Wehling Axel, Walla Peter J
Technical University of Braunschweig, Institute for Physical and Theoretical Chemistry, Department for Biophysical Chemistry, Hans-Sommerstr. 10, D-38106 Braunschweig, Germany.
J Phys Chem B. 2005 Dec 29;109(51):24510-6. doi: 10.1021/jp053890j.
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.
我们展示了用嗜热栖热放线菌光系统I（PS I）测量的时间分辨飞秒双光子泵浦-探测数据。在类胡萝卜素光学禁戒的第一激发单重态（Car S1）的光谱区域进行双光子激发（λ(exc)/2 = 575 nm），会产生Car S1→S(n)激发态吸收的800 fs和9 ps衰减成分，其幅度分别约为47±16%和53±10%。在完全相同的条件下测量纯β-胡萝卜素溶液时，只能观察到9 ps的衰减成分。通过单光子激发（λ(exc) = 575 nm）在完全相同的光谱区域激发PS I，也未显示任何亚皮秒成分。我们将观察到的800 fs常数归因于约47±16%的β-胡萝卜素态，这些态有可能通过光学暗态Car S1将其能量有效地转移到叶绿素色素上。我们将这些数据与传统的单光子泵浦-探测数据进行了比较，后者激发的是光学允许的第二激发态Car S2。这种比较表明，仅通过传统的单光子激发很难揭示光学禁戒态的快速动力学，因为在Car S2→Car S1内转换后，相应的Car S1布居太小。对Car S2激发后观察到的PS I和β-胡萝卜素的Car S1→S(n)激发态吸收幅度进行直接比较，可以确定PS I中Car S1形成的量子产率为44±5%。总之，在室温下观察到从Car S2到叶绿素的总能量转移效率约为69±5%，其中56±5%通过Car S2和可能的非常热的Car S1态转移，13±5%通过热的和“冷”的Car S1态转移。
