Protonation state and structural changes of the tetrapyrrole chromophore during the Pr → Pfr phototransformation of phytochrome:: A resonance Raman spectroscopic study

The photoconversion of phytochrome (phytochrome A from Avena satina) from the inactive (P-r) to the physiologically active form (P-fr) was studied by near-infrared Fourier transform resonance Raman spectroscopy at cryogenic temperatures, which allow us to trap the intermediate states. Nondeuterated and deuterated buffer solutions were used to determine the effect of H/D exchange on the resonance Raman spectra. For the first time, reliable spectra of the "bleached" intermediates meta-R-A and meta-R-C were obtained. The vibrational bands in the region 1300-1700 cm(-1), which is particularly indicative of structural changes in tetrapyrroles, were assigned on the basis of recent calculations of the Raman spectra of the chromophore in C-phycocyanin and model compounds [Kneip, C., Hildebrandt, P., Nemeth, K., Mark, F., Schaffner, K. (1999) Chem. Phys. Lett. 311, 479-485]. The experimental resonance Raman spectra P, are compatible with the Raman spectra calculated for the protonated ZZZasa configuration, which hence is suggested to be the chromophore structure in this parent state of phytochrome. Furthermore, marker bands could be identified that are of high diagnostic value for monitoring structural changes in individual parts of the chromophore. Specifically, it could be shown that not only in the parent states P, and P-fr but also in all intermediates the chromophore is protonated at the pyrroleninic nitrogen. The spectral changes observed for lumi-R confirm the view that the photoreaction of P-r is a Z --> E isomerization of the CD methine bridge. The subsequent thermal decay reaction to meta-RA includes relaxations of the CD methine bridge double bond, whereas the formation of meta-R-C is accompanied by structural adaptations of the pyrrole rings B and C in the protein pocket. The far-reaching similarities between the chromophores of meta-R-A, and P-fr suggest that in the step meta-R-A --> P-fr the ultimate structural changes of the protein matrix occur.