Getoff, N. Pulse Radiolysis Studies of β-Carotene in Oxygenated DMSO Solution. Formation of β-Carotene Radical Cation.
The spectroscopic and kinetic characteristics of β-carotene radical cation (β-carotene·+) were studied by pulse radiolysis in aerated DMSO solution. The buildup of β-carotene·+ with k1 = (4.8 ± 0.2) × 108 dm3 mol–1 s–1 [λmax = 942 nm, ɛ = (1.6 ± 0.1) × 104 dm3 mol–1 cm–1] results from an electron transfer from β-carotene to DMSO·+. The β-carotene·+ species decays exclusively by first-order reaction, k = (2.1 ± 0.1) × 103 s–1, probably by two processes: (1) at low substrate concentration by hydrolysis and (2) at high concentrations also by formation of dimer radical cation (β-carotene)2·+. Under the experimental conditions, a small additional β-carotene triplet-state absorption (3β-carotene) in the range of 525 to 660 nm was observed. This triplet absorption is quenched by oxygen (k = 7 × 104 s–1), resulting in singlet oxygen (1O2), whose reactions can also lead to additional formation of β-carotene·+.