Binding of the Oxo−Rhenium(V) Core to Methionine and to N-Terminal Histidine Dipeptides

• Published in: Inorganic chemistry Vol. 43; no. 23; pp. 7463 - 7473
• Main Authors: Tessier, Christian, Rochon, Fernande D, Beauchamp, André L
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 15.11.2004
• Subjects: Amino Acids - chemistry; Crystallography, X-Ray; Dipeptides - chemistry; Histidine - chemistry; Methionine - chemistry; Molecular Conformation; Molecular Structure; Nuclear Magnetic Resonance, Biomolecular; Organometallic Compounds - chemistry; Rhenium - chemistry; Stereoisomerism
• DOI: 10.1021/ic048776e
• ISSN: 0020-1669

The ReOX2(met) compounds (X = Cl, Br) adopt a distorted octahedral structure in which a carboxylato oxygen lies trans to the ReO bond, whereas the equatorial plane is occupied by two cis halides, an NH2, and an SCH3 group. Coordination of the SCH3 unit creates an asymmetric center, leading to two diastereoisomers. X-ray diffraction studies reveal that the crystals of ReOBr2(d,l-met)·1/2H2O and ReOBr2(d,l-met)·1/2CH3OH contain only the syn isomer (S−CH3 bond on the side of the ReO bond), whereas ReOCl2(d-met) and ReOCl2(d,l-met) consist of the pure anti isomer. 1H NMR spectroscopy shows that both isomers coexist in equilibrium in acetone (anti/syn ratio = 1:1 for X = Br, 3:1 for X = Cl). Exchange between these two isomers is fast above room temperature, but it slows down below 0 °C, and the sharp second-order spectra of both isomers at −20 °C were fully assigned. The coupling constants are consistent with the solid-state conformations being retained in solution. Complexes of the type [ReOX2(His-aa)]X (X = Cl, Br) are isolated with the dipeptides His-aa (aa = Gly, Ala, Leu, and Phe). X-ray diffraction work on [ReOBr2(His-Ala)]Br reveals the presence of distorted octahedral cations containing the ReO3+ core and a dipeptide coordinated through the histidine residue via the imidazole nitrogen, the terminal amino group, and the amide oxygen, the site trans to the ReO bond being occupied by the oxygen. The alanine residue is ended by a protonated carboxylic group that does not participate in the coordination. The constant pattern of the1H NMR signals for the protons in the histidine residue confirms that the various dipeptides adopt a similar binding mode, consistent with the solid-state structure being retained in CD3OD solution.