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Resonance Raman spectra of a number of protocatechuate 3,4-dioxygenase-inhibitor complexes were studied by use of the available lines of an argon and a krypton laser. Three types of inhibitors were investigated-hydroxybenzoates, dicarboxylates, and 4-nitrocatechol. The hydroxybenzoate study shows that the hydroxy group in 3-hydroxybenzoate does not coordinate to the active site iron, in agreement with earlier suggestions, and confirms the coordination of the hydroxy group in the isomeric 4-hydroxybenzoate. The dicarboxylate study demonstrates that both glutarate and terephthalate perturb the active-site environment, shifting the charge-transfer interaction to lower energy. The pH dependence of terephthalate binding as well as the spectral similarities of the dicarboxylate complexes to the ESO2 intermediate provides further evidence for the suggestion that this intermediate is a tightly bound enzyme-product complex. The 4-nitrocatechol study indicates that, unlike the substrate catechols, 4-nitrocatechol does not bind to the iron; a binding configuration wherein the acidic phenolate group interacts with the carboxylate binding site has been suggested by others. Finally the spectra of the 4-hydroxybenzoate and terephthalate complexes demonstrate the presence of two tyrosines coordinated to the active-site iron as suggested by others; these tyrosines have different vCO's and excitation profiles.

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