CORE EXPANSION AND ELECTRONIC-STRUCTURE OF THE PORPHYRIN IN THE NEUTRAL PH FORM OF COPPER CYTOCHROME-C
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The pH-dependent resonance Raman and absorption spectra of copper cytochrome c are examined. Large shifts in the Raman core-size marker lines are observed in the neutral pH form of copper cytochrome c relative to the protein at pH extremes (pH 2 and 13). The 5-coordinate copper-porphyrin complexes exhibit similar, but somewhat smaller, shifts in the core-size marker lines. The Raman shifts represent a significant expansion (0.03 A) of the porphinato core in the neutral form. Further, analysis of the -* absorption spectral changes using the four-orbital model indicates a substantial destabilization of the top-filled a2u() orbital if the neutral pH form of the protein has 5-coordinate copper. The result is a red shift of all * bands, a decrease in the separation of the -band and Soret band, and a reversal in the - and -band absorbance ratio from ~1.4 to ~0.8. Iterative extended Hiickel molecular orbital (MO) calculations for model 4-, 5-, and 6-coordinate copper porphyrins show considerable destabilization of the a2u orbital caused by addition of a -donating axial ligand. Thus, the calculations support previous interpretations of spectral changes based on addition of an axial fifth ligand in the copper-modified heme proteins and copper porphyrins in coordinating solvents. The MO calculations are also consistent with studies associating a2u orbital energy with the frequency of Raman marker lines. They further predict an observed new, strong UV band near 350 nm in the neutral form of copper cytochrome c and 5-coordinate copper-porphyrin complexes to be a [formula-omitted] charge-transfer band. No evidence for 6-coordination of copper is found. Finally, changes in Raman and absorption spectra at the pH extremes are consistent with dimerization of the porphyrins in the copper cytochrome c dimers. 1984, American Chemical Society. All rights reserved.