Distance-dependent electron- and energy transfer between tryptophan- and flavin-substituents in biomimetic oligo-proline diads

Oligo-proline-based diads with chain lengths between 3 and 12 prolines (Pro) between a tryptophan (Trp) and an isoalloxazine residue were studied for their photochemical properties. The proline backbone was chosen due to its capacity to adopt a stable helical (poly-pro-II) structure. Steady-state and time-resolved absorption and fluorescence measurements reveal energy- and electron transfer from Trp to the isoalloxazine moiety with efficiencies depending on the proline chain length. The quantum yield of triplet formation (φT) was similar for the longer compounds but was considerably lower for Pro-3 and Pro-4. In all diads, a small absorbance at 660 nm indicated the formation of a long-lived neutral flavin radical. Steady-state fluorescence measurements (λex = 450 nm) showed a quantum yield (φF) for Pro-3 smaller than that of flavin mononucleotide. That for Pro-4 matched exactly the value of FMN, whereas φF for Pro-6, Pro-9, and Pro-12 grew gradually. Exciting the Trp component (λex = 295 nm) revealed that φF, Trp was smaller for all diads than that of flavin-devoided Pro-6, pointing to energy transfer to the flavin moiety. The flavin fluorescence, instead, increased up to Pro-6, then decreased for Pro-9 and Pro-12. The fluorescence lifetime for all diads was ca. 6.7 ns (λex = 450 nm), except for Pro-3 and Pro-4, which showed a biexponential decay with an additional, shorter component of 0.9 and 2.6 ns, respectively. The biexponential decays of these two shortest diads, together with their lower φT, point to deactivation channels by electron transfer and/or Förster Resonance Energy Transfer (FRET) from Trp to the singlet excited state of the flavin.