SINGLET OXYGEN QUENCHING ABILITIES OF CAROTENOIDS

The bimolecular rate constants k(q) for quenching of singlet oxygen (( 1) Delta(g) state) by 26 different natural and novel synthetic caroten oids were determined at 37 degrees C in a mixture of chloroform and et hanol. The steady-state technique used involves the generation of O-1( 2) by thermal decomposition of disodium 3,3'-naphtalene-1,4-diyl-dipro pionate endoperoxide (NDPO2) and the detection of its luminescence int ensity at 1270 nm. Excitation energies (pi, pi, 1(1)A(g) --> 1(1)B(u) ) and absorption maxima (430-590 nn) vary in the broadest range. Deepl y coloured blue carotenoids are also included in the studies for the f irst time. An empirical correlation between the pi,pi (1(1)A(g) --> 1 (1)B(u)) excitation energy and carotenoid structure (effective chain l ength N-eff) was found: E(S) = 12642 cm(-1) + 92027 cm(-1) x 1/N-eff. The quenching ability of the investigated carotenoids depends on the e xcitation energy of their transition at long wavelengths in a characte ristic way showing as Limiting factors either the thermal Arrhenius ac tivation or the diffusion-controlled rate. This dependence and the sus pected relationship between singlet E(S) and triplet E(T) energies, re spectively, are discussed.