STRUCTURE AND INTERACTIONS OF PHYCOCYANOBILIN CHROMOPHORES IN PHYCOCYANIN AND ALLOPHYCOCYANIN FROM AN ANALYSIS OF THEIR RESONANCE RAMAN-SPECTRA

Raman spectra of phycocyanobilin, phycocyanin, and allophycocyanin wer e obtained at resonance with their visible and near-UV transitions. Th ese spectra were empirically assigned with the help of N-14- and N-15- isotopic substitutions and comparisons with resonance Raman spectra of phycoerythrin. These results confirm the previously suggested assignm ent of a conformation-sensitive band around 1239-1246 cm(-1) to a mode involving nu CmH and nu CN coordinates. Computer-assisted decompositi on of the complex, conformation-sensitive 1580-1670-cm(-1) region yiel ded five components that we labeled I-V. The previously described spec tral changes observed upon monomerization and denaturation in resonanc e Raman spectra of phycocyanin and allophycocyanin essentially arise f rom changes in the relative intensities of these components. Component I (around 1649-1651 cm(-1)) and component III (1621-1624 cm(-1)) orig inate predominantly from nu C=C at C-15 of the chromophore. Their rela tive intensity ratio reflects the relative amounts of C-15-Z-anti and C-15-Z-syn methine bridge conformations, respectively. Component II (1 633-1638 cm(-1)) is ascribed to a nu C=C mode of pyrrole rings; it is not sensitive to the chromophore conformation. Component IV is also co nformation-insensitive and originates from nu C=N and nu C=C coordinat es, most likely from ring C. Component V (1591-1594 cm(-1)) involves a nu C=N coordinate in ring D, coupled to a nu C=C coordinate of the C- 15 methine bridge. The implications of the present assignments on thos e of resonance Raman active modes of phytochrome are discussed. A cons istent set of correlations between chromophore conformations and reson ance Raman data is obtained for both phycobiliproteins and phytochrome .