Dynamics of Uniformly 13C-Enriched Cell Wall Polysaccharide of Streptococcus mitis J22 Studied by 13C Relaxation Rates

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Abstract
We have studied the dynamics of the motion of a complex polysaccharide having seven sugar residues in the repeating subunit and which is a receptor for lectin interaction in the coaggregation of oral bacteria. Measurements of the longitudinal and the rotating frame relaxation rates and the heteronuclear nuclear Overhauser effects were carried out on a uniformly 13C-enriched sample using pulse sequences chosen to minimize the effects of 13C−13C coupling and cross relaxation. T1 and T1ρ measurements both showed single exponential decay for the anomeric carbon atom resonances of the polysaccharide. The results show the polymer to be highly flexible with a hinge at the (1→6)-linked galactofuranoside residue. Since there is no generally accepted scheme for interpreting polysaccharide dynamics, several different methods of data analysis were used including a reduced spectral density function method as well as several different methods in which a series of isotropically decaying rotational correlation functions are assumed. The different analyses all show that there are differing amounts of internal motion in the different residues of the polysaccharide. One possible interpretation of the data, which uses an extended version of the model-free treatment, indicates that picosecond motion is exhibited to a similar degree by all the residues in addition to a slower motion on the nanosecond time scale whose amplitude is greatest in the hinge region around the (1→6)-linked galactofuranoside residue in the polysaccharide.