Diatoms are one of the most abundant photosynthetic organisms on earth and contribute largely to the atmospheric oxygen production. They contain fucoxanthin and chlorophyll-a/c binding proteins (FCPs) as light-harvesting complexes with a remarkable adaptation to the fluctuating light on ocean surfaces. To understand the basis of the photosynthetic process in diatoms, the excitation energy funneling within FCPs needs to be probed. A state-of-the-art multiscale analysis within a quantum mechanics/molecular mechanics framework has been employed. To this end, the chlorophyll (Chl) excitation energies within the FCP complex from the diatom Phaeodactylum tricornutum have been determined. The Chl-c excitation energies were found to be significantly lower than those in organic solvents. This finding challenges the general belief that the excitation energy of Chl-c is higher than that of Chl-a in FCP proteins and reveals that Chl-c molecules are much more sensitive to electric fields within protein scaffolds than Chl-a pigments. A direct connection to experiment is obtained by analysis of the linear absorption spectrum of FCP.
See the supplementary material for extra figures of excitation energy ladders and distributions, density differences, effects of external electric fields, excitonic couplings as well as spectral densities.