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A flexible model for the cell wall polysaccharide of Streptococcus mitis J22 determined by three‐dimensional 13 C edited nuclear overhauser effect spectroscopy and 13 C‐ 1 H long‐range coupling constants combined with molecular modeling
Author(s) -
Xu Qiuwei,
Mohan S.,
Bush C. Allen
Publication year - 1996
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/(sici)1097-0282(199603)38:3<339::aid-bip7>3.0.co;2-v
Subject(s) - chemistry , tetrasaccharide , two dimensional nuclear magnetic resonance spectroscopy , glycosidic bond , nuclear overhauser effect , heteronuclear molecule , nuclear magnetic resonance spectroscopy , spectroscopy , dihedral angle , stereochemistry , polysaccharide , crystallography , molecule , organic chemistry , hydrogen bond , physics , quantum mechanics , enzyme
We report on the conformation of a tetrasaccharide fragment in the repeating subunit of the cell wall polysaccharide of Streptococcus mitis J22, a receptor for the lectin of Actinomyces viscosus T14V in a bacterial coaggregation that is important in the ecological interactions of oral bacteria. Although there is considerable overlap of the 1 H‐nmr signals, some cross peaks can be extracted from conventional two‐dimensional nuclear Overhauser effect spectroscopy (NOESY) data on the polysaccharide. These data cannot be fit to a single conformation of the tetrasaccharide fragment. Therefore we have prepared a polysaccharide sample fully enriched in 13 C from which we have determined accurate NOESY cross‐peak volumes in a three‐dimensional heteronuclear‐resolved spectrum that allows accurate determination of many more NOESY cross peaks than does conventional two‐dimensional spectroscopy. We have also used the 13 C enriched polysaccharide to measure accurate values of long‐range 13 C‐ 1 H coupling constants that can be correlated with glycosidic dihedral angles. Molecular modeling calculations on the polysaccharide fragment, including molecular dynamics simulations, identify multiple low‐energy conformations. This result is to be contrasted with previous calculations on blood group oligosaccharides in our laboratory using similar methods that showed relatively rigid conformations with little flexibility of the glycosidic linkages. The present NOESY and 3 J CH data can be reconciled with a model for the antigenic tetrasaccharide in which three distinct conformations are in fast exchange. We propose that some carbohydrate epitopes such as those of the blood group oligosaccharides are relatively rigid while others such as the tetrasaccharide fragment in these studies exhibit much greater flexibility. © 1996 John Wiley & Sons, Inc.