Carotenoid radical formation: dependence on conjugation length.
Carotenoid radical formation: dependence on conjugation length.
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DOI：
10.1021/jp204787b
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发表时间：
2011-08-04
影响因子：
3.3
通讯作者：
Kispert, Lowell D.
中科院分区：
文献类型：
--
作者：
Focsan, A. Ligia;Bowman, Michael K.;Molnar, Peter;Deli, Jozsef;Kispert, Lowell D.
从线性类胡萝卜素的自由基阳离子的质子损失形成的类胡萝卜素中性自由基的相对能量已被检查作为共轭长度的函数从n = 15到n = 9。对于n = 15的最大共轭长度（双去氢番茄红素，一种对称化合物），质子损失可以从十个甲基中的任何一个发生，其中从位置C1或C1'的甲基的质子损失是最有利的。相比之下，从番茄红素、脉孢烯、拟球烯、拟球蛋白酮、野黄素和脱水紫红质的自由基阳离子的最有利的能量质子损失发生在延伸共轭体系的亚甲基。例如，通过饱和双去氢番茄红素的C3-C4和C3 ′-C4 ′双键将共轭长度降低到n = 11（番茄红素）有利于C4或C4 ′亚甲基上的质子损失。在脉孢烯、拟球烯、拟球烯酮（n = 9、10、11）的情况下，在C7 '-C8'处的饱和有利于在C8'亚甲基处形成中性自由基。通过将甲氧基添加到n = 12或13的共轭的去水解酶样结构（脱水紫红质，赤藓黄素）上而使C1-C2饱和有利于C2亚甲基处的质子损失。作为自由基阳离子去质子化的结果，未成对电子自旋分布发生变化，使得中性自由基（13-16 MHz）比自由基阳离子（7-10 MHz）发生更大的β-甲基质子耦合，从而提供了通过Mims ENDOR鉴定生物系统中可能的类胡萝卜素自由基的方法。
The relative energy of carotenoid neutral radicals formed by proton loss from the radical cations of linear carotenoids has been examined as a function of conjugation length from n = 15 to n = 9. For a maximum conjugation length of n = 15 (bisdehydrolycopene, a symmetrical compound) proton loss can occur from any of the ten methyl groups, with proton loss from the methyl group at position C1 or C1' being the most favorable. In contrast, the most energetically favorable proton loss from the radical cations of lycopene, neurosporene, spheroidene, spheroidenone, spirilloxanthin and anhydrorhodovibrin occurs from methylene groups that extend the conjugated system. For example, decreasing the conjugation length to n = 11 (lycopene) by saturation of the double bonds C3–C4 and at C3'–C4' of bisdehydrolycopene favors proton loss at C4 or C4' methylene groups. Saturation at C7'–C8' in the case of neurosporene, spheroidene, spheroidenone (n = 9, 10, 11) favors formation of a neutral radical at the C8' methylene group. Saturation of C1–C2 by addition of a methoxy group to a dehydrolycopene-like structure with conjugation of n = 12 or 13 (anhydrorhodovibrin, spirilloxanthin) favors proton loss at the C2 methylene group. As a consequence of deprotonation of the radical cation, the unpaired electron spin distribution changes so that larger β-methyl proton couplings occur for the neutral radicals (13–16 MHz) than for the radical cation (7–10 MHz) providing a means to identify possible carotenoid radicals in biological systems by Mims ENDOR.
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