β-Carotene As a Lipophilic Scavenger of Nitric Oxide.

The efficient bleaching following continuous bubbling of gaseous nitric oxide (NO(•)) to β-carotene (β-Car) dissolved in n-hexane under anaerobic conditions results from an initial addition of two NO(•) followed by fragmentation coupled with further NO(•) addition as shown by mass spectrometry (MS). Density functional theory (DFT) calculations demonstrated that hydrogen atom transfer (HAT) and electron transfer (ET) from β-Car to NO(•) are strongly energetically unfavorable in contrast to radical adduct formation (RAF) followed by degradation. The results indicated the lowest energy for addition of the first NO(•) at C7 with an activation free energy of ΔG(≠) = 74.40 kJ mol(-1) and a rate constant of 0.56 s(-1), followed by trans-addition of a second NO(•) at C8 with ΔG(≠) = 55.51 kJ mol(-1). MS confirmed the formation of a dinitrosyl-β-Car (596.6 m/z), and of a β-Car fragment (400.4 m/z) formed by C7/C8 bond cleavage and suggested to be of importance for progression of bleaching. Up to eight reaction products with increasing mass of 28 m/z are assigned to continuous addition of NO(•) to the initially formed fragment forming nitroxides. Continuous wave photolysis of sodium nitroprusside (SNP) as a NO(•) source dissolved together with β-Car in 4:1 (v/v) methanol:tetrahydrofuran gradually bleached β-Car. Nanosecond laser flash photolysis at 355 nm followed by transient absorption spectroscopy showed a β-Car derived intermediate with an absorption maximum around 420 nm in agreement with a prediction (425 nm) from time-dependent DFT (TDDFT) for the trans-C7,8 dinitrosyl adduct of β-Car. The NO(•) adduct of β-Car decays with a rate constant of ∼10(7) s(-1) at 25 °C.