The binding of Mg2+ to the ribitol teichoic acid of Staphylococcus aureus H walls was examined by equilibrium dialysis in solution and in the intact wall; the influence of alanyl ester groups on binding was determined. In solution the ribitol polymer had a lower affinity than did a glycerol teichoic acid and bound Mg2+ in the ratio Mg2+/P of 1:1. The presence of alanyl ester residues caused a decrease in the amount of cations bound in stoicheiometric proportion to the ratio Ala/P, but the affinity constant was unaltered. It is concluded that in solution the ribitol teichoic acid binds Mg2+ univalently to phosphate groups and univalently to a counter-ion. In the intact wall the binding of Mg2+ was different. The affinity constant was higher and resembled that of a glycerol teichoic acid. It is concluded that Mg2+ forms bridges across phosphate groups in teichoic acid chains lying adjacent to each other in the wall. The effect of alanyl esters was similar to that in solution, but Scatchard plots were not linear at low concentrations of Mg2+ where it was shown that the difference in affinities between walls with and without alanyl ester residues was much greater than it was at higher concentrations of Mg2+. Thus at very low concentrations of Mg2+ effective binding to the wall is markedly improved by loss of alanyl ester residues.

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