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The thermal and oxidative degradation of carotenoids was studied in an oil model system to determine their relative stabilities and the major β-carotene isomers formed during the reaction. All-trans β-carotene, 9-cis β-carotene, lycopene, and lutein were heated in safflower seed oil at 75, 85, and 95 °C for 24, 12, and 5 h, respectively. The major isomers formed during heating of β-carotene were 13-cis, 9-cis, and an unidentified cis isomer. The degradation kinetics for the carotenoids followed a first-order kinetic model. The rates of degradation were as follows: lycopeneall-trans β-carotene≈9-cis β-carotenelutein. The values for the thermodynamic parameters indicate that a kinetic compensation effect exists between all of the carotenoids. These data suggest that lycopene was most susceptible to degradation and lutein had the greatest stability in the model system of the carotenoids tested. Furthermore, there was no significant difference in the rates of degradation for 9-cis and all-trans β-carotene under the experiment conditions.

In comparison with the all-trans and other cis isomers of β-carotene, the 11-cis and 11,11′-di-cis isomers exhibited the following unique properties. (1) The wavelengths of the Bu+←Ag- (0-0) absorption of these

Rates of thermal, cis-trans rearrangement among all-trans-, 15-cis-, and 13-cis-β-carotene have been measured at temperatures in the range 37-69°C. From the resulting specific rate constants, Arrhenius and Eyring parameters are derived. Positions of equilibrium are estimated experimentally and by force field calculations based on the Allinger MM2 program as improved by Roth (MM2-ERW), while enthalpies of activation for cis-trans isomerization to 11-, 9-, and 7-cis-β-carotenes are estimated by application of the Roth program augmented by the inclusion of the quantum mechanical program of Malrieu et al., EVBH (effective valence-bond Hamiltonian), expanded to encompass longer polyenes. Implications of the interaction of strain and delocalization in the rotation about the 7,8 double bond are presented. A procedure has been developed for the small scale preparation of 13-cis-β-carotene by heating all-trans-β-carotene at 80°C for 8 h. Kinetically and thermodynamically accessible at 37°C, 15-cis- or 13-cis-β-carotene or both become candidates for the role of true anticarcinogenic agent, whereupon all-trans-β-carotene would be relegated to the role of reservoir for the active species.

The products of triplet-sensitized photoisomerization (excitation at 337 nm of the sensitizer, anthracene) and thermal isomerization of β-carotene in n-hexane, starting from the all-trans, 7-cis, 9-cis, 13-cis, and 15-cis isomers, were analyzed by HPLC.Direct photoisomerization (excitation at 488 and 337 nm) was also examined for comparison.Three different isomerization patterns were found in both triplet-sensitized and thermal isomerization: pattern A, cis to trans isomerization around each cis bond; pattern B, trans to cis isomerization in the central part of the conjugated chain; and pattern C, cis to another cis isomerization.In the T1 state, the pattern A isomerization was predominant even for the peripheral-cis (7-cis and 9-cis) isomers and its efficiency was extremly high for the central-cis (13-cis and 15-cis) isomers.In the S0 state, the pattern B isomerization, instead, was predominant for the peripheral-cis isomers, and the pattern A isomerization was predominant only for the central-cis isomers.The quantum yields of triplet-sensitized isomerization (decrease of the starting isomer per triplet species generated) were determined to be as follows: all-trans, 0.04; 7-cis, 0.12; 9-cis, 0.15; 13-cis, 0.87; and 15-cis, 0.98.In direct photoisomerization, the quantum yield of isomerization at 488-nnm (337 nm) excitation was 4 (3) orders of magnitude lower than the above values, the relative values among the isomers being similar to the above.Further, the overall isomerization patterns of direct photoexcitation were similar to those of triplet-sensitized isomerization, supporting the idea that isomerization takes place via the T1 state even in the case of direct photoexcitation.Carbon-carbon ? bond orders of model polyenes in the T1 and S0 states were calculated by using the Pariser-Parr-Pople CI theorie; bond lengths were optimized by using a bond order-bond length relationship.Isomerization characteristics in the T1 and S0 states observed were discussed based on the results of the calculations.

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