title: Mediation of ultrafast light-harvesting by a central dimer in phycoerythrin 545 studied by transient absorption and global analysis.
abstract: We report ultrafast femtosecond transient absorption measurements of energy-transfer dynamics for the antenna protein phycoerythrin 545, PE545, isolated from a unicellular cryptophyte Rhodomonas CS24. The phycoerythrobilins are excited at both 485 and 530 nm, and the spectral response is probed between 400 and 700 nm. Room-temperature measurements are contrasted with measurements at 77 K. An evolution-associated difference spectra (EADS) analysis is combined with estimations of bilin spectral positions and energy-transfer rates to obtain a detailed kinetic model for PE545. It is found that sub pulse-width dynamics include relaxation between the exciton states of a chromophore dimer (beta 50/60) located in the core of the protein. Energy transfer from the lowest exciton state of the phycoerythrobilin (PEB) dimer to one of the periphery 15,16-dihydrobiliverdin (DBV) bilins is found to occur on a time scale of 250 fs at room temperature and 960 fs at 77 K. A host of energy-transfer dynamics involving the beta 158, beta 82, and alpha 19 bilins occur on a time scale of 2 ps at room temperature and 3 ps at 77 K. A final energy transfer occurs between the red-most DBV bilins with a time scale estimated to be approximately 30 ps. The role of the centrally located phycoerythrobilin dimer is seen as crucial-spectrally as it expands the cross-section of absorption of the protein; structurally as it sits in the middle of the protein acting as an intermediary trap; and kinetically, as the internal conversion and subsequent red-shift of the excitation is extremely fast.
authors.affiliation: Lash Miller Chemical Laboratories, 80 St. George Street, University of Toronto, Ontario, M5S 3H6 Canada.
authors.forename: Alexander B
authors.initials: AB
authors.lastname: Doust
authors.forename: Ivo H M
authors.initials: IH
authors.lastname: van Stokkum
authors.forename: Delmar S
authors.initials: DS
authors.lastname: Larsen
authors.forename: Krystyna E
authors.initials: KE
authors.lastname: Wilk
authors.forename: Paul M G
authors.initials: PM
authors.lastname: Curmi
authors.forename: Rienk
authors.initials: R
authors.lastname: van Grondelle
authors.forename: Gregory D
authors.initials: GD
authors.lastname: Scholes
mesh_terms: D000042:Absorption; D019281:Dimerization; D004735:Energy Transfer / Q000528:radiation effects; D056890:Eukaryota / Q000737:chemistry; D007700:Kinetics; D007834:Lasers; D008027:Light; D008954:Models, Biological; D015394:Molecular Structure; D010799:Phycoerythrin / Q000737:chemistry* / Q000528:radiation effects*; D013057:Spectrum Analysis; D013696:Temperature; D013997:Time Factors
chemical_list: C045714:phycoerythrin 545; D010799:Phycoerythrin