Carotenoid radical ions: A laser flash photolysis study
Laser excitation of a single precursor, namely 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (HHEMP), has been used for generating the radical cations and radical anions of various carotenoids in methanol. In the presence of oxygen, laser excitation of HHEMP undergoes an efficient α-cleavage reaction (Norrish type I) to form acyl radicals, which react with O2, in a nearly diffusion-controlled reaction, to form their corresponding strong oxidizing acylperoxyl radicals (RO2 •) (E = ~1.1 V (v SHE)), which are capable of oxidizing almost all carotenoids. Under argon-saturated conditions and in the presence of strong base (0.01 M NaOH or tetrabutylammonium hydroxide (TBAOH)), the initially formed 2-hydroxy-2-propyl radical (ACH•), generated after LFP of HHEMP, is deprotonated to form the strong reducing acetone ketyl radical (AC•-) (E {acetone/ AC•-} = −2.1 V (v SHE)), which is capable of reducing all carbonyl-containing carotenoids. To validate this new proposed approach, retinal and β-apo-8′-carotenal (APO), with known spectroscopic data, were investigated in methanol, acetonitrile and tetrahydrofuran (THF). In addition, the radical ions of newly investigated carotenoids, namely 4-oxo-β-apo-15′-carotenoic acid (4-oxo-15′), crocetindial, 4-oxo-β-apo-10′-carotenoic acid ethyl ester (4-oxo-10′) and 4-oxo-β-apo-8′-carotenoic acid ethyl ester (4-oxo-8′) have been reported. Moreover, the scope of this approach has been extended to investigate the radical ions of chlorophyll b.