Carotenoid radical ions: A laser flash photolysis study

Ali El-Agamey; Thor B Melø; Maha A El-Hagrasy; Vassilia Partali; Shunichi Fukuzumi

doi:10.1016/j.jphotobiol.2020.112023

Carotenoid radical ions: A laser flash photolysis study

J Photochem Photobiol B. 2020 Nov:212:112023. doi: 10.1016/j.jphotobiol.2020.112023. Epub 2020 Sep 10.

Authors

Ali El-Agamey¹, Thor B Melø², Maha A El-Hagrasy³, Vassilia Partali⁴, Shunichi Fukuzumi⁵

Affiliations

¹ Department of Physics, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway; Chemistry Department, Faculty of Science, Damietta University, New Damietta, Damietta, Egypt; Department of Chemistry, Faculty of Science, King Faisal University, Al-Ahsa, Saudi Arabia. Electronic address: aelagamey@kfu.edu.sa.
² Department of Physics, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway.
³ Chemistry Department, Faculty of Science, Damietta University, New Damietta, Damietta, Egypt.
⁴ Department of Chemistry, Norwegian University of Science and Technology (NTNU), N-7491 Trondheim, Norway.
⁵ Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Republic of Korea; Faculty of Science and Technology, Meijo University, ALCA and SENTAN, Japan Science and Technology Agency (JST), Nagoya, Aichi 468-8502, Japan.

PMID: 32980657
DOI: 10.1016/j.jphotobiol.2020.112023

Abstract

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.

Keywords: Carotenoid; Chlorophyll b; Flash photolysis; Ketyl radical; Radical anion; Radical cation.

MeSH terms

Carotenoids / chemistry*
Lasers*
Oxidation-Reduction
Photolysis*

Substances

Carotenoids