Interexciton-State Relaxation and Exciton Localization in Allophycocyanin Trimers

Abstract

We have employed dynamic absorption, transient hole-burning, and discrete two-color pump−probe femtosecond spectroscopic methods to obtain a time-resolved view of exciton dynamics in trimers of allophycocyanin, the major component of the core of the phycobilisome in cyanobacteria. Allophycocyanin trimers contain a C3-symmetric array of phycocyanobilin (open-chain tetrapyrrole) dimers. The femtosecond time-resolved pump−probe spectra and single-wavelength transients observed in allophycocyanin preparations were interpreted with the aid of a series of calculated spectra, which are based on an assignment of the ground-state absorption spectrum in terms of exciton-coupled chromophore dimers. The observed and calculated time-resolved pump−probe spectra indicate that interexciton-state relaxation, a radiationless transfer of population between dimer exciton states, is responsible for a red shift of the spectra on the ∼30-fs time scale. The time evolution of the pump−probe spectra is inconsistent with a model ac...