title: Unraveling the intrinsic color of chlorophyll.
abstract: The exact color of light absorbed by chlorophyll (Chl) pigments, the light-harvesters in photosynthesis, is tuned by the protein microenvironment, but without knowledge of the intrinsic color of Chl it remains unclear how large this effect is. Experimental first absorption energies of Chl a and b isolated in vacuo and tagged with quaternary ammonium cations are reported. The energies are largely insensitive to details of the tag structure, a finding supported by first-principles calculations using time-dependent density functional theory. Absorption is significantly blue-shifted compared to that of Chl-containing proteins (by 30-70 nm). A single red-shifting perturbation, such as axial ligation or the protein medium, is insufficient to account even for the smallest shift; the largest requires pigment-pigment interactions.
authors.affiliation: Nano-Bio Spectroscopy group and ETSF Scientific Development Centre, Department of Materials Physics, University of the Basque Country, CFM CSIC-UPV/EHU-MPC and DIPC, Avenida de Tolosa 72, E-20018 Donostia (Spain); Centre for Computational Physics, Department of Physics, University of Coimbra, Rua Larga, 3004-516 Coimbra (Portugal). bruce@teor.fis.uc.pt.
authors.forename: Bruce F
authors.initials: BF
authors.lastname: Milne
authors.forename: Yoni
authors.initials: Y
authors.lastname: Toker
authors.forename: Angel
authors.initials: A
authors.lastname: Rubio
authors.forename: Steen Brøndsted
authors.initials: SB
authors.lastname: Nielsen
mesh_terms: D064751:Ammonium Compounds / Q000737:chemistry; D002734:Chlorophyll / Q000737:chemistry*; D000077194:Chlorophyll A; D003116:Color; D004396:Coloring Agents / Q000737:chemistry*; D008958:Models, Molecular; D013053:Spectrophotometry
chemical_list: D064751:Ammonium Compounds; D004396:Coloring Agents; D002734:Chlorophyll; C037184:chlorophyll b; D000077194:Chlorophyll A
keywords: action spectroscopy; chlorophyll; color tuning; photosynthesis; time-dependent density functional theory