title: Strong Impact of an Axial Ligand on the Absorption by Chlorophyll a and b Pigments Determined by Gas-Phase Ion Spectroscopy Experiments.
abstract: The microenvironments in photosynthetic proteins affect the absorption by chlorophyll (Chl) pigments. It is, however, a challenge to disentangle the impact on the transition energies of different perturbations, for example, the global electrostatics of the protein (nonbonded environmental effects), exciton coupling between Chl's, conformational variations, and binding of an axial ligand to the magnesium center. This is needed to distinguish between the two most commonly proposed mechanisms for energy transport in photosynthetic proteins, relying on either weakly or strongly coupled pigments. Here, on the basis of photodissociation action spectroscopy, we establish that the redshift of the Soret absorption band due to binding of a negatively charged carboxylate (as present in aspartic acid and glutamic acid residues) is 0.1-0.2 eV for Chl a and b. This effect is almost enough to reproduce the well-known green color of plants and can account for the strong spectral variation between Chl's. The experimental data serve to benchmark future high-level calculations of excited-state energies. Finally, we demonstrate that complexes between Chl a and histidine, tagged by a quaternary ammonium ion, can be made in the gas phase by electrospray ionization, but more work is needed to produce enough ions for gas-phase spectroscopy.
authors.affiliation: Department of Physics and Astronomy, Aarhus University , DK-8000 Aarhus, Denmark.
authors.forename: Christina
authors.initials: C
authors.lastname: Kjær
authors.affiliation: Department of Physics and Astronomy, Aarhus University , DK-8000 Aarhus, Denmark.
authors.forename: Mark H
authors.initials: MH
authors.lastname: Stockett
authors.affiliation: Department of Physics and Astronomy, Aarhus University , DK-8000 Aarhus, Denmark.
authors.forename: Bjarke M
authors.initials: BM
authors.lastname: Pedersen
authors.affiliation: Department of Physics and Astronomy, Aarhus University , DK-8000 Aarhus, Denmark.
authors.forename: Steen Brøndsted
authors.initials: SB
authors.lastname: Nielsen
mesh_terms: D000109:Acetylcholine / Q000737:chemistry; D001224:Aspartic Acid / Q000737:chemistry; D002734:Chlorophyll / Q000737:chemistry*; D000077194:Chlorophyll A; D004735:Energy Transfer; D005561:Formates / Q000737:chemistry; D018698:Glutamic Acid / Q000737:chemistry; D006639:Histidine / Q000737:chemistry; D007700:Kinetics; D008024:Ligands; D008274:Magnesium / Q000737:chemistry; D008958:Models, Molecular; D018532:Pisum sativum / Q000737:chemistry* / Q000502:physiology; D045332:Photosystem II Protein Complex / Q000737:chemistry*; D017433:Protein Structure, Secondary; D013057:Spectrum Analysis / Q000379:methods; D055672:Static Electricity; D013816:Thermodynamics
chemical_list: D005561:Formates; D008024:Ligands; D045332:Photosystem II Protein Complex; C030544:formic acid; D002734:Chlorophyll; D001224:Aspartic Acid; D018698:Glutamic Acid; D006639:Histidine; C037184:chlorophyll b; D008274:Magnesium; D000109:Acetylcholine; D000077194:Chlorophyll A