title: The reaction of triplet flavin with indole. A study of the cascade of reactive intermediates using density functional theory and time resolved infrared spectroscopy.
abstract: As a model for riboflavin, lumiflavin was investigated using density functional theory methods (B3LYP/6-31G* and B3LYP/6-31+G**) with regard to the proposed cascade of intermediates formed after excitation to the triplet state, followed by electron-transfer, proton-transfer, and radical[bond]radical coupling reactions. The excited triplet state of the flavin is predicted to be 42 kcal/mol higher in energy than the singlet ground state, and the pi radical anion lies 45.1 kcal/mol lower in energy than the ground-state flavin and a free electron in the gas phase. The former value compares to a solution-phase triplet energy of 49.8 kcal/mol of riboflavin. For the radical anion, the thermodynamically favored position to accept a proton on the flavin ring system is at N(5). A natural population analysis also provided spin density information for the radicals and insight into the origin of the relative stabilities of the six different calculated hydroflavin radicals. The resulting 5H-LF* radical can then undergo radical[bond]radical coupling reactions, with the most thermodynamically stable adduct being formed at C(4'). Vibrational spectra were also calculated for the transient species. Experimental time-resolved infrared spectroscopic data obtained using riboflavin tetraacetate are in excellent agreement with the calculated spectra for the triplet flavin, the radical anion, and the most stable hydroflavin radical.
authors.affiliation: Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, USA.
authors.forename: Christopher B
authors.initials: CB
authors.lastname: Martin
authors.forename: Meng-Lin
authors.initials: ML
authors.lastname: Tsao
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Hadad
authors.forename: Matthew S
authors.initials: MS
authors.lastname: Platz
mesh_terms: D005415:Flavins / Q000737:chemistry*; D007211:Indoles / Q000737:chemistry*; D015394:Molecular Structure; D010777:Photochemistry; D011506:Proteins / Q000737:chemistry; D012256:Riboflavin / Q000737:chemistry*; D013055:Spectrophotometry, Infrared / Q000379:methods; D013816:Thermodynamics
chemical_list: D005415:Flavins; D007211:Indoles; D011506:Proteins; C001922:lumiflavin; C030374:indole; D012256:Riboflavin