EPR/ESEEM SPECTROMETERY OF SINGLE CRYSTAL MODELS FOR PROTEIN ACTIVE SITES

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Electron paramagnetic resonance (EPR) and electron spin eco envelope modulation (ESEEM) spectroscopic studies will be performed on copper doped crystal systems of simple amino acids that serve as models for active sites in copper metalloproteins. The tensor parameters derived from these studies provide structural and bonding insight into the nature of the metal coordination in these systems. These investigations have a great deal of importance because of their ability to provide unambiguous and accurate spectral parameters in model systems of well defined structure and can be utilized to analyze and interpret results of many similar studies of metalloproteins in frozen solution samples. Experimental advances in EPR methods have allowed the measurement of increasingly weaker interactions between an unpaired electron, mostly localized on a transition metal, and neighboring but distant magnetic nuclei. Thus, the impaired electron wave- function at the proteins active sites are becoming defined to an increasingly finer detail. For example, such studies have detected both backbone peptide nitrogen couplings and glutamine side chain nitrogen couplings in blue copper proteins, anc couplings from nitrogens hydrogen bonded to iron ligands in iron-sulfur proteins. However, solid interpretation of the measured interactions are still lacking due to the absence of a good model systems where the coupling between the paramagnetic ion and distant non- imidazole nitrogens have been unambiguously and accurately quantified. The proposed research aims to accomplish this by performing CW-EPR and ESEEM measurements on single crystals on Cu(II)-doped into creatine, glycine, glycylglycine and Zn(I methionine)2. A second focus will be on the development of a fitting method whereby a limited but select set of single crystal orientational ESEEM data can be used to accurately determine 14N coupling tensor parameters. The long term are to provide a basis both in terms of experimental methodology and analysis for similar EPR and ESEEM studies on single crystals on the actual metalloproteins.