Metal atoms in proteins and enzymes provide sites for numerous vital biological processes. We have interested in the interaction of oxygen with metalloproteins and metalloenzymes having iron and copper at their active sites. Oxygen transport by the iron protein hemerythrin in lower invertebrates and by the copper protein hemocyanin in mollusks and arthropods requires a binuclear metal site for reversible oxygen binding. The electronic structure of the binding site is determined by the nature and orientation of the directly coordinating amino acids as well as by the properties unique to a binuclear metal group. Oxygen-utilizing iron and copper enzymes such as ribonucleotide reductase and tyrosinase contain active sites similar to the oxygen-transport proteins. Our long-range objectives are the assignment of high-energy charge-transfer transitions and thus, a characterization of the active site electronic structure in copper and non-heme iron proteins and enzymes. Proteins are rich in intense Pi electron transitions in the UV region, which obscure the active site metal charge-transfer transitions. In order to identify these transitions, it is necessary to use inorganic analogues in which the immediate environments of the metals are the same as in the protein but without the spectral interference.
Our specific aims, therefore, are to obtain the polarized electronic spectra of suitable binuclear iron, copper and cobalt analogues under conditions of varying coordination geometries and temperatures in order to assign the inorganic charge-transfer spectra. The charge-transfer transitions of interest are very intense and, since inorganic crystals have high concentrations of chromophores, the spectra cannot be studied directly by usual absorption techniques. Our principal experimental technique will therefore be single-crystal polarized specular reflectance spectroscopy which, because specular reflectance is greatest in regions of highest absorption, is particularly well-suited to this problem. In addition, X-ray crystallography will be used both in our laboratory for determination of molecular alignment and in a collaborating laboratory for complete structure determination as needed. Complementary spectroscopic techniques including resonance Raman, MCD, EPR and ligand field polarized absorption will be applied by our collaborators to provide comprehensive views of the electronic structures.
