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Determination of the Effective Charge Density of pH-Responsive Keplerate Polyoxometalate Clusters by Means of Agarose Gel Electrophoresis


  • Dedicated to Michael T. Pope on the occasion of his 80th birthday


The effective (net) charge densities of the Keplerate {Mo72Fe30}, {Mo72V30}, and {W72Mo30} macroanions in dilute aqueous solution were determined from electrophoresis measurements, which provided direct, quantitative evidence of the counterion association behavior around macroions. The latter two clusters are strong electrolytes, and their charge densities show no clear pH dependence. The surface charge density of {Mo72Fe30} increased with increasing solution pH owing to the partial deprotonation of the weak acid {Mo72Fe30} macroions. A close connection between the surface charge density of {Mo72Fe30} and their assembled blackberry structure sizes was observed.