Surface-enhanced resonance Raman spectroscopy of phycocyanin and allophycocyanin

High quality surface-enhanced resonance Raman (SERR) spectra were recorded from native and denatured phycocyanin and allophycocyanin on ascorbic acid treated silver hydrosols. The visible-excited SERR and resonance Raman (RR) spectra of the phycobiliproteins were very similar, indicating a predominantly electromagnetic surface enhancement mechanism. Investigation of pH-induced denaturation ofx allophycocyanin has shown that even small differences in protein/chromophore conformational are sensitively reflected by the SERR spectra. Concerning the adsorption of the protein to the metal surface, the experiments have shown that: (i) there is limited possibility for changing protein conformation during the adsorption process, (ii) there are no changes after the protein has been adsorbed onto the silver surface and (iii) for each protein an optimal activation of the silver sol has to be found for recording proper SERR spectra. The results obtained on phycobiliproteins are also discussed in connection with the interpretation of phytochrome Raman spectra.