Quantitative Numerical Analysis of g Strain in the EPR of Distributed Systems and Its Importance for Multicenter Metalloproteins

A method for simulation of inhomogeneously broadened EPR of metallo-proteins based on recent theoretical advances is surveyed critically in terms of efficiency and accuracy. From the quality of the experimental spectrum, minimal boundary conditions are established for the spatial integration over the g-strained polycrystal. Computational efficiency is achieved by generating the S = 12 spectrum as an absorption in g space, reducing the number of molecular orientations computed by filtering mosaic artifacts from the Fourier-transformed spectrum, and generating the lineshape due to g strain from a tabulated distribution function. These techniques provide a reduction in computation time by some two orders of magnitude and make the data analysis of EPR of metalloproteins by minimization practical. The resulting simulation program is superior to current approaches in that it does not introduce artifactual multiplicities, and it is expected to require a smaller number of fitting parameters for the quantitative analysis of most cases. To illustrate its potential, the method is applied to EPR data from the iron-sulfur centers in NADH:Q oxidoreductase and in QH2:ferricytochrome c oxidoreductase, clarifying existing controversies on the stoichiometries of these centers.