Groot M L, Peterman E J, van Kan P J, van Stokkum I H, Dekker J P, van Grondelle R
Department of Physics and Astronomy, Vrije Universiteit, Amsterdam, The Netherlands.
Biophys J. 1994 Jul;67(1):318-30. doi: 10.1016/S0006-3495(94)80483-7.
A key step in the photosynthetic reactions in photosystem II of green plants is the transfer of an electron from the singlet-excited chlorophyll molecule called P680 to a nearby pheophytin molecule. The free energy difference of this primary charge separation reaction is determined in isolated photosystem II reaction center complexes as a function of temperature by measuring the absolute quantum yield of P680 triplet formation and the time-integrated fluorescence emission yield. The total triplet yield is found to be 0.83 +/- 0.05 at 4 K, and it decreases upon raising the temperature to 0.30 at 200 K. It is suggested that the observed triplet states predominantly arise from P680 but to a minor extent also from antenna chlorophyll present in the photosystem II reaction center. No carotenoid triplet states could be detected, demonstrating that the contamination of the preparation with CP47 complexes is less than 1/100 reaction centers. The fluorescence yield is 0.07 +/- 0.02 at 10 K, and it decreases upon raising the temperature to reach a value of 0.05-0.06 at 60-70 K, increases upon raising the temperature to 0.07 at approximately 165 K and decreases again upon further raising the temperature. The complex dependence of fluorescence quantum yield on temperature is explained by assuming the presence of one or more pigments in the photosystem II reaction center that are energetically degenerate with the primary electron donor P680 and below 60-70 K trap part of the excitation energy, and by temperature-dependent excited state decay above 165 K. A four-compartment model is presented that describes the observed triplet and fluorescence quantum yields at all temperatures and includes pigments that are degenerate with P680, temperature-dependent excited state decay and activated upward energy transfer rates. The eigenvalues of the model are in accordance with the lifetimes observed in fluorescence and absorption difference measurements by several workers. The model suggests that the free energy difference between singlet-excited P680 and the radical pair state P680+l- is temperature independent, and that a distribution of free energy differences represented by at least three values of about 20, 40, and 80 meV, is needed to get an appropriate fit of the data.
绿色植物光系统II光合作用反应中的一个关键步骤是，将电子从名为P680的单重态激发叶绿素分子转移到附近的去镁叶绿素分子。通过测量P680三重态形成的绝对量子产率和时间积分荧光发射产率，在分离的光系统II反应中心复合物中确定该初级电荷分离反应的自由能差与温度的函数关系。发现总三重态产率在4K时为0.83±0.05，在温度升至200K时降至0.30。据推测，观察到的三重态主要源于P680，但在较小程度上也源于光系统II反应中心中存在的天线叶绿素。未检测到类胡萝卜素三重态，这表明制备物中CP47复合物的污染小于1/100反应中心。荧光产率在10K时为0.07±0.02，在温度升至60 - 70K时降至0.05 - 0.06，在温度升至约165K时升至0.07，在进一步升高温度时又下降。荧光量子产率对温度的复杂依赖性可通过假设光系统II反应中心中存在一种或多种与初级电子供体P680能量简并且在60 - 70K以下捕获部分激发能的色素，以及高于165K时温度依赖的激发态衰减来解释。提出了一个四室模型，该模型描述了在所有温度下观察到的三重态和荧光量子产率，并包括与P680简并的色素、温度依赖的激发态衰减和活化的向上能量转移速率。该模型的本征值与几位研究人员在荧光和吸收差异测量中观察到的寿命一致。该模型表明，单重态激发的P680与自由基对态P680 + l - 之间的自由能差与温度无关，并且需要由至少三个约20、40和80meV的值表示的自由能差分布才能对数据进行适当拟合。
