Ultrafast Excited-state Deactivation of Flavins Bound to Dodecin

Abstract

Background: Dodecin prevents riboflavin from autodegradation and exerting photo-induced cellular stress. Results: Ultrafast depopulation of excited states and ground state recovery is observed by transient spectroscopy. Conclusion: Ultrafast electron transfer in combination with proton transfer is responsible for deactivation of flavin excited states. Significance: A comprehensive study of parameters in the binding pocket affecting the riboflavin quenching mechanism is given.

Directed Manipulation of a Flavoprotein Photocycle

“…Riboflavin (Rf) is stacked to W36 at a distance of 3.3 Å. Hydrogen bonding to Q55 is highlighted in gray. An observed deuteration effect on the lifetimes of the intermediate states suggested that a proton transfer underlies the photocycle hν → τ 1 → τ 2 3. This is in line with quantum‐chemical calculations performed in this study, which indicate that the riboflavin is stabilized in its neutral form by proton exchange with neighboring water molecules, both in the semiquinone as well as in the oxidized ground state (see the Supporting Information).…”

“…In the key step of the relaxation process of the light‐activated riboflavin, an electron of tryptophan W36 is transferred to the excited flavin, generating a charge‐separated intermediate state that subsequently recombines to the ground state. Recently, we were able to assign time constants to the individual processes in the photocycle of dodecin: 1) charge separation faster than the resolution of the experiment (<0.2 ps, τ 1 ); 2) electron back‐transfer with a time constant of 0.9 ps ( τ 2 ); (3) a relaxation process with 6 ps parallel to (2) with an intermediate absorbing at 500 nm ( τ 3 ); and (4) proton transfer from the surrounding water coupled with the electron‐transfer/back‐transfer cycle (Scheme ) 3. Based on high‐resolution X‐ray structural data and a concise functional characterization, establishing a system of extraordinarily well‐defined structure–function relationships, we considered dodecin as excellently suited for modulating biological electron‐transfer reactions by rational protein design, thereby studying the protein in a manipulative manner.…”

“…In a previous work where we had characterized wild‐type dodecin in complex with the different flavins, we did not find spectral differences in transient data, and thus we assume that the even smaller structural changes in noncanonical dodecin analogues similarly do not affect spectroscopic properties (Figure 2 C). 3 In other words, we use a series of highly isomorphous dodecins that only differ in the electronic properties modulated by the ionization potential of W36.…”

Resonance energy transfer between FMN molecules in the presence of dimers: A review

“…Other potential functions might be light harvesting or protection against UV radiation by aromatic tetrade which is formed by sandwiching of the six riboflavin dimers [33]. Recently Staudt et al [34] showed that riboflavin forms antiparallel stacked dimers in archeal dodecin in which the distance between the two isoalloxazine moieties is 3.5 Å. Exactly the same distance between the monomer units in flavin dimers was obtained by Grajek et al in solutions (3.5 ± 0.3 Å in water [35] and 3.2 ± 0.3 Å in PVA [36,37]).…”

“…Flavins take part in electron transport [34,77,78] and Förster nonradiative excitation energy transfer. Numerous spectroscopic examinations have shown that flavins participate in Förster nonradiative excitation energy transfer processes from indole to flavin [77], from tryptophan to FAD [79,80] and between salicylic acid and riboflavin [81].…”