Proteins as solvents: Blue copper proteins as a molecular rules for solvent effects on resonance Raman intensities

Resonance Raman spectra, excited within the S-Cu charge-transfer transition, are presented for poplar a, spinach, zucchini, and cucumber plastocyanin, type I "blue" copper proteins (cupredoxins) found in the photosynthetic electron transport apparatus of plants. The degree of similarity of the resulting spectra increases as the amino acid homology of the proteins increases. For example, the resonance Raman spectrum of spinach plastocyanin exhibits frequencies and relative intensities significantly different from those of poplar a plastocyanin, with which it is 78% homologous, while the resonance Raman spectra of zucchini and cucumber plastocyanin, which are 91% homologous, are identical. The spectra exhibit good signal/noise ratios, are reproducible from different protein preparations, and are unaffected by solvent ionic strength, indicating that the observed spectral differences reflect actual differences in ground-state or excited-state (or both) structures as a result of protein composition. The relative intensities appear to be more sensitive to amino acid composition than the frequencies are, although frequency shifts are observed in the spectra that also exhibit the largest changes in relative intensity. These spectral differences are correlated with molecular structure by performing molecular modeling of the structure and electrostatic environment of the copper site in the four plastocyanins based on the experimentally determined crystal structure of poplar a plastocyanin. These correlations indicate that the resonance Raman intensities are sensitive to structural or electrostatic differences (or both) in the protein environment as far as 12 Angstrom from the copper site. This long-range sensitivity dies off with amino acid composition changes at distances >15 Angstrom. Possible mechanisms for this long-range sensitivity are discussed.