Resolution enhancement of EPR spectra using the Fourier transform technique. Analysis of nitrosyl cytochrome c oxidase in frozen solution

Abstract

A computer program is described which applies the Fourier transform technique to electron paramagnetic resonance spectroscopy, in order to improve the signalto-noise ratio and to differentiate or integrate the EPR spectra. Resolution enhancement is performed by transforming the lineshape from Lorentzian to Gaussian and by computing higher derivatives using the experimental first derivative spectrum. This procedure avoids overmodulation of the spectrum when higher derivatives are measured. Furthermore, a simulation procedure for spectra of randomly oriented anisotropic systems is combined with these Fourier transform facilities. As an example, the complete determination of the EPR parameters of nitrosyl cytochrome c oxidase complexes was accomplished. The hexacoordinated and the pentacoordinated complexes (without axially bound histidine) show hyperfine constants in the x and y directions (heme plane) which are equivalent for nitrosyl and pyrrole nitrogen atoms, respectively. Both complexes show a larger hyperfine splitting of 15NO in the z direction than in the x and y directions. The interaction of the electron spin with the pyrrole nitrogens influences the resonances in the x and y directions significantly whereas the contribution to the resonances in the z direction seems to be very small. The hexacoordinated complex exhibits splitting constants of N ϵ of the histidine in all principal directions; the largest splitting is found in the z direction.