[Pt(dien)](2+) migrates intramolecularly from methionine S to imidazole N-epsilon 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 methion ine- and histidine containing peptides H-His-Gly-Met-OH and Ac-His-Ala-Ala- AIa-Met-NHPh at 313 K has been investigated by HPLC and NMR spectroscopy. F or both peptides, initial relatively rapid formation of the kinetically fav oured methionine S-bound complex is followed by slow intramolecular migrati on of the [Pt(dien)](2+) fragment to imidazole N-epsilon 2 (or, in the case of H-His-Gly-Met-OH, to a much lesser extent to the competing imidazole N- delta 1) of the histidine side chain over a period of 500 h. Time-dependent studies for the pentapeptide at pH8.0 demonstrate that this isomerization can take place by either direct S-->N-epsilon 2 migration or by a two-step mechanism involving initial N-epsilon 2 coordination of a second [Pt(dien)] (2+) fragment and subsequent cleavage of the orginal Pt-S bond in the resul ting dinuclear complex. The rate of kappa S/kappa N-epsilon 2 isomerization is markedly reduced on lowering the pH to 5.1.