We have recently shown that structural investigations of regular polysaccharides can be performed rapidly by computer-assisted analysis using the computer program CASPERQ. This uses data from sugar and methylation analysis together with the unassigned 'H-or 13C-n.m.r. spectrum or the two-dimensional C,H-correlation n.m.r. spectrum. The program generates all possible structures and simulates their n.m.r. spectra. The latter are ranked according to their fit to the experimental spectrum and, for some of these spectra, a high degree of similarity is observed. For several linear and branched polysaccharides of known structure, with repeating units of three to five sugar residues, an exclusive structure can be selected',*.
Among the polysaccharides of known structure used for testing CASPER was the O-antigen polysaccharide from Escherichia cali 018ac, the structure of which was investigated a few years ago3. A pentasaccharide repeating-unit was proposed from composition, fragmentation procedures, methylation analysis, and n.m.r. spectroscopy.
In the computer-assisted analysis, information on components and linkages together with the 13C-n.m.r. data were used as input to CASPER. That the material was identical to that used earlier was evident from a comparison with the published 13C-n.m.r. spectrum as well as by comparison of methylation analysis data. The result of the analysis with CASPER is shown in Scheme 1. Structures with spectra in which the C-l-H-l and H-l-H-2 coupling constants were not compatible with the observed values2 were excluded from the analysis. For the suggested structure 1, a spectrum with a considerably lower deviation from the experimental spectrum than that for structure 2 was obtained, the difference being so large that it and the following structures could be considered as being incompatible with the experimental spectrum. The deviation termed deltasum is the difference between the experimental spectrum and the simulated spectrum obtained by a peak-by-peak comparison. The structure proposed earlier appears as suggested structure 11, with a spectrum