On the exciton coupling between two chlorophyll pigments in the absence of a protein environment: Intrinsic effects revealed by theory and experiment

Exciton coupling between two or more chlorophyll (Chl) pigments is a key mechanism associated with the color tuning of photosynthetic proteins but it is difficult to disentangle this effect from shifts that are due to the protein microenvironment. Herein, we report the formation of the simplest coupled system, the Chl a dimer, tagged with a quaternary ammonium ion by electrospray ionization. Based on action spectroscopic studies in vacuo, the dimer complexes were found to absorb 50-70 meV to the red of the monomers under the same conditions. First-principles calculations predict shifts that somewhat depend on the relative orientation of the two Chl units, namely 50 and 30 meV for structures where the Chl rings are stacked and unstacked, respectively. Our work demonstrates that Chl association alone can produce a large portion of the color shift observed in photosynthetic macromolecular assemblies. How strong is the coupling between two adjacent chlorophyll molecules in the absence of a protein environment? Mass spectrometry experiments now provide an answer to this question as chlorophyll a dimers tagged by quaternary ammonium ions could be produced in the gas phase by electrospray ionization.