Electron paramagnetic resonance study of the complex formed between copper and 1‐[2(S)‐3‐mercapto‐2‐methylpropionyl]‐ L ‐proline (Captopril) in solid and aqueous solution

EPR spectroscopy was used to elucidate the nature of the interaction between copper and the antihypertensive agent 1‐[2(S)‐3‐mercapto‐2‐methylpropionyl]‐L‐proline (Captopril) in both solid and aqueous solutions. The EPR spectra indicate that binding to copper(II) occurs via the ligand oxygens rather than the thiol moieties, and that the EPR‐detectable copper is greatly reduced on complexation. The EPR parameters for the solid copper(II) complexes are in the range g∥ = 2.374−2.386, g⟂ = 2.076−2.086 and A = 13.5 mT, and the parameters for the aqueous solutions are giso = 2.107−2.129 and A = 7.1−8.6 mT.