title: On the Exciton Coupling between Two Chlorophyll Pigments in the Absence of a Protein Environment: Intrinsic Effects Revealed by Theory and Experiment.
abstract: 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.
authors.affiliation: CFisUC, Department of Physics, University of Coimbra, Rua Larga, 3004-516, Coimbra, Portugal. bfmilne@uc.pt.
authors.forename: Bruce F
authors.identifier: 0000-0002-5522-4808
authors.initials: BF
authors.lastname: Milne
authors.affiliation: Department of Physics and Astronomy, Aarhus University, Ny Munkegade, DK-8000, Aarhus C, Denmark.
authors.forename: Christina
authors.initials: C
authors.lastname: Kjaer
authors.affiliation: Department of Physics and Astronomy, Aarhus University, Ny Munkegade, DK-8000, Aarhus C, Denmark.
authors.forename: Jørgen
authors.initials: J
authors.lastname: Houmøller
authors.affiliation: Department of Physics and Astronomy, Aarhus University, Ny Munkegade, DK-8000, Aarhus C, Denmark.
authors.forename: Mark H
authors.initials: MH
authors.lastname: Stockett
authors.affiliation: Institute of Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan, 5290002, Israel.
authors.forename: Yoni
authors.initials: Y
authors.lastname: Toker
authors.affiliation: Max Planck Institute for the Structure and Dynamics of Matter, Luruper Chaussee 149, 22761, Hamburg, Germany.|Nano-Bio Spectroscopy Group and ETSF, Universidad del País Vasco, CFM CSIC-UPV/EHU-MPC & DIPC, 20018, San Sebastián, Spain.
authors.forename: Angel
authors.initials: A
authors.lastname: Rubio
authors.affiliation: Department of Physics and Astronomy, Aarhus University, Ny Munkegade, DK-8000, Aarhus C, Denmark. sbn@phys.au.dk.
authors.forename: Steen Brøndsted
authors.initials: SB
authors.lastname: Nielsen
mesh_terms: D064751:Ammonium Compounds / Q000737:chemistry; D002734:Chlorophyll / Q000737:chemistry*; D000077194:Chlorophyll A; D019281:Dimerization; D008958:Models, Molecular; D013053:Spectrophotometry; D013816:Thermodynamics
chemical_list: D064751:Ammonium Compounds; D002734:Chlorophyll; D000077194:Chlorophyll A
keywords: absorption spectroscopy; chlorophyll; density functional calculations; mass spectrometry; photosynthesis