Glutathione-coordinated [2Fe-2S] cluster is stabilized by intramolecular salt bridges

Halide salts of alkali and alkaline earth metals were used to probe the contributions of intramolecular salt bridge formation on the stability of glutathione-coordinated [2Fe-2S] cluster toward hydrolysis. The effect of ionic strength on cluster stability was quantitatively investigated by application of Debye-Hückel theory to the rates of hydrolysis. Results from this study demonstrate that ionic strength influences the stability of the cluster, with the rate of cluster degradation depending on the charge density, hydrated ionic radius, and hydration energy. The identity of the salt ions was also observed to be correlated with the binding affinity toward the cluster. Based on the modified Debye-Hückel equation and counterion screening effect, these results suggest that interactions between glutathione molecules in the [2Fe-2S](GS)4 cluster is via salt bridges, in agreement with our previous results where modifications of glutathione carboxylates and amines prevented solution aggregation and cluster formation. These results not only provide a rationale for the stability of such clusters under physiological conditions, but also suggest that the formation of glutathione-complexed [2Fe-2S] cluster from a glutathione tetramer may be facilitated by salt bridge interactions between glutathione molecules prior to cluster assembly, in a manner consistent with Nature's equivalent of dynamic combinatorial chemistry.