Electronic excited states and excitation transfer kinetics in the Fenna-Matthews-Olson protein of the photosynthetic bacterium Prosthecochloris aestuarii at low temperatures

The molecular structure-function relationship of the Fenna-Matthews-Olson l ight-harvesting complex of the photosynthetic green bacterium Prosthecochlo ris aestuarii has been investigated. It has been assumed that the electroni c excited states responsible for the function (transfer of electronic excit ation energy) result from the dipole-dipole interactions between the bacter iochlorophyll molecules bound to the polypeptide chain of the complex at a specific three-dimensional geometry. The molecular structure-electronic exc ited states relationship has been addressed on the basis of simultaneous si mulations of several spectroscopic observations. Current electronic excited state models for the Fenna-Matthews-Olson complex have generally been base d on obtaining an optimal match between the information contents of the opt ical steady-state spectra and the bacteriochlorophyll organization. Recent kinetic and spectral information gathered from ultrafast time-resolved meas urements have not yet been used effectively for further refinement of the e xcited state models and for quantification of the relation between the exci ted states and the energy transfer processes. In this study, we have search ed for a model that not only can explain the key features of several steady -state spectra but also the temporal and spectral evolution observed in a r ecent absorption difference experiment and we have discussed the implicatio ns of this model for equilibration of the electronic excitation energy in s ystems at low temperatures.