Determination of the electron relaxation rates in paramagnetic metal complexes: applicability of available NMR methods

Abstract

Four different approaches for determining the electron relaxation rates in paramagnetic metallo-proteins are investigated, using a paramagnetic Ni 2+ complex of a protein as an example. All four approaches rely on the determination of the longitudinal paramagnetic relaxation enhancements, R 1p, of the 1H nuclei and the backbone 15N nuclei. Three of the methods utilize the field dependence of the R 1p rates. It is found that the applicability of each of these methods depends on whether the fast-motion condition, ω S 2 τ 2≪1, applies to the electron relaxation, ω S being the Larmor frequency of the electron spin S and τ the correlation time of the electron relaxation. If the fast-motion condition is fulfilled, the electron relaxation rate can be obtained from the ratio of the R 1p rates of one or more protons at two magnetic field strengths (method A). On the other hand, if the fast-motion condition does not apply, more elaborate methods must be used that, in general, require a determination of the R 1p rates over a larger range of magnetic field strengths (method C). However, in the case of paramagnetic metal ions with relatively slow electron relaxation rates only two magnetic field strengths suffice, if the R 1p rates of a hetero nucleus are included in the analysis (method B). In the fourth method (method D), the electron relaxation is estimated as a parameter in a structure calculation, using distance constraints derived from proton R 1p rates at only one magnetic field strength. In general, only methods B and C give unambiguous electron relaxation rates.