STRUCTURAL DIFFERENCES BETWEEN PHYCOCYANIN AND ALLOPHYCOCYANIN FROM THEIR RESONANCE RAMAN-SPECTRA

Resonance Raman spectra of the chromophores of the cyanobacterial ligh t-harvesting proteins phycocyanin (CPC) and allophycocyanin (APC) were recorded using 364 nm excitation. The 1500-1700 cm(-1) regions of the se spectra were analyzed for the pH-induced structural changes accompa nying the disruption of the native trimers into monomers as well as th e progressive denaturation of these monomers. Computer-assisted decomp osition of the 1642 cm(-1) marker bands of these spectra yielded up to four components (named I-IV), the frequencies of which were constant within 5 cm(-1) (CPC) and 10 cm(-1) (APC). The relative intensities of two of these components, namely I and III, were sensitive to chromoph ore conformations. The previously reported downshift of the 1642 cm(-1 ) band upon folding of the chromophores was shown to result from a wea kening of component I and a concomitant enhancement of component III. Components I-IV had different relative intensity patterns in CPC and A PC spectra. In particular, the higher relative intensity of component I at 1646 cm(-1) indicated more extended average conformations of the chromophores in trimeric APC than in trimeric CPC. This difference lik ely resulted from the extra beta-155 chromophore present in CPC. Compo nent III was sizably active in RR spectra of monomeric APC but was not observed either in those of monomeric CPC or in those of trimeric APC and CPC. This indicated that, in APC monomers, chromophore(s) did not assume the native conformations found in the trimer, while monomer fo rmation did not sizably alter the structures of the CPC chromophores.