Energy transfer in monomeric phycoerythrocyanin.

Phycoerythrocyanin (PEC) is part of the phycobilisome of cyanobacteria. Its monomer carries one phycoviolobilin and two phycocyanobilins (PCB) as chromophores. For an understanding of the complicated energy transfer in phycobilisomes, a detailed knowledge of the processes in the constituting building proteins is indispensable. We report the experimental data necessary for the description of Förster energy transfer in monomeric PEC, including fluorescence lifetimes and quantum yields of the two subunits. The bulk experiments are complemented by studies of single PEC molecules. Förster energy calculations and Monte Carlo simulations based on the bulk data are presented. They reveal that earlier experimental findings of energy transfer heterogeneities in single PEC molecules originate in spectral shifts between the contributing chromophores.