NMR Studies of Bis(O-ethyl-L-cysteinato)nickel(II)

Nickel can replace beneficial zinc atoms in sulfur-rich proteins such as zinc fingers.  This metal exchange may lead to cancer, oxidative damage, and other health issues.  The NiN₂S₂ center as well as the derivatized cysteine amino acid make the title compound, Ni(cysE)₂, a suitable model for nickel transmetallated zinc fingers.  It is important to fully characterize model compounds before they can be used in further studies.  As part of our structural characterization of our model compound in the solution state, we obtained ¹H-NMR spectra in various solvents.  ¹H-NMR spectra in d-methanol and d-DMSO show significant broadening.  This line broadening may be caused by paramagnetism, poor instrument resolution, or dynamic processes such as proton exchange or conformational changes.  Magnetic susceptibility studies confirm that broadening is not due to paramagnetism since the complex is diamagnetic in both solution and solid state.  Two dimensional TOCSY ¹H-NMR studies revealed the connectivity of the protons to adjacent protons and thus allowed us to positively assign the proton signals to the structure.  A series of 1D ¹H-NMR spectrum were obtained at various temperatures in d-DMSO to determine any structural changes observable on the NMR timescale.  The lack of resolved peaks with decreasing temperatures indicates there is no dynamic process.  Molecular modeling showed that the proton on the chiral carbon has a complex splitting pattern of overlying peaks, which results in a single broad peak.  This conclusion is consistent with our findings that increasing the resolution of the NMR from 200 to 400 MHz minimized line broadening.