Cryptococcus neoformans, the major etiological agent of Cryptococcosis, is a primary cause of opportunistic infections in AIDS patients. Four serotypes (A-D) of C. neoformans are distinguished; they differ in the structure of the glucuronoxylomannans (GXMs), capsular polysaccharides on their outer surface. Primary structural characterization of these GXMs is the first step in understanding the epitope specificity corresponding to the individual serotypes. The primary structure, conformation, and dynamics of the GXM serotype D, labeled with 13C, was studied by NMR spectroscopy. In order to aid the analysis, molecular dynamics (MD) simulations are being performed on the pentasaccharide repeating unit of serotype D using the GLYCAM 93 parameter set developed for AMBER. NOE build-up rates are being used in a restrained MD protocol to derive a solution structure of this repeating unit. Alternative approaches to comparing the results of long (~ ns) MD simulations to NMR data are being explored. Analysis of the 13C-NMR heteronuclear relaxation rates are being analyzed by the """"""""model-free"""""""" formalism of Lipari and Szabo to reveal internal motions in the polysaccharide that occur on a timescale faster than the rotation correlation time of the molecule. We are mapping the spectral density of the molecule through a more detailed measurement of the relaxation rates of different coherences to compare the results with the Lipari-Szabo formalism an d thus evaluate its usefulness in studying the dynamics of polysaccharides. This conformational analysis of the repeating unit of serotype D will set the stage for attempting similar studies on the most virulent serotype A strain of C. neoformans.
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