Chromophore interactions in allophycocyanin.

Allophycocyanin, which is normally isolated as a trimer (alpha 3 beta 3), has now been successfully dissociated into a monomer (alpha beta) with strikingly different spectroscopic properties. In particular, upon dissociation the characteristic 650-nm absorption and 661-nm fluorescence emission bands of the trimer are completely lost and its fluorescence polarization properties are sharply altered. The spectroscopic characteristics of allophycocyanin monomers are much closer to those of C-phycocyanin than to trimeric allophycocyanin. A model for trimeric allophyocyanin is presented in which the appearance of the 650-nm absorption band is induced by a particular kind of chromophore-chromophore interaction. Similar results are found for both allophycocyanin II and III.