β-胡萝卜素在乙腈体系中的激光光解研究

The Triplet Properties of β-Carotene in Acetonitrile Solution:A Laser Flash Photolysis Study

利用纳秒级激光光解瞬态吸收光谱装置,研究了以乙腈作为溶剂,以2-萘乙酮作为敏化剂的体系在355nm激光作用下敏化产生β-胡萝卜素激发三重态的机制,并进一步研究了β-胡萝卜素激发三重态的性质。研究显示2-萘乙酮和β-胡萝卜素的二元体系在355nm激光作用下,2-萘乙酮首先被激发为其激发三重态(420nm),2-萘乙酮激发三重态与β-胡萝卜素发生激发能转移,产生β-胡萝卜素激发三重态(510nm)。通过激发能转移的方法测得了β-胡萝卜素激发三重态在最大吸收波长510nm处的摩尔消光系数为23000L·mol-1·cm-1。改变β-胡萝卜素的浓度测得了其激发三重态在乙腈体系中的衰变反应速率常数6·5×104s-1,其在乙腈体系中的三重态寿命为15·6μs。同时获得了激发态2-萘乙酮与β-胡萝卜素之间激发能转移反应的速率常数1·5×1010L·mol-1·s-1。

The representative of carotenoids,β-carotene,can scavenge reactive oxygen radicals like singlet molecular oxygen,nitrogen dioxide radical and peroxyl radical due to the effective antioxidative properties.In medicine,β-carotene is used to alleviate the disease erythropoietic protoporphyria（EPP）,by intercepting the triplet state of protoporphyrin（a porphyrin lacking a central metalion,a precursor to haem） therefore preventing the formation of singlet oxygen.Epidemiological evidence has suggested that dietary β-carotene may inhibit certain types of cancer.Much of work has been carried out in benzene,toluene,or chloroform as most caroienoids are sufficiently soluble in these nonpolarity solvents.In the present paper,the generation and properties of triplet β-carotene in acetonitrile solution were investigated with 355 nm laser flash photolysis.2-acetonaphthone was used as an excitation energy donor to sensitize the production of the triplet state of β-carotene.Excitation of the solution containing 2-acetonaphthone and β-carotene upon 355 nm laser flash produced the triplet of 2-acetonaphthone（420 nm） firstly.Subsequently,the excitation energy of triplet 2-acetonaphthone was transferred to β-carotene generating triplet β-carotene.Characteristic absorption spectra of triplet β-carotene（510 nm） were recorded.By means of transfer of excitation energy,the molar absorption coefficients of triplet β-carotene were determined to be 23 000 dm3 mol-1·cm-1 at 510 nm.The triplet lifetime for β-carotene in acetonitrile solution was observed to be 15.6 μs.The rate constant for the reaction of triplet energy transfer from triplet 2-acetonaphthone to β-carotene was calculated to be 1.5×1010 dm3·mol-1·s-1.Obviously,the triplet β-carotene has very low excitation energy.Taking the advantage of the photochemical properties of triplet β-carotene,β-Carotene has been widely used as energy acceptor to determine the excited state characteristic of other substance.This work extends the understanding of photochemical properties of β-carotene.