Determination of configuration of arsenite-glutathione complexes using ECSTM

Inorganic arsenicals such as arsenite [As(III)] and arsenate [As(V)] are known human carcinogens. The interactions of As(III) with sulfhydryl groups of peptides and proteins are very important mechanisms for the toxicity and metabolism of arsenic in mammals. The present study was designed to explore the application of electrochemical scanning tunneling microscopy (ECSTM) for determining the configuration of complexes formed between As(III) and glutathione (GSH) in solution. The configurations of GSH and As(III)-GSH complexes were imaged on the Au(111) surface in a 0.1M NaClO(4) solution. High-resolution STM images revealed that the As(III) and GSH formed a As(GS)(3) complex. The orientation and packing arrangement of the molecular adlayers were also seen clearly from the images and molecular models constructed using the Chemical Window and Hyperchem software package. The configuration of GSH in As(GS)(3) was found to be different from single GSH. UV-vis spectra indicated the emergence of an absorption shoulder in the range 250-280 nm for the aged As(III)-GSH solution, compared to the spectra of single As(III) and GSH solutions. MS spectra showed the presence of a new peak for the aged As(III)-GSH solution at m/z 992 corresponding to the As(GS)(3) complex. The results obtained by the last two methods verify the compound imaged by using STM is As(GS)(3). Studying the interactions of As(III) and peptides and knowing the structure details of the complexes are a significant step toward a better understanding of the interactions between As(III) and proteins and the mechanism of arsenic toxicology. ECSTM will be especially valuable for the determination of competitive interactions of GSH and proteins with arsenic.