Flexibility in a tetrasaccharide fragment from the high mannose type of N-linked oligosaccharides

An analysis has been carried out of the three-dimensional structure of a tetrasaccharide, Man(alpha 1-3)Man(alpha 1-6)Man(beta 1-4)GlcN Ac beta 1-OCD3, which is a fragment from the high mannose type of N-linked oligosaccharides. Although earlier work had suggested that this fragment might adopt a stable three-dimensional structure, both n.m.r. and conformational energy calculations support the existence of an ensemble of structures. The conformational entropy calculated from the ensemble and the distribution of distances between the terminal Man(alpha 1-3) and GlcN Ac residues, however, suggests that a significant fraction of the ensemble has the two terminal residues in close proximity.