Organoarsenicals are being designed for the purification and visualization of proteins that contain vicinal cysteine residues. In collaboration with Professor Thorpe at the University of Delaware, fluorescent probes have been synthesized that selectively bind to dithiol motifs. Importantly, these probes exhibit minimal fluorescence free in solution, but upon binding to protein, display enhanced fluorescence.

In the course of this work, it became clear that very little is actually known concerning the effects of arsenic binding to protein structure. This is surprising since arsenic(III) species exert a wide array of effects in biological systems, in part because of selective interactions with vicinal thiols in proteins and peptides. For example, inorganic As(III) derivatives can be both acute or chronic toxicants as well as inducers of apoptosis. Although certain arsenic species are classified as human carcinogens, some have a venerable history as chemotherapeutics: for example in Ehrlich's treatment of trypanosomiasis (e.g. African sleeping sickness) and arsenic trioxide (As₂O₃) has been used in the treatment of acute promyelocytic leukemia.

In light of the importance of arsenic in biology, we are studying the effects of As(III) binding on protein structure. Our approach entails studying the structural consequences of As(III) binding to individual secondary structural elements that comprise globular proteins (helix, sheet, and turn). We began by studying the effects of As(III) binding on helical structure (J. Am. Chem. Soc. 2003, in press). This study utilizes model dicysteine-containing, a-helical peptides to study the structural effects of arsenic binding. With i, and i+1, i+2 or i+3 arrangements, CD spectroscopy shows that As(III) coordination causes helical destabilization when Cys residues are located at central or C-terminal regions of the helix. In contrast, helical stabilization is observed for peptides containing i, i+4 Cys residues (Figures 1 and 2), with corresponding pseudo pairwise interaction energies (DG_pw^o) of -1.0 and -0.7 kcal/mole for C-terminal and central placements, respectively. Binding affinities and association rate constants show that As(III) binding is comparatively insensitive to the location of the Cys residues within these moderately stable helices. Interestingly, As(III) binding of i, i+3 Cys-containing helical peptides results in the total elimination of helical structure and the formation of an alternate fold that is quite stable to heat denaturation. This is an intriguing result when one contemplates the preponderance of proteins that contain Cys residues in this arrangement (many redox active proteins) and their possible role in arsenic induced apoptosis and toxicity. Studies are in progress to study the effects of As(III) binding on both sheet and turn structure. Collectively, these studies will afford a complete assessment of the structural ramifications induced by As(III) binding. In addition, this knowledge should enable our group to prepare designer oranoarsenic-based probes that are protein selective in that dithiol motifs displayed in different types of secondary structure may be selectively targeted.