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Isomerization of Cholecalciferol through Energy Transfer as a Protective Mechanism Against Flavin-Sensitized Photooxidation

Scurachio, Regina S., Santos, Willy G., Nascimento, Eduardo S. P. do, Skibsted, Leif H., Cardoso, Daniel R.
Journal of agricultural and food chemistry 2015 v.63 no.18 pp. 4629-4637
alcohols, cholecalciferol, energy transfer, fluorescence emission spectroscopy, hydrophobicity, isomerization, nutrients, oxygen, photolysis, photooxidation
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 × 108 L mol–1 s–1 in 7/3 tert-butyl alcohol/water (v/v) at 25 °C, assigned to energy transfer from 3FMN* 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.