Fig. 4.
Scheme showing the two possible orientations of As(III) within a three-stranded coiled coil. (A) In the exo configuration, the As(III) ion would be located above both Sγ atoms and β-methylene protons of the cysteine side chains, and the lone pair would point toward the N terminus. (B) In the endo configuration, which is observed in the structure of As(CSL9C)3, the As(III) ion would be located below the Sγ atoms and approximately level with the β-methylene protons of the cysteine side chains. The lone pair would then be directed toward the C terminus.