Primary photoreactions of phylloquinone (vitamin K1) and plastoquinone-1 in solution

Abstract

The photoreactions of electron transport quinones vitamin K₁ (1) and plastoquinone-1 (2) were studied by picosecond pump–probe spectroscopy, nanosecond flash photolysis, step-scan FTIR spectroscopy, and by irradiation in glassy solvents at 77 K with optical and EPR detection. In polar solvents, charge transfer from the β,γ-double bond to the quinone moiety initiates intramolecular proton transfer from the side chain, k = 2.7 × 10¹¹ s⁻¹, yielding 1,3-quinone methide diradicals, which establish a metastable equilibrium between the singlet and the triplet state. Subsequent proton transfer through the solvent forms 1,2-quinone methides. In apolar solvents the predominant primary photoreaction is formation of cyclic ‘preoxetane’ diradicals (k_form >3 × 10¹¹ s⁻¹, k_decay ≈1 × 10⁹ s⁻¹), which revert to a photostationary E/Z mixture of the starting materials, unless they are trapped by oxygen. The β,γ-double bond in the isoprenoid side chain thus provides two efficient deactivation processes, which prevent the intermolecular photoreactions commonly observed with the parent quinones. A combined mechanistic scheme rationalises the known photoreactions of 1, 2 and related compounds in solution.