter-glycosidic (3)J(CH)] back-calculated from the Mn simulation were in close agreement with experimentally measured val ues for the free reducing oligosaccharide in D2O solution.