Galinato Mary Grace I, Niedzwiedzki Dariusz, Deal Cailin, Birge Robert R, Frank Harry A
Department of Chemistry, University of Connecticut, U-3060, 55 North Eagleville Road, Storrs, CT 06269-3060, USA.
Photosynth Res. 2007 Oct;94(1):67-78. doi: 10.1007/s11120-007-9218-5. Epub 2007 Jul 19.
Carotenes and xanthophylls are well known to act as electron donors in redox processes. This ability is thought to be associated with the inhibition of oxidative reactions in reaction centers and light-harvesting pigment-protein complexes of photosystem II (PSII). In this work, cation radicals of neoxanthin, violaxanthin, lutein, zeaxanthin, beta-cryptoxanthin, beta-carotene, and lycopene were generated in solution using ferric chloride as an oxidant and then studied by absorption spectroscopy. The investigation provides a view toward understanding the molecular features that determine the spectral properties of cation radicals of carotenoids. The absorption spectral data reveal a shift to longer wavelength with increasing pi-chain length. However, zeaxanthin and beta-cryptoxanthin exhibit cation radical spectra blue-shifted compared to that of beta-carotene, despite all of these molecules having 11 conjugated carbon-carbon double bonds. CIS molecular orbital theory quantum computations interpret this effect as due to the hydroxyl groups in the terminal rings selectively stabilizing the highest occupied molecular orbitals of preferentially populated s-trans-isomers. The data are expected to be useful in the analysis of spectral results from PSII pigment-protein complexes seeking to understand the role of carotene and xanthophyll cation radicals in regulating excited state energy flow, in protecting PSII reaction centers against photoinhibition, and in dissipating excess light energy absorbed by photosynthetic organisms but not used for photosynthesis.
众所周知，胡萝卜素和叶黄素在氧化还原过程中充当电子供体。人们认为这种能力与抑制光系统II（PSII）反应中心和捕光色素 - 蛋白质复合物中的氧化反应有关。在这项工作中，使用氯化铁作为氧化剂在溶液中生成了新黄质、紫黄质、叶黄素、玉米黄质、β - 隐黄质、β - 胡萝卜素和番茄红素的阳离子自由基，然后通过吸收光谱进行研究。该研究为理解决定类胡萝卜素阳离子自由基光谱特性的分子特征提供了一个视角。吸收光谱数据显示，随着π链长度的增加，波长向更长方向移动。然而，尽管所有这些分子都有11个共轭碳 - 碳双键，但玉米黄质和β - 隐黄质的阳离子自由基光谱与β - 胡萝卜素相比发生了蓝移。CIS分子轨道理论量子计算将这种效应解释为由于末端环中的羟基选择性地稳定了优先占据的s - 反式异构体的最高占据分子轨道。这些数据有望用于分析PSII色素 - 蛋白质复合物的光谱结果，以了解胡萝卜素和叶黄素阳离子自由基在调节激发态能量流动、保护PSII反应中心免受光抑制以及消散光合生物吸收但未用于光合作用的多余光能方面的作用。
