RESONANCE RAMAN SPECTRA OF THE Pr-FORM OF PHYTOCHROME

Abstract— Resonance Raman spectra of the P_r-form of oat phytochrome have been obtained at 77 K. Interference from phytochrome fluorescence is avoided by employing far-red 752 nm excitation. Vibrational assignments are suggested for the tetrapyrrole chromophore in phytochrome by comparison with previously published model compound spectra and by examining the characteristic shifts induced by deuteration of the pyrrole nitrogens. The lack of carbonyl intensity, the frequencies of the 1626 and 1644 cm^-1 C=C stretching modes, and the presence of an intense mode at 1326 cm^-1 are all consistent with a protonated structure for the tetrapyrrole chromophore in P_r. This suggests that the -50 nm red-shift of the protein-bound chromophore absorption compared to the chromophore in vitro is caused by protonation of the pyrrole nitrogen.