Systems containing equal metal ions, J kT

It is convenient to divide this section into systems containing equal or different metal ions. Each of the two cases will be treated according to whether J is larger or smaller than kT. 

Proton hyperfine shifts (ppm) and T1M values (s 1, estimated errors in parentheses) for MM (PMK)3+ complexes [13] 

When the dimetallic center is embedded in a protein, the correlation time is given by the electronic relaxation time. A studied case is provided by the 

With the PMK ligand a CoCu derivative has been obtained [7] (Fig. 6.7). From the temperature dependence of the shifts (and magnetic susceptibility measurements in solution), the value of J appears to be positive and much smaller than kT [13]. As expected, the hyperfine shifts are the sum of those of the CuZn and ZnCo systems for each proton (Table 6.2). The NMR lines of the copper domain are now quite sharp, even sharper than those of the cobalt domain (Fig. 6.7). Qualitatively, the data can be accounted for if xs of copper is sizably reduced and approaches that of cobalt (and thus the Redfield limit is reached). 

The above system is similar to that built into the protein superoxide dismutase (Fig. 6.2). The zinc can be replaced by several metal ions. Cobalt(II) and nickel(II) are pertinent here. The cobalt-copper system has J 17 cm-1 with S = 1 ground state [25]. The electronic relaxation times for the two ions are expected to be either equal or similar. The H NMR spectrum is shown in Fig. 6.8A. The assignment has been performed through several steps. 

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