Exciton interaction in allophycocyanin.

The absorption and circular dichroism (CD) spectra of allophycocyanin II in the trimer and monomer (dissociated) forms were resolved into four and two components, respectively. The short-wavelength region of the visible spectra was approximated by a chimera of Lorentzian- and Gaussian-shaped bands having a bandwidth of ca. 65 nm. The rest of the bands have a pure Gaussian form. The characteristic 652-nm band in the absorption spectrum (656 nm in the CD spectrum) is shown to arise from exciton interaction between two fluorescent phycocyanobilin chromophores.