SIMULATION OF MULTIFREQUENCY EPR-SPECTRA FROM MN(III) MN(IV) CATALASEOF LACTOBACILLUS-PLANTARUM USING A NEW APPROACH BASED ON PERTURBATION-THEORY/

The 16-line EPR signal from the Mn(III)/Mn(IV) state of Lactobacillus plantarum Mn catalase was studied at three microwave frequencies: S-ba nd (3.0 GHz), X-band (9.2 GHz), and P-band (15.5 GHz). The spectra wer e simulated using a program that calculated the hyperfine splitting as a perturbation of the Zeeman term to third order. The transition ener gies were calculated numerically from the perturbation terms rather th an from an explicit expression derived from perturbation theory, as ha s been done previously for spectra from multinuclear Mn complexes. Nin e Mn catalase spectra were fit using a nonlinear least-squares minimiz ation routine assuming an axial S = 1/2 system. Axial g and hyperfine matrices were found to fit the spectra well. Second-order perturbation theory was sufficient to fit the X- and P-band spectra, but third-ord er perturbation terms were necessary to adequately fit the S-band spec trum and give parameters that agreed with those found at the higher fr equencies. The EPR parameters found for this biological Mn dimer (g(z) = 1.990, g(x) = g(y) = 2.008, A1z = 104.1 x 10(-4) cm-1, A1x = A1y = 141.6 x 10(-4) cm-1, A2z = 83.5 x 10(-4) cm-1, A2x = A2y = 76.0 x 10(- 4) cm-1) compare well with those for synthetic Mn-(III)/Mn(IV) complex es and with estimates by vector projection using literature values of the independent Mn ions. The success of the simulation method employed here, which can accommodate rhombic systems and carry the perturbatio n calculation to third or higher orders, will have utility for simulat ion of the S2 multiline signal at g = 2 from the O2-evolving complex o f higher plants and algae.