Luther E. Erickson

Stereochemistry of platinum complexes of the neutral amino acids allylglycine, S-methylcysteine, methionine, and corresponding sulfoxides

Abstract The stereochemistry—both configuration and conformation—of platinum complexes of the neutral amino acids allylglycine, methionine, S-methylcysteine and their sulfoxides, which all contain a potential metal binding site, X, has been investigated by 1-D and 2-D proton NMR spectroscopy, molecular mechanics calculations, and X-ray crystallography. The configuration of each diastereomer of square-planar complexes of general formula N, XPt(amino acidH)Cl2 has been assigned for all five of the amino acids. Relative equilibrium concentrations in solution of isomers of allylglycine (2S,4S/2S,4R=2R,4R/2R,4S=5), S-methylcysteine (2S,SR/2S,SS=2), and methionine (2S,SR/2S,SS=1.1) have been determined. The vicinal proton–proton coupling constants and NOESY spectra employed to assign configurations to the isomers also permitted an assessment of chelate ring conformations for the coordinated ligands. X-ray structures of kinetically stable sulfoxide complexes were employed to assign their configurations, and subsequent decomposition of isolated methionine and S-methylcysteine sulfoxide chelate isomers isomers was used to assign the very similar proton spectra of the two diastereomers of the corresponding free sulfoxides amino acids. With the significant exception of the methionine sulfoxide complexes, molecular mechanics calculations generally yield low energy conformations and isomer ratios in qualitative agreement with the NMR and X-ray structure data for these platinum complexes.

A PROTON NMR STUDY OF THE STEREOCHEMISTRY AND KINETICS OF THE REACTIONS OF DIMETHYL SULFOXIDE WITH POTASSIUM DICLOROGLYCINATOPLATINATE(II) AND ITS METHYL DERIVATIVES

Abstract Amino acid complexes of general formula K[Pt(NO)Cl2], where NO denotes the metal bonded atoms of the amino acid, react completely with solvent DMSO to yield two products, cis- and trans-Pt(NO) (DMSO)Cl, where cis and trans refer to positions of DMSO relative to coordinated nitrogen. The products were identified and kinetic data were obtained from changes in the proton nmr spectra of the amino acid, when DMSO-d6 was the solvent, or of both amino acid and coordinated DMSO, when ordinary DMSO was the solvent. For glycine and π-aminoisobutyric acid complexes, the rate of displacement of trans chloride exceeds that of cis chloride by a factor of 3. However, subsequent equilibration favors the cis isomer over the trans isomer by a factor of 10. By contrast, for the corresponding N, N-dimethyl derivatives, the rates of formation of the two isomers are more nearly the same and the equilibrium ratio does not differ from the kinetic ratio. In addition to providing a sensitive technique for evaluating small...

Hands-on Use of High-Field NMR without the NMR

The authors describe how and why they provide students with free induction decay data for an unknown and the students work on the unknown on a personal computer using spectral analysis software.