Question

In zinc enzymes, 'spectroscopically silent' Zn(II) can often be replaced by Co(ll) with high retention of activity. Explain the principles by which this substitution can be exploited to obtain structural and mechanistic information. inorganic chem

Answer 1

In zinc Enzymes, you have zinc 2 plus and this is d-10 system, no d-d transitions and no magnetic properties;So, therefore, it is a silent system.Zinc has a specific role in bioinorganic processes because of the peculiar properties of the coordination compounds of the zinc(II) ion. Zinc(II) can easily be four-, five-, or six-coordinate, without a marked preference for six coordination. The electronic configuration of zinc(II) is 3d 10 with two electrons per orbital. In coordination compounds, there is no ligandfield stabilization energy.Direct spectroscopic investigation of zinc enzymes is difficult, because zinc(II) is colorless and diamagnetic; so it cannot be studied by means of electronic or epr spectra.The zinc ion can be replaced by other ions, and sometimes the enzymatic activity is retained fully or partially. So metallosubstitution is a convenient tool for monitoring the Zn enzyme and its function by means of spectroscopic techniques

Cobalt(II)-substituted zinc proteins often show about as much activity as the native zinc enzymes. This is a general characteristic of the cobaltsubstituted zinc enzymes, since the coordination chemistry of cobalt(II) is very similar to that of zinc(II). The two ions also show virtually identical ionic radii. Cobalt(II) derivatives generally display useful electronic spectra. High-spin cobalt(II) ions are paramagnetic, containing three unpaired electrons ; thus they can also give rise to EPR spectra. On the other hand, as the paramagnetic broadening of the NMR lines in such systems is inversely proportional to the electronic relaxation times, room-temperature IH NMR spectra of cobalt(II) complexes can be easily detected, even in the absence of chemical exchange. Therefore cobalt(II) is an exceptional probe to monitor the structure and reactivity of zinc enzymes. Of course, the transfer of information from the artificial to the native enzyme must be done with caution. However, if we can understand the functioning of the cobalt enzyme, we then have a reference frame by which to understand the kinetic properties of the native Zn enzyme.