Complexation of captopril with gold(I) and its exchange reactions with thiomalate and cyanide

Captopril {Hcap, 1-[(2S)-3-mercapto-2-methylpropionyl]-L-proline} with gold(I) forms a 1:1 crystalline complex [Au(cap)]. The exchange reactions of thiomalate and cyanide with [Au(cap)] have been studied using ¹³C NMR spectroscopy. It is found that [Au(cap)] forms a very high-molecular-weight polymer compared to gold(I) thiomalate. Thiomalate and CN^- both bind to [Au(cap)], however, as expected, CN^- binds more strongly. When ¹³CN^- was added to [Au(cap)] solution, ¹³C NMR resonances due to [Au(cap) (¹³CN)]^- (two isomers containing cis or trans cap) and [Au(¹³CN)₂]^- were observed. Evidence is presented which shows that the cis isomer of [Au(cap) (¹³CN)]^- is more stable than the trans.