Exploring the reactivity of retinol radical cation toward organic and biological molecules: A laser flash photolysis study

Vitamin A (retinol) and various natural retinoids are essential for life. Under oxidative conditions, vitamin A rad- ical cation (RET•+) can be formed. Many deleterious effects were reported about the formation of carotenoid rad- ical cations in biological environments, on the other hand, little is known about the consequences of the RET•+ formation in these environments. Therefore, it is important to explore the reactivity of RET•+ toward various bi- ological substrates. Here, we employed nanosecond laser flash photolysis (LFP) to generate RET•+ (λmax=580nm in methanol) and examine its reactivity toward a wide range of biological molecules including amino acids, vita- mins, carotenoids, naturally-occurring phenols, neurotransmitters such as catecholamines, wide range of phenol derivatives and some selected electron-donors. The results show that the reactivity of RET•+ toward various sub- strates is strongly dependent on the polarity of solvent. In addition, RET•+ is able to oxidize amino acids, which subsequently can lead to protein damage. However, the presence of vitamins (vitamins E and C), carotenoids and naturally-occurring phenols (e.g. resveratrol, vanillin, dopamine hydrochloride and L-Dopa) can inhibit the dam- aging effect of retinol•+ by reducing it back to retinol. Vitamin E and carotenoids are the most efficient quenchers for the RET•+ (diffusion-controlled reactions). Importantly, our results clearly indicate that the reactivity of RET•+ is as strong as that of the powerful trichloromethylperoxyl radical (CCl3O2•). Thereby, formation of RET•+ in biological media is expected to induce bio-damage.