The emission spectra of the porphyrin in metal-free and Zn cytochrome c and in metal-free mesoporphyrin derivatives of horseradish peroxidases A and C, leghemoglobin, and myoglobin were examined as a function of temperature and excitation wavelength. At room temperature, the emission spectra were unresolved and were independent of excitation wavelength. At low temperature (4.2 K), the spectra depended upon excitation wavelength: using narrow-band excitation into the high-energy side of the 0-1 and 0-0 bands gave unresolved emission spectra whereas excitation into the low-energy side produced quasi-line spectra. The resolved spectra were different for the five proteins and further varied with pH, indicating chromophore-protein interactions. The spectra are interpreted in terms of site selection and phonon interactions.
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