title: Cofactor-specific photochemical function resolved by ultrafast spectroscopy in photosynthetic reaction center crystals.
abstract: High-resolution mapping of cofactor-specific photochemistry in photosynthetic reaction centers (RCs) from Rhodobacter sphaeroides was achieved by polarization selective ultrafast spectroscopy in single crystals at cryogenic temperature. By exploiting the fixed orientation of cofactors within crystals, we isolated a single transition within the multicofactor manifold, and elucidated the site-specific photochemical functions of the cofactors associated with the symmetry-related active A and inactive B branches. Transient spectra associated with the initial excited states were found to involve a set of cofactors that differ depending upon whether the monomeric bacteriochlorophylls, BChl(A), BChl(B), or the special pair bacteriochlorophyll dimer, P, was chosen for excitation. Proceeding from these initial excited states, characteristic photochemical functions were resolved. Specifically, our measurements provide direct evidence for an alternative charge separation pathway initiated by excitation of BChl(A) that does not involve P*. Conversely, the initial excited state produced by excitation of BChl(B) was found to decay by energy transfer to P. A clear sequential kinetic resolution of BChl(A) and the A-side bacteriopheophytin, BPh(A), in the electron transfer proceeding from P* was achieved. These experiments demonstrate the opportunity to resolve photochemical function of individual cofactors within the multicofactor RC complexes using single crystal spectroscopy.
authors.affiliation: Notre Dame Radiation Laboratory, University of Notre Dame, Notre Dame, IN 46556, USA. lhuang2@nd.edu
authors.forename: Libai
authors.initials: L
authors.lastname: Huang
authors.forename: Nina
authors.initials: N
authors.lastname: Ponomarenko
authors.forename: Gary P
authors.initials: GP
authors.lastname: Wiederrecht
authors.forename: David M
authors.initials: DM
authors.lastname: Tiede
mesh_terms: D000042:Absorption; D003067:Coenzymes / Q000378:metabolism*; D003460:Crystallization; D007700:Kinetics; D055668:Photochemical Processes; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism*; D012242:Rhodobacter sphaeroides / Q000378:metabolism; D013057:Spectrum Analysis / Q000379:methods*
chemical_list: D003067:Coenzymes; D045322:Photosynthetic Reaction Center Complex Proteins