2.2. Determination of Singlet Oxygen Quenching by Dehydrolutein, Lutein, and Zeaxanthin

To determine the rate of singlet oxygen quenching, singlet oxygen was generated by photoexcitation of all-trans-retinal solution in benzene with a 5-ns laser pulse of the third harmonic of a Q-switched Nd:YAG laser (Continuum Surelite II-10, Photonic Solutions Plc., Edinburgh, UK), and monitored by detecting its infrared emission at 1270 nm by a liquid-nitrogen-cooled germanium diode (Applied Detectors Co., Fresno, CA, USA), connected to an Agilent 54830B digitizer (Agilent Technologies UK Ltd., Cheadle, UK) and analyzed by a RISC workstation operating the LKS.60 nanosecond time-resolved laser photolysis spectrometer (Applied Photophysics, Leatherhead, UK) [19,20,40]. The absorbance of 5.3 µM all-trans-retinal in a 1-cm square cuvette was about 0.17 at the excitation wavelength of 355 nm for all the solutions. The rate constant for the quenching of singlet oxygen by carotenoids was determined by quantifying the rate of decay of its infrared phosphorescence in the presence of different carotenoid concentrations [18].