ABSTRACT
Excitation energy can be transferred from chromophore to chromophore by a mechanism described in general by Förster. The transfer occurs by dipole-dipole interaction between donor and acceptor molecules. The spectra of the donor and acceptor are assumed to be unaffected by interaction. The mechanism is a radiationless, resonance interaction between the two dipoles. The rate of energy transfer depends on a coupling of energy levels between donor and acceptor, that is, the fluorescence emission of the donor should overlap the absorption spectrum of the acceptor. The fluorescence properties of a bihprotein with a complex absorption spectrum, Rphycoerythrin, enabled the suggestion to be offered that excitation energy is passed from the higher-energy absorbers to the lowest-energy absorber. The lowest-energy absorber then was responsible for fluorescence emission. Allophycocyanin is a trimer when isolated near neutral pH. Its fluorescence polarization spectrum was very low 29,40,41,49 and constant 29,40,41 across its visible absorption band.
