Acceleration of the mercury-induced aquation of bromopentammine Co(III) by naturally occurring glycosaminoglycans

Abstract

The glycosaminoglycans are naturally occurring anionic polysaccharides whose polyelectrolyte properties and counterion interactions may underlie a number of biological activities. We have investigated the ability of the sulfated glycosaminoglycans to accelerate the reaction of Br(NH3)5Co2+ with Hg2+. Chondroitin sulfates A, B, and C, heparin, and heparan sulfates all accelerate this reaction to nearly the same extent, but do so at different optimal concentrations, above which inhibition is observed. This phenomenon of rate acceleration, sometimes called "polyelectrolyte catalysis", has previously been explained by a two-phase model, in which the reactant counterions are thought to equilibrate rapidly between the polymer domain and the bulk phase. Rate effects would then be due to higher local concentrations in the neighbourhood of the polymer. Our results show that this is an oversimplified view for the glycosaminoglycans. Rather, they demonstrate similar rate accelerations independent of net structural charge density. It is argued that counterion condensation, as described by Manning, reduces the surface potential of all glycosaminoglycans to nearly constant values. Different optimal concentrations of the glycosaminoglycans may arise in part from the unreactivity of a subpopulation of bound ions.