Variable-temperature platinum-195 NMR spectroscopy, a new technique for the study of stereodynamics. Sulfur inversion in a platinum(II) complex with methionine

Abstract

This paper introduces /sup 195/Pt NMR spectroscopy as a method for the study of stereodynamics and apparently represents only the third report on variable-temperature /sup 195/Pt NMR spectroscopy. Displacement of a Cl/sup -/ ligand from PtCl/sub 4//sup 2 -/ by N-acetyl-L-methionine (AcMetH) yields (Pt(AcMetH)Cl/sub 3/)/sup -/ (1), which is a model for the binding of PtCl/sub 3//sup -/ label to proteins. The complex in solution is characterized by the method of preparation and by its uv-vis and /sup 1/H, /sup 13/C, and /sup 195/Pt NMR spectra. The AcMetH ligand is coordinated to the Pt(II) atom through the S atom in the side chain; coordination of the amide N atom, which would result in a six-membered ring, does not occur. It is concluded that the coordination of the amide nitrogen to platinum is facilitated if it yields a five-membered ring or a larger chelate containing such a ring. On account of its chiral S atom, complex 1 exists in two diastereomeric forms, which are undetectable in the /sup 1/H and /sup 13/C NMR spectra at convenient temperatures but are clearly evident in the /sup 195/Pt NMR spectrum. The /sup 195/Pt NMR spectra at nine temperatures spanning 86 deg show reversible, intramolecularmore » inversion of configuration at the S atom; ..delta..G double dagger = 63.7 kJ mol/sup -1/ at 335 K. Other mechanisms for the interconversion of the diastereomers are ruled out. Since the chiral carbon and sulfur atoms are three bonds apart, there is virtually no stereochemical discrimination and the two diastereomers of 1 exist in equal concentrations. This finding is discussed in terms of the known molecular structures of methionine and its derivatives. It is pointed out that /sup 195/Pt NMR spectroscopy is uniquely suited to the study of dynamic processes involving relatively complex biomolecules and processes causing subtle changes in molecular structure. 47 references, 1 figure, 1 table.« less