Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA, JST, Suita, Japan.
Chemistry. 2012 Nov 12;18(46):14660-70. doi: 10.1002/chem.201201328. Epub 2012 Sep 25.
In this paper, nanosecond laser flash photolysis has been used to investigate the influence of metal ions on the kinetics of radical cations of a range of carotenoids (astaxanthin (ASTA), canthaxanthin (CAN), and β-carotene (β-CAR)) and various electron donors (1,4-diphenyl-1,3-butadiene (14DPB), 1,6-diphenyl-1,3,5-hexatriene (16DPH), 4-methoxy-trans-stilbene (4 MeOSt), and trans-stilbene (trans-St)) in benzonitrile. Radical cations have been generated by means of photosensitized electron-transfer (ET) using 1,4-dicyanonaphthalene (14DCN) and biphenyl (BP). The kinetic decay of CAR(· +) shows a strong dependence on the identity of the examined metal ion. For example, whereas NaClO(4) has a weak effect on the kinetics of CAR(· +), Ni(ClO(4))(2) causes a strong retardation of the decay of CAR(· +). It is also interesting to note that Mn(2+), which is a biologically relevant metal ion, shows the strongest effect of all the investigated metal ions (e.g., in the presence of Mn(2+) ions, the half-life (t(1/2)) of CAN(· +) (t(1/2) >90 ms) is more than three orders of magnitude higher than in the absence of the metal ions (t(1/2) ≈16 μs)). Furthermore, the influence of metal-ion and oxygen concentrations on the kinetics of CAR(· +) reveals their pronounced effect on the kinetic decay of CAR(· +). However, these remarkable effects are greatly diminished if either oxygen or metal ions are removed from the investigated solutions. Therefore, it can be concluded that oxygen and metal ions interact cooperatively to induce the observed substantial effects on the stabilities of CAR(· +). These results are the first direct observation of the major role of oxygen in the stabilization of radical cations, and they support the earlier mechanism proposed by Astruc et al. for the role of oxygen in the inhibition of cage reactions. On the basis of these results, the factors that affect the stability of radical cations are discussed and the mechanism that shows the role of oxygen and metal ions in the enhancement of radical-cation stability is described.
本文采用纳秒激光闪光光解技术研究了金属离子对一系列类胡萝卜素（虾青素（ASTA）、角黄素（CAN）和β-胡萝卜素（β-CAR））和各种电子供体（1,4-二苯基-1,3-丁二烯（14DPB）、1,6-二苯基-1,3,5-己三烯（16DPH）、4-甲氧基-反式-二苯乙烯（4-MeOSt）和反式-二苯乙烯（trans-St））在苯甲腈中自由基阳离子动力学的影响。自由基阳离子是通过 1,4-二氰基萘（14DCN）和联苯（BP）敏化电子转移（ET）生成的。CAR(·+)的动力学衰减强烈依赖于所研究的金属离子的特性。例如，尽管 NaClO(4) 对 CAR(·+)的动力学影响较弱，但 Ni(ClO(4))(2) 会强烈延迟 CAR(·+)的衰减。同样有趣的是，Mn(2+)，这是一种与生物相关的金属离子，显示出所有研究的金属离子中最强的影响（例如，在 Mn(2+)离子存在下，CAN(·+)的半衰期（t(1/2)）（t(1/2)>90 ms）比没有金属离子时高三个数量级（t(1/2)≈16 μs））。此外，金属离子和氧气浓度对 CAR(·+)动力学的影响表明它们对 CAR(·+)的动力学衰减有显著影响。然而，如果从研究溶液中除去氧气或金属离子，这些显著的影响会大大减小。因此，可以得出结论，氧气和金属离子协同作用，导致观察到的 CAR(·+)稳定性的显著影响。这些结果是直接观察氧气在稳定自由基阳离子中的主要作用的首次观察结果，并且支持 Astruc 等人提出的氧气在抑制笼反应中的作用的早期机制。基于这些结果，讨论了影响自由基阳离子稳定性的因素，并描述了表明氧气和金属离子在增强自由基阳离子稳定性中的作用的机制。
