SPECTRAL AND PHOTOCHEMICAL PROPERTIES OF CURCUMIN

Curcumin, hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione, is a natu ral yellow-orange dye derived from the rhizome of Curcuma longa, an Ea st Indian plant. In order to understand the photobiology of curcumin b etter we have studied the spectral and photochemical properties of bot h curcumin and 4-(4-hydroxy-3-methoxyphenyl)-3-buten-2-one (hC, half c urcumin) in different solvents. In toluene, the absorption spectrum of curcumin contains some structure, which disappears in more polar solv ents, e.g. ethanol, acetonitrile. Curcumin fluorescence is a broad ban d in acetonitrile (lambda(max) = 524 nm), ethanol (lambda(max) = 549 n m) or micellar solution (lambda(max) = 557 nm) but has some structure in toluene (lambda(max) = 460, 488 nm). The fluorescence quantum yield of curcumin is low in sodium dodecyl sulfate (SDS) solution (phi = 0. 011) but higher in acetonitrile (phi = 0.104). Curcumin produced singl et oxygen upon irradiation (lambda > 400 nm) in toluene or acetonitril e (phi = 0.11 for 50 mu M curcumin); in acetonitrile curcumin also que nched O-1(2) (k(q) = 7 x 10(6) M(-1) s(-1)). Singlet oxygen production was about 10 times lower in alcohols and was hardly detectable when c urcumin was solubilized in a D2O micellar solution of Triton X-100. In SDS micelles containing curcumin no singlet oxygen phosphorescence co uld be observed. Curcumin photogenerates superoxide in toluene and eth anol, which was detected using the electron paramagnetic resonance/spi n-trapping technique with 5,5-dimethyl-pyrroline-N-oxide as a trapping agent. Unidentified carbon-centered radicals were also detected. Thes e findings indicate that the spectral and photochemical properties of curcumin are strongly influenced by solvent. In biological systems, si nglet oxygen, superoxide and products of photodegradation may all part icipate in curcumin phototoxicity depending on the environment of the dye.