for Le a , Le x , sLe a and sLe x respectively. This agrees with the ubiquity of closed forms in both free and bound structures observed from X-ray spectroscopy (Figures and). Several clearly defined energy minima were identified on the torsional landscapes of the Gal-GlcNAc and Fuc-GlcNAc glycosidic linkages, corresponding to a large ensemble of non-closed conformational states. These minima have a range of stabilities, with those closer to the main closed (F 1 and G 1 ) wells, e.g. G 2/3 and F 2/3 , lying within 1-2 kcal mol when sialylation is absent. The msesMD simulations of Le a /sLe a suggest a similar behavior may also be true for these two systems too, albeit possibly to a lesser extent, especially considering the lack of agreement with the unbiased MD simulations. Interestingly, the K d has been measured for the binding of a Le x tetrasaccharide and sLe x pentasaccharide to RSL by Topin et al. These sugars have the primary structure Gal14)(Fuc13))GlcNAc3)Gal and Neu5Ac(23)Gal14) (Fuc13))GlcNAc3)Gal respectively, differing from Le x and sLe x in possessing an extra galactose residue distal to the sialylation site. The K d of Le x tetrasaccharide was found to be approximately half that of sLe x pentasaccharide, at 26 and 58