Avital Shlomo, Brumfeld Vlad, Malkin Shmuel
Department of Biological Chemistry, The Weizmann Institute of Science, Rehovot, Israel.
Biochim Biophys Acta. 2006 Jul;1757(7):798-810. doi: 10.1016/j.bbabio.2006.05.038. Epub 2006 Jun 7.
To get an insight to the mechanism of the zeaxanthin-dependent non-photochemical quenching in photosystem II of photosynthesis, we probed the interaction of some xanthophylls with excited chlorophyll-a by trapping both pigments in micelles of triton X-100. Optimal distribution of pigments among micelles was obtained by proper control of the micelle concentration, using formamide in the reaction mixture, which varies the micellar aggregation number over three orders of magnitude. The optimal reaction mixture was obtained around 40% (v/v) formamide in 0.2-0.4% (v/v) triton X-100 in water. Zeaxanthin in the micellar solution exhibited initially absorption and circular dichroism spectral features corresponding to a J-type aggregate. The spectrum was transformed over time (half-time values vary-an average characteristic figure is roughly 20 min) to give features representing an H-type aggregate. The isosbestic point in the series of spectral curves favors the supposition of a rather simple reaction between two pure J and H-types dimeric species. Violaxanthin exhibited immediately stable spectral features corresponding to a mixture of J-type and more predominately H-type dimers. Lutein, neoxanthin and beta-carotene did not show any aggregated spectral forms in micelles. The spectral features in micelles were compared to spectra in aqueous acetone, where the assignment to various aggregated types was established previously. The specific tendency of zeaxanthin to form the J-type dimer (or aggregate) could be important for its function in photosynthesis. The abilities of five carotenoids (zeaxanthin, violaxanthin, lutein, neoxanthin and beta-carotene) to quench chlorophyll-a fluorescence were compared. Zeaxanthin, in its two micellar dimeric forms, and beta-carotene were comparable good quenchers of chlorophyll-a fluorescence. Violaxanthin was a much weaker quencher, if at all. Lutein and neoxanthin rather enhanced the fluorescence. The implications to non-photochemical quenching process in photosynthesis are discussed.
为深入了解光合作用光系统II中玉米黄质依赖的非光化学猝灭机制，我们通过将两种色素捕获在Triton X-100胶束中，探究了一些叶黄素与激发态叶绿素a的相互作用。通过适当控制胶束浓度，在反应混合物中使用甲酰胺，使胶束聚集数在三个数量级范围内变化，从而实现了色素在胶束间的最佳分布。在水中0.2 - 0.4%（v/v）的Triton X-100中，当甲酰胺含量约为40%（v/v）时可得到最佳反应混合物。胶束溶液中的玉米黄质最初呈现出与J型聚集体相对应的吸收光谱和圆二色光谱特征。随着时间推移（半衰期值各不相同，平均特征值约为20分钟），光谱发生转变，呈现出代表H型聚集体的特征。一系列光谱曲线中的等吸收点支持了两种纯J型和H型二聚体物种之间存在相当简单反应的假设。紫黄质立即呈现出与J型和更主要的H型二聚体混合物相对应的稳定光谱特征。叶黄素、新黄质和β-胡萝卜素在胶束中未表现出任何聚集光谱形式。将胶束中的光谱特征与丙酮水溶液中的光谱进行了比较，此前已在丙酮水溶液中确定了各种聚集类型的归属。玉米黄质形成J型二聚体（或聚集体）的特定倾向可能对其在光合作用中的功能很重要。比较了五种类胡萝卜素（玉米黄质、紫黄质、叶黄素新黄质和β-胡萝卜素）猝灭叶绿素a荧光的能力。玉米黄质的两种胶束二聚体形式以及β-胡萝卜素都是叶绿素a荧光的良好猝灭剂。紫黄质即使能猝灭荧光，其能力也弱得多。叶黄素和新黄质反而增强了荧光。文中讨论了这些结果对光合作用中非光化学猝灭过程的影响。
