Ziolek Marcin, Pawlowicz Natalia, Naskrecki Ryszard, Dobek Andrzej
Center for Ultrafast Laser Spectroscopy, A. Mickiewicz University, Umultowska 85, 61-614 Poznan, Poland.
J Phys Chem B. 2005 Sep 29;109(38):18171-6. doi: 10.1021/jp050682i.
Electron transfer at the reaction center of the purple photosynthetic bacterium Rb. sphaeroides R-26 was measured at room temperature by the time-resolved transient absorption spectroscopy technique with 200 fs temporal resolution. The absorbance changes characteristic of the excited state of the primary donor and extending over the whole spectral range investigated from 350 nm up to 720 nm appeared after excitation with a laser pulse of about 100 fs duration at 800 nm. The time evolution of the spectra reflected the excitation of bacteriochlorophylls (BChl) M and L and the subsequent transfer of this excitation to the primary electron donor (P), with the time constant shorter than 1 ps. The decay time constant of the excited primary donor P was determined as about 3 ps, and it was faster than the rise of the reduced intermediary acceptor bacteriopheophytin (BPhe(L)). Photoreduction of BPhe(L) and its further reoxidation was clearly observed as an increase in its bleaching band intensity at around 540 nm in about 4 ps and its decrease in about 200 ps. Our findings support the theoretical model assuming the involvement of the intermediate state P(+)BChl- in the so-called "two-step" model. In this model an electron is transferred in a sequence from the excited special pair P* to bacteriochlorophyll, BChl(L), then to bacteriopheophytin, BPhe(L), and further on to quinone, Q(A). The branched charge separation, partially via P and partially via BChl(L), was also observed.
利用具有200 fs时间分辨率的时间分辨瞬态吸收光谱技术，在室温下测量了紫色光合细菌球形红杆菌Rb. sphaeroides R-26反应中心的电子转移。在用800 nm处约100 fs持续时间的激光脉冲激发后，出现了初级供体激发态的吸光度变化，其特征在从350 nm到720 nm的整个研究光谱范围内都有体现。光谱的时间演化反映了细菌叶绿素（BChl）M和L的激发以及随后这种激发向初级电子供体（P）的转移，时间常数短于1 ps。激发的初级供体P的衰减时间常数确定为约3 ps，且比还原的中间受体细菌脱镁叶绿素（BPhe(L)）的上升速度快。BPhe(L)的光还原及其进一步的再氧化在约4 ps内表现为其在540 nm左右的漂白带强度增加，在约200 ps内强度降低，这一点清晰可见。我们的研究结果支持了假设中间态P(+)BChl-参与所谓“两步”模型的理论模型。在该模型中，电子按顺序从激发的特殊对P*转移到细菌叶绿素BChl(L)，然后到细菌脱镁叶绿素BPhe(L)，再进一步到醌Q(A)。还观察到了部分通过P和部分通过BChl(L)的分支电荷分离。
