APPLICATION OF 2-DIMENSIONAL NMR-SPECTROSCOPY AND MOLECULAR-DYNAMICS SIMULATIONS TO THE CONFORMATIONAL-ANALYSIS OF OLIGOSACCHARIDES CORRESPONDING TO THE CELL-WALL POLYSACCHARIDE OF STREPTOCOCCUS GROUP-A

This paper describes the use of a protocol for conformational analysis of oligosaccharide structures related to the cell-wall polysaccharide of Streptococcus group A. The polysaccharide features a branched structure with an L-rhamnopyranose (Rhap) backbone consisting of alternating alpha-(1 --> 2) and alpha-(1 --> 3) links and D-N-acetylglucosamine (GlcpNAc) residues beta-(1 --> 3)-connected to alternating rhamnose rings: [GRAPHICS] Oligomers consisting of three to six residues have been synthesized and nuclear magnetic resonance (NMR) assignments have been made. The protocol for conformational analysis of the solution structure of these oligosaccharides involves experimental and theoretical methods. Two-dimensional NMR spectroscopy methods (TOCSY, ROESY and NOESY) are utilized to obtain chemical shift data and proton-proton distances. These distances are used as constraints in 100 ps molecular dynamics simulations in water using QUANTA and CHARMm. In addition, the dynamics simulations are performed without constraints. ROE build-up curves are computed from the averaged structures of the molecular dynamics simulations using the CROSREL program and compared with the experimental curves. Thus, a refinement of the initial structure may be obtained. The alpha-(1 --> 2) and the beta-(1 --> 3) links are unambiguously defined by the observed ROE cross peaks between the A-B', A'-B and C-B, C'-B' residues, respectively. The branch-point of the trisaccharide CBA' is conformationally well-defined. Assignment of the conformation of the B-A linkage (alpha-(1 --> 3)) was problematic due to TOCSY relay, but could be solved by NOESY and T-ROESY techniques. A conformational model for the polysaccharide is proposed.