Determination of the electron self-exchange rates of blue copper proteins by super-WEFT NMR spectroscopy.

An NMR approach for determining the electron self-exchange (ESE) rate constants in blue copper proteins is presented. The approach uses the paramagnetic relaxation enhancement of resonances in 1D 1H super-WEFT spectra of partly oxidized (paramagnetic) proteins. These spectra allow a more precise determination of the relevant paramagnetic linebroadenings than conventional 1D 1H spectra and, thus, permit a more detailed investigation of the applicability of the linebroadenings for determining the electron exchange rates. The approach was used to estimate the ESE rate constant of plastocyanin from Anabaena variabilis. It was found that, although the rate constant can be determined accurately from a series of resonances, precise but erroneous constants are obtained from the resonances of the copper-bound residues, unless a narrow splitting of these resonances caused by the presence of two conformations is taken into account. As demonstrated here, this complication can be overcome by a correct analysis of the paramagnetic broadening of the combined double signals. Because of the high resolution and specific sensitivity of the approach it should be generally applicable to estimate electron transfer rates, k, if the paramagnetic relaxation enhancement R2p of the resonances can be determined, and the conditions k << R2p or delta omega(p) >> k >> R2p are fulfilled, delta omega(p) being the frequency separation between corresponding diamagnetic and paramagnetic sites.