New 13C NMR Methods for Determining the Structure of Algal Polysaccharides. Part 1. The Effect of Substitution on the Chemical Shifts of Simple Diad Galactans

The 13 C nuclear magnetic resonance spectra of unsubstituted algal galactans of simple diad structure, together with their sulfated, methylated and permethylated analogues, are recorded as sets of signals, and provisional assignments are presented in matrix form for those spectra previously unassigned. This use of discrete mathematics permits a concise and transparent presentation of changes in chemical shifts arising from various substitutions. Regularities in the chemical shifts due to substitution, represented in matrix form, are observed for methylation and to a lesser extent for sulfate ester. Different reference matrices are required for each conformational state about linkage bonds, and some irregularities, particularly where there is sulfate ester near the linkage bond, can be interpreted in terms of changes in conformation brought about by the substitution. The β-shifts due to substitution are provisionally interpreted in terms of changes of the magnetic vector potential from residual magnetization fields from the region of substitution. The chemical shifts of methoxyl carbon atoms vary by approximately 6 ppm, are characteristic of their location on the polymer and may be of use for structural analysis. Changes of chemical shift due to substitution are sufficiently regular for the examples described here that they should be useful for predicting spectra of unknown red algal galactans.