Han Rui-Min, Tian Yu-Xi, Wu Yi-Shi, Wang Peng, Ai Xi-Cheng, Zhang Jian-Ping, Skibsted Leif H
State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing, PR China.
Photochem Photobiol. 2006 Mar-Apr;82(2):538-46. doi: 10.1562/2005-09-30-RA-705R.1.
The C-40 xanthophylls zeaxanthin and astaxanthin were confirmed to form radical cations, Car.+, in the electron-accepting solvent chloroform by direct excitation using subpicosecond time-resolved absorption spectroscopy in combination with spectroelectrochemical determination of the near-infrared absorption of Car.+. For the singlets, the S2(1B(u+) state and most likely the S(x)(3A(g)-) state directly eject electrons to chloroform leading to the rapid formation of Car.+ on a timescale of approximately 100 fs; the lowest-lying S1(2A(g)-) state, however, remains inactive. Standard reduction potential for Car.+ was determined by cyclic voltametry to have the value 0.63 V for zeaxanthin and 0.75 V for astaxanthin from which excited state potentials were calculated, which confirmed the reactivity toward radical cation formation. On the other hand, Car.+ formation from the lowest triplet excited state T1 populated through anthracene sensitization is mediated by a precursor suggested to be a solute-solvent complex detected with broad near-infrared absorption to the shorter wavelength side of the characteristic Car.+ absorption. However, ground state carotenoids are able to react with a secondary solvent radical to yield Car.+, a process occurring within 16 micros for zeaxanthin and within 21 mus for astaxanthin. Among the two xanthophylls together with lycopene and beta-carotene, all having 11 conjugated double bonds, zeaxanthin ranks with the highest reactivity in forming Car.+ from either the S2(1B(u+)) or the ground state. The effects of substituent groups on the reactivity are discussed.
通过亚皮秒时间分辨吸收光谱直接激发，并结合对Car.+近红外吸收的光谱电化学测定，证实了C - 40叶黄素玉米黄质和虾青素在电子接受溶剂氯仿中形成自由基阳离子Car.+。对于单线态，S2(1B(u+)态以及很可能的S(x)(3A(g)-)态直接将电子喷射到氯仿中，导致在大约100飞秒的时间尺度上快速形成Car.+；然而，能量最低的S1(2A(g)-)态保持不活跃。通过循环伏安法测定，玉米黄质的Car.+标准还原电位为0.63 V，虾青素为0.75 V，据此计算出激发态电位，这证实了其对自由基阳离子形成的反应活性。另一方面，通过蒽敏化产生的最低三线态激发态T1形成Car.+是由一种前体介导的，该前体被认为是一种溶质 - 溶剂复合物，在特征性Car.+吸收的较短波长一侧有宽泛的近红外吸收。然而，基态类胡萝卜素能够与二级溶剂自由基反应生成Car.+，对于玉米黄质，这个过程在16微秒内发生，对于虾青素则在21微秒内发生。在这两种叶黄素以及番茄红素和β - 胡萝卜素中，它们都有11个共轭双键，玉米黄质在从S2(1B(u+))态或基态形成Car.+方面具有最高的反应活性。文中讨论了取代基对反应活性影响。
