Single molecule spectroscopy has opened up a whole new vista on the photophysics and photochemistry of proteins whose functionality depends on optically active chromophores. A Nature Chemistry paper by Goldsmith and Moerner reports their study of the allophycocyanin (APC) protein in solution. It consists of a trimer, each of which contains two phycocyanonobilin (PCB) chromophores (shown in red below). There is a strong excitonic (Forster) coupling within each pair.
I found this particularly interesting because this is a symmetric methine dye, similar to those studied in this recent J. Chem. Phys. paper by Seth Olsen and I.
They observe significant fluctuations in the radiative lifetimes of the chromophore, arguing that this is due to changes in the local environment due to conformational change of the protein. They suggest there are four relevant electronic states.
The results can be compared to A Microscopic Model for the Fluctuations of Local Field and Spontaneous Emission of Single Molecules in Disordered Media
I thank Seth Olsen for bringing the paper to my attention.