Exciton interactions in phycoerythrin.

Upon assembly of the phycoerythrin trimer into hexamer and the hexamer into dodecamer, marked spectral changes are observed. The absorption and circular dichroism spectra of the various phycoerythrin aggregates were resolved into Gaussian components representing individual electronic transitions of phycoerythrobilin chromophores within these proteins. While the contribution of a broad, sensitizing band (at 525 nm) is constant, with increasing aggregate size, a short-wavelength pair of bands centered at 555 nm decreases concomitantly with a dramatic increase in the intensity of a long-wavelength pair of chromophore transitions centered at 563 nm. The implications of these spectral changes for efficient energy transfer in the phycobilisome are discussed.