Mortensen A, Skibsted L H
Department of Dairy and Food Science, Royal Veterinary and Agricultural University, Frederiksberg C, Denmark.
Free Radic Res. 1997 Jun;26(6):549-63. doi: 10.3109/10715769709097826.
Carotenoids in chloroform and carbon tetrachloride photobleach upon nanosecond laser flash photolysis in two steps: instantaneously and in a second-order reaction. The rate constant for second-order reaction (first-order in a solvent derived radical and first-order in (excess) carotenoid) is largest for carotenes (9.8.10(8) M-1 for beta-carotene), intermediate for hydroxylated carotenoids, and smallest for carbonyl containing carotenoids (1.0.10(8) M-1 S-1 for astaxanthin) in chloroform at 20 degrees C. Near infrared absorbing transients are formed concomitant with photobleaching in chloroform (not detected in carbon tetrachloride). A species formed instantaneously is tentatively identified as either a carotenoid/solvent adduct or an ion-pair. A second species is formed by decay of instantaneously formed species and is identified as the carotenoid radical action. This species is formed in a first-order reaction with a rate constant of approx. 5.10(4) S-1 and absorbing at longer wavelength than the precursor. The lifetime (second-order decay) of the intermediates appears to be longest for the carotenoids with the longest conjugated system. The results indicate that carotenes are better antioxidants than xanthophylls as the carotenes, at least in the present lipophilic solvents, react faster with free radicals.
瞬间发生以及通过二级反应。二级反应的速率常数（对溶剂衍生自由基为一级，对（过量的）类胡萝卜素为一级）对于胡萝卜素来说最大（β-胡萝卜素在20℃的氯仿中为9.8×10⁸ M⁻¹），对于羟基化类胡萝卜素处于中间水平，而对于含羰基类胡萝卜素最小（虾青素在20℃的氯仿中为1.0×10⁸ M⁻¹ s⁻¹）。在氯仿中光漂白的同时会形成近红外吸收瞬态（在四氯化碳中未检测到）。瞬间形成的一种物质初步鉴定为类胡萝卜素/溶剂加合物或离子对。第二种物质由瞬间形成的物质衰变形成，被鉴定为类胡萝卜素自由基。该物质通过一级反应形成，速率常数约为5×10⁴ s⁻¹，吸收波长比前体更长。对于具有最长共轭体系的类胡萝卜素，中间体的寿命（二级衰变）似乎最长。结果表明，胡萝卜素比叶黄素是更好的抗氧化剂，因为至少在目前的亲脂性溶剂中，胡萝卜素与自由基反应更快。
