Pulse electron paramagnetic resonance (EPR) spectroscopy is employed at two very different excitation frequencies 9 77 and 30 67 GHz in the study of the nitrogen coordination environment of the Mn(III)Mn(IV) state of the dimanganese-containing catalases from Lactobacillus plantarum and Theimus thermophilus Consistent with previous studies the lower-frequency results reveal one unique histidine nitrogen-Mn cluster interaction For the first time, a second, more strongly hyperfine-coupled N-14 atom is unambiguously observed through the use of higher frequency/higher field EPR spectroscopy The low excitation frequency spectral features are rationalized as arising from the interaction of a histidine nitrogen that is bound to the Mn(IV) ton and the higher excitation frequency features are attributed to the histidine nitrogen bound to the Mn(III) ion These results allow for the computation of intrinsic hyperfine coupling constants which range from 2 2 to 2 9 MHz for sp(2)-hybriclized nitrogens coordinating equatorially to high valence Mn ions The relevance of these findings is discussed in the context of recent results from analogous higher fiequency EPR studies of the Mn cluster in photosystem II and other exchange-coupled transition metal-containing systems