Interconversion between S- and N-bound L-methionine adducts of pt(dien)2+ (dien = diethylenetriamine) via dien ring-opened intermediates

The reaction between [Pt(N-15-dien)Cl](+) 1 and L-methionine (L-MetH) has been studied using [H-1,N-15] 2D NMR spectroscopy and HPLC, together with N-15-labelled amine ligands. The complex [Pt(dien)(L-MetH-S)](2+) 2 was dominant at neutral pH and converted partially and reversibly into [Pt(dien)(L-Met-N)](+) 3 at pH > 8. When the pH was lowered from 8 to 3, the dien ring-opened intermediate [Pt(dienH-N,N')(L-Met-S,N)](2+) 4 was formed which converted slowly into complex 2. Complex 4 was separated by HPLC and characterized by NMR spectroscopy as a mixture of four diastereomers (due to chiral centres at S and NH) present in a 2:2:1:1 molar ratio. This isolated intermediate had a surprisingly long lifetime (days) in 0.55 M NH4H2PO4 (pH 4.0).