Binding of the oxo-rhenium(V) core to methionine and to N-terminal histidine dipeptides.
The ReOX(2)(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 NH(2), and an SCH(3) group. Coordination of the SCH(3) unit creates an asymmetric center, leading to two diastereoisomers. X-ray diffraction studies reveal that the crystals of ReOBr(2)(d,l-met).1/2H(2)O and ReOBr(2)(d,l-met).1/2CH(3)OH contain only the syn isomer (S-CH(3) bond on the side of the Re=O bond), whereas ReOCl(2)(d-met) and ReOCl(2)(d,l-met) consist of the pure anti isomer. (1)H 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 degrees C, and the sharp second-order spectra of both isomers at -20 degrees C were fully assigned. The coupling constants are consistent with the solid-state conformations being retained in solution. Complexes of the type [ReOX(2)(His-aa)]X (X = Cl, Br) are isolated with the dipeptides His-aa (aa = Gly, Ala, Leu, and Phe). X-ray diffraction work on [ReOBr(2)(His-Ala)]Br reveals the presence of distorted octahedral cations containing the Re=O(3+) 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 the(1)H 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 CD(3)OD solution.
Tessier
Christian
C
Département de Chimie, Université de Montréal, C. P. 6128, Succ. Centre-ville, Montréal, Québec, Canada H3C 3J7.
Rochon
Fernande D
FD
Beauchamp
André L
AL
D000596:Amino Acids / Q000737:chemistry; D018360:Crystallography, X-Ray; D004151:Dipeptides / Q000737:chemistry*; D006639:Histidine / Q000737:chemistry*; D008715:Methionine / Q000737:chemistry*; D008968:Molecular Conformation; D015394:Molecular Structure; D019906:Nuclear Magnetic Resonance, Biomolecular; D009942:Organometallic Compounds / Q000737:chemistry*; D012211:Rhenium / Q000737:chemistry*; D013237:Stereoisomerism
D000596:Amino Acids; D004151:Dipeptides; D009942:Organometallic Compounds; D006639:Histidine; D012211:Rhenium; D008715:Methionine