title: Time-resolved two-photon spectroscopy of photosystem I determines hidden carotenoid dark-state dynamics.
abstract: We present time-resolved fs two-photon pump-probe data measured with photosystem I (PS I) of Thermosynechococcus elongatus. Two-photon excitation (lambda(exc)/2 = 575 nm) in the spectral region of the optically forbidden first excited singlet state of the carotenoids, Car S1, gives rise to a 800 fs and a 9 ps decay component of the Car S1 --> S(n) excited-state absorption with an amplitude of about 47 +/- 16% and 53 +/- 10%, respectively. By measuring a solution of pure beta-carotene under exactly the same conditions, only a 9 ps decay component can be observed. Exciting PS I at exactly the same spectral region via one-photon excitation (lambda(exc) = 575 nm) also does not show any sub-ps component. We ascribe the observed constant of 800 fs to a portion of about 47 +/- 16% beta-carotene states that can potentially transfer their energy efficiently to chlorophyll pigments via the optically dark Car S1 state. We compared these data with conventional one-photon pump-probe data, exciting the optically allowed second excited state, Car S2. This comparison demonstrates that the fast dynamics of the optically forbidden state can hardly be unravelled via conventional one-photon excitation only because the corresponding Car S1 populations are too small after Car S2 --> Car S1 internal conversion. A direct comparison of the amplitudes of the Car S1 --> S(n) excited-state absorption of PS I and beta-carotene observed after Car S2 excitation allows determination of a quantum yield for the Car S1 formation in PS I of 44 +/- 5%. In conclusion, an overall Car S2 --> Chl energy-transfer efficiency of approximately 69 +/- 5% is observed at room temperature with 56 +/- 5% being transferred via Car S2 and probably very hot Car S1 states and 13 +/- 5% being transferred via hot and "cold" Car S1 states.
authors.affiliation: Technical University of Braunschweig, Institute for Physical and Theoretical Chemistry, Department for Biophysical Chemistry, Hans-Sommerstr. 10, D-38106 Braunschweig, Germany.
authors.forename: Axel
authors.initials: A
authors.lastname: Wehling
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Walla
mesh_terms: D001429:Bacteriochlorophylls / Q000378:metabolism; D002338:Carotenoids / Q000737:chemistry / Q000502:physiology*; D000458:Cyanobacteria / Q000737:chemistry / Q000502:physiology*; D008027:Light; D017785:Photons; D045331:Photosystem I Protein Complex / Q000737:chemistry / Q000502:physiology*; D013053:Spectrophotometry; D019207:beta Carotene / Q000378:metabolism
chemical_list: D001429:Bacteriochlorophylls; D045331:Photosystem I Protein Complex; D019207:beta Carotene; D002338:Carotenoids