Intramolecular Excited-State Hydrogen Transfer in Rutin and Quercetin

Steady-state and time-resolved optical spectroscopy techniques were employed to study the excited-state intramolecular hydrogen transfer (ESIHT) in rutin, a flavonol that contains a disaccharide and is a natural product in plants. The results are compared with those of a similar natural compound, quercetin, which also has a flavonol structure. The fluorescence decay signal of the normal form of these two compounds is composed of three time components. The ESIHT rate in both compounds has a time constant of 70 fs or less. The ESIHT processes of both compounds show a distinctive kinetic isotope effect of 1.5 or more. The intermediate and long-time components are about 300 fs and a few picoseconds, respectively, for both compounds. The amplitude of the intermediate component in rutin is twice that of quercetin. We explain this difference as arising from the hydrogen bonding of the glucose in rutin to the ESIHT active site.