| 17854178 |
Desrochers PJ, Duong DS, Marshall AS, Lelievre SA, Hong B, Brown JR, Tarkka RM, Manion JM, Holman G, Merkert JW, Vicic DA: Nickel-cysteine binding supported by phosphine chelates. . Inorg Chem. 2007 Oct 29;46(22):9221-33. Epub 2007 Sep 14.
The effect of chelating phosphines was tested on the structure and pH-dependent stability of nickel-cysteine binding. (1,2-Bis (diphenylphosphino) ethane (dppe) and 1,1,1-tris [(diphenylphosphino) methyl] ethane (triphos) were used with three different cysteine derivatives (L-cysteine, Cys; L-cysteine ethyl ester, CysEt; cystamine, CysAm) to prepare complexes of the form (dppe) NiCysR (n+) and (triphos) NiCysR (n+) (n = 0 for Cys; n = 1 for CysEt and CysAm). Similar 31P {1H} NMR spectra for all (dppe) NiCysRn+ confirmed their square-planar P2NiSN coordination spheres. The structure of [(dppe) NiCysAm] PF6 was also confirmed by single-crystal X-ray diffraction methods. The (triphos) NiCysAm+ and (triphos) NiCysEt+ complexes were fluxional at room temperature by 31P NMR. Upon cooling to -80 degrees C, all gave spectra consistent with a P2NiSN coordination sphere with the third phosphorus uncoordinated. Temperature-dependent 31P NMR spectra showed that a trans P-Ni-S pi interaction controlled the scrambling of the coordinated triphos. In aqueous media, (dppe) NiCys was protonated at pH approximately 4-5, leading to possible formation of a nickel-cysteinethiol and eventual cysteine loss at pH < 3. The importance of N-terminus cysteine in such complexes was demonstrated by preparing (dppe) NiCys-bead and trigonal-bipyramidal Tp*NiCys-bead complexes, where Cys-bead represents cysteine anchored to polystyrene synthesis beads and Tp*- = hydrotris (3,5-dimethylpyrazolyl) borate. Importantly, results with these heterogeneous systems demonstrated the selectivity of these nickel centers for cysteine over methionine and serine and most specifically for N-terminus cysteine. The role of Ni-S pi bonding in nickel-cysteine geometries will be discussed, including how these results suggest a mechanism for the movement of electron density from nickel onto the backbone of coordinated cysteine. |
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