The triplet properties of beta-carotene in acetonitrile solution: A laser flash photolysis study

The representative of carotenoids, beta-carotene, can scavenge reactive oxygen radicals like singlet molecular oxygen, nitrogen dioxide radical and peroxyl radical due to the effective antioxidative properties. In medicine, beta-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 beta-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 beta-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 beta-carotene. Excitation of the solution containing 2-acetonaphthone and beta-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 beta-carotene generating triplet beta-carotene. Characteristic absorption spectra of triplet beta-carotene (510 nm) were recorded. By means of transfer of excitation energy, the molar absorption coefficients of triplet beta-carotene were determined to be 23 000 dm(3) mol(-1).cm(-1) at 510 nm. The triplet lifetime for beta-carotene in acetonitrile solution was observed to be 15.6 mu s. The rate constant for the reaction of triplet energy transfer from triplet 2-acetonaphthone to beta-carotene was calculated to be 1.5 X 10(10) dm(3).mol(-1).s(-1). Obviously, the triplet beta-carotene has very low excitation energy. Taking the advantage of the photochemical properties of triplet beta-carotene, beta-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 beta-carotene.