Triplets, radical cations and neutral semiquinone radicals of lumiflavin and riboflavin: An overhaul of previous pump–probe data and new multichannel absolute absorption spectra

Though long-lived transient photoproducts of flavins have received much attention over the last five decades, their spectroscopic characterization remains rudimentary and altogether inconclusive. Lumiflavin and riboflavin have therefore been reexamined, using nanosecond laser photolysis and multichannel detection over a wide spectral range (230–900 nm); differential absorption spectra of flavin solutions in aqueous buffer at neutral pH have been recorded; radical cations and semiquinone radicals were generated by quenching the triplets with C (NO2)4 (electron acceptor) and NO 2 - (electron donor), respectively. The problem of estimating the extent of ground state depletion after a single flash has been solved by developing a new strategy that provides reliable lower and upper bounds for the depletion (and for the molar absorption coefficients of the transient species of interest). The spectra of the neutral radicals and triplets of the two flavins are almost identical, but those of the two cations are not.