Steric inhibition of phenylboronate complex formation of 2'-(5"-phosphoribosyl)-5'-AMP

Unlike all other adenine nucleotides which contain cis-diols in their ribose groups, the degradation product of polyadenosine diphosphoribose, 2'-(5"-phosphoribosyl)-5'-AMP, does not bind to boronate affinity columns. This anomalous behavior occurs even under conditions of high ionic strength (1 M salt), indicating that the absence of binding is not due to generalized repulsion between the negatively charged phosphate groups of 2'-(5"-phosphoribosyl)-5'-AMP and the negatively charged boronate sites of the resin. Despite the lack of boronate binding the presence of a chemically functional cis-diol group in 2'-(5"-phosphoribosyl)-5'-AMP was verified by quantitative periodate oxidation. Subsequent dephosphorylation rendered the cis-diol in ribosyladenosine accessible to binding to the boronate sites; therefore, one or both of the phosphate groups sterically interfere with complexation of the cis-diol with boronate. Computer model-building studies based on NMR data implicate the 5'-phosphate group of the molecule, is located sterically near the cis-diol group, inhibiting the complex formation with boronate.