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Bioassay-guided fractionation of a crude extract of Guazuma ulmifolia bark led to the isolation of polymeric proanthocyanidins which inactivated cholera toxin (CT). The average degree of polymerization (DP) of the active compounds ranged from 14.4 to 32.0. The polymers consisted mainly of (−)-epicatechin units. In polymers of a representative fraction, the flavanol units were connected by [4 → 8] bonds and, less frequently, by [4 → 6] bonds. Inhibition of CT by tannins increased with Mr and conformation flexibility of the tannin molecule. Several known procyanidin oligomers were also isolated. 1H NMR shift rules to distinguish between [4 → 8] and [4 → 6] linked proanthocyanidin peracetates, that have been proposed for dimers, were extended to trimers and a tetramer. A further diagnostic shift parameter to determine the interflavanoid bonding position is presented and the conformation of oligomeric proanthocyanidin peracetates is discussed. |
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