Interaction of Arsenous Acid with the Dithiol-Type Chelator British Anti-Lewisite (BAL): Structure and Stability of Species Formed in an Unexpectedly Complex System

British anti-Lewisite (2,3-dimerkaptopropan-1-ol, dimercaprol, BAL) is one of the best-known chelator-type therapeutic agents against toxic metal ions and metalloids, especially arsenicals. Surprisingly, the mechanisms of action at the molecular level, as well as the coordination features of this traditional drug toward various arsenicals, are still poorly revealed. The present study on the interaction of arsenous acid (H₃AsO₃) with BAL, involving UV and NMR titrations, electrospray ionization mass spectrometry, and 2D NMR experiments combined with MP2 calculations, demonstrates that the reaction of H₃AsO₃ with BAL at pH = 7.0 results in a more complex speciation than was assumed before. The three reactive hydroxyl groups of H₃AsO₃ allow for interaction with three thiol moieties via condensation reaction, leading to the observed AsBAL₂ and As₂BAL₃ complexes besides the AsBAL species. This indicates the strong propensity of inorganic As(III) to saturate its coordination sphere with thiolate groups. The alcoholic hydroxyl group of the ligand may also directly bind to As(III) in AsBAL. Compared to dithiothreitol or dithioeritritol, the preference of BAL to form complexes with such a tridentate binding mode is much lower owing to the more strained bridged bicyclic structure with an α_AsSC < 90° bond angle and an unfavorable condensed boat-type six-membered ring. On the basis of the NMR data, the predominating, bidentately bound AsBAL species, including a five-membered chelate ring, exists in rapidly interconverting envelope forms of E and Z stereoisomers. The conditional stability constants calculated for the three macrospecies from a series of UV data [log β^pH=7.0 = 6.95 (AsBAL), 11.56 (AsBAL₂), and 22.73 (As₂BAL₃)] reflect that BAL is still the most efficient, known, dithiol-type chelator of H₃AsO₃.