Electron spin echo modulation spectra were recorded from a blue copper protein whose EPR spectrum resembles that of stellacyanin. As with stellacyanin, the cw-EPR and optical spectra of the cucumber protein protein differ at low and high pH, with the exception that the latter protein is stable at high pH. Electron spin echo studies were initiated in order to investigate the ligand structure and charge symmetry of the two forms of the protein. ESEEM were recorded at 7.8, 9.5, and 10.8 GHz. The electron spin echo spectra suggest that there are two imidazoles coordinated to copper, and the coupling to these two nitrogens (a measure of spin delocalization for the Cu d9 configuration) differs. The ESEEM spectra of high and low pH forms of the protein are qualitatively identical. There is no indication of new ligands at high pH. Furthermore, the quadrupole parameters of the remote imidazole nitrogen atoms are unchanged. The only apparent difference between the two proteins is the magnitude of the nuclear hyperfine coupling, which seems to be less in the high pH form of the protein.
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