[Pt(dien)]2+ migrates intramolecularly from methionine S to imidazole Nz 2 in the peptides H-His-Gly-Met-OH and Ac-His-Ala-Ala-Ala-Met-NHPh

The pH- and time-dependent reaction of Pt(dien)(H₂O)]²⁺ with the methionine- and histidine-containing peptides H-His-Gly-Met-OH and Ac-His-Ala-Ala-Ala-Met-NHPh at 313 K has been investigated by HPLC and NMR spectroscopy. For both peptides, initial relatively rapid formation of the kinetically favoured methionine S-bound complex is followed by slow intramolecular migration of the Pt(dien)]²⁺ fragment to imidazole N_{ε 2} (or, in the case of H-His-Gly-Met-OH, to a much lesser extent to the competing imidazole N_{δ 1}) of the histidine side chain over a period of 500 h. Time-dependent studies for the pentapeptide at pH 8.0 demonstrate that this isomerization can take place by either direct S→N_{ε 2} migration or by a two-step mechanism involving initial N_{ε 2} coordination of a second Pt(dien)]²⁺ fragment and subsequent cleavage of the orginal Pt-S bond in the resulting dinuclear complex. The rate of κS/κN _{ε 2} isomerization is markedly reduced on lowering the pH to 5.1. Received: 26 February 1999 / Accepted: 14 April 1999