Multifrequency EPR investigation of dimanganese catalase and related Mn(III)Mn(IV) complexes

The superoxidized (MnMnIV)-Mn-III state of dimanganese catalase from Thermus thermophilus and a series of structurally similar (MnMnIV)-Mn-III complexes were investigated using continuous wave (CW) and pulsed EPR spectroscopy at X- (9 GHz), Q- (34 GHz), and W-band (94 GHz) frequencies. The bis(mu-oxo) or bis(mu-oxo)(mu-carboxylato) bridged complexes exhibit strong antiferromagnetic coupling (\J\ > 100 cm(-1)). Relevant EPR parameters (G- and Mn-55 hyperfine coupling tensors) are obtained by spectral simulations and yield a consistent data set for all frequency bands. Two mechanisms that lead to EPR line broadening are discussed. The advantage of our fitting strategy, i.e., precise determination of the G-tensor from high-field/frequency and hyperfine couplings from low-frequency EPR spectra, over conventional procedures is outlined. Comparison of the G- and Mn-55 hyperfine tensors of the model complexes with those of dimanganese catalase show that both values are sensitive probes for small structural changes.