title: Isomerization of Cholecalciferol through Energy Transfer as a Protective Mechanism Against Flavin-Sensitized Photooxidation.
abstract: 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.
authors.affiliation: †Instituto de Química de São Carlos, Universidade de São Paulo, Avenida Trabalhador São Carlense 400, CP 780, 13560-470 São Carlos, Brazil.
authors.forename: Regina S
authors.initials: RS
authors.lastname: Scurachio
authors.affiliation: †Instituto de Química de São Carlos, Universidade de São Paulo, Avenida Trabalhador São Carlense 400, CP 780, 13560-470 São Carlos, Brazil.
authors.forename: Willy G
authors.initials: WG
authors.lastname: Santos
authors.affiliation: †Instituto de Química de São Carlos, Universidade de São Paulo, Avenida Trabalhador São Carlense 400, CP 780, 13560-470 São Carlos, Brazil.
authors.forename: Eduardo S P do
authors.initials: ES
authors.lastname: Nascimento
authors.affiliation: ‡Food Chemistry, Department of Food Science, University of Copenhagen, Rolighedsvej 30, DK-1958 Frederiksberg C, Denmark.
authors.forename: Leif H
authors.initials: LH
authors.lastname: Skibsted
authors.affiliation: †Instituto de Química de São Carlos, Universidade de São Paulo, Avenida Trabalhador São Carlense 400, CP 780, 13560-470 São Carlos, Brazil.
authors.forename: Daniel R
authors.initials: DR
authors.lastname: Cardoso
mesh_terms: D002762:Cholecalciferol / Q000737:chemistry*; D004735:Energy Transfer / Q000528:radiation effects; D005415:Flavins / Q000737:chemistry*; D007536:Isomerism; D007700:Kinetics; D008027:Light; D010084:Oxidation-Reduction / Q000528:radiation effects; D010777:Photochemistry
chemical_list: D005415:Flavins; D002762:Cholecalciferol
keywords: 5,6-trans-vitamin D; photochemistry; riboflavin; vitamin D