• Energy Research

Cholecalciferol, vitamin D3, was found to isomerize to 5,6-trans-vitamin-D3 with a quantum yield of 0.15 ± 0.01 in air-saturated 7/3 tert-butyl alcohol/water (v/v) at 25 °C, increasing to 0.32 ± 0.02 in the absence of oxygen, through quenching of triplet excited state flavin mononucleotide, FMN, rather than becoming oxidized. The quenching was found by laser flash photolysis to have a rate constant of 1.4 × 10(8) L mol(-1) s(-1) in 7/3 tert-butyl alcohol/water (v/v) at 25 °C, assigned to energy transfer from (3)FMN* to form a reactive vit.D3 diradical. vit.D3 forms a 1/1 precomplex with FMN by hydrophobic stacking with ΔH° = -36 ± 7 kJ mol(-1) and ΔS° = -4 ± 3 J mol(-1) K(-1), as shown by single photon counting fluorescence spectroscopy and steady-state fluorescence spectroscopy. Both ground-state precomplex formation and excited-state energy transfer seem important for vit.D3 protection against flavin-sensitized photooxidation of nutrients in food and biological systems.