The reaction of triplet flavin with indole. A study of the cascade of reactive intermediates using density functional theory and time resolved infrared spectroscopy.
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.
Martin
Christopher B
CB
Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, USA.
Tsao
Meng-Lin
ML
Hadad
Christopher M
CM
Platz
Matthew S
MS
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
D005415:Flavins; D007211:Indoles; D011506:Proteins; C001922:lumiflavin; C030374:indole; D012256:Riboflavin