Directed Electron Transfer in Flavin Peptides with Oligoproline‐Type Helical Conformation as Models for Flavin‐Functional Proteins

Abstract

To mimic the charge separation in functional proteins we studied flavin‐modified peptides as models. They were synthesized as oligoprolines that typically form a polyproline type‐II helix, because this secondary structure supports the electron transfer properties. We placed the flavin as photoexcitable chromophore and electron acceptor at the N‐terminus. Tryptophans were placed as electron donors to direct the electron transfer over 0–3 intervening prolines. Spectroscopic studies revealed competitive photophysical pathways. The reference peptide without tryptophan shows dominant non‐specific ET dynamics, leading to an ion pair formation, whereas peptides with tryptophans have weak non‐specific ET and intensified directed electron transfer. By different excitation wavelengths, we can conclude that the corresponding ion pair state of flavin within the peptide environment has to be energetically located between the S1 and S4 states, whereas the directed electron transfer to tryptophan occurs directly from the S1 state. These photochemical results have fundamental significance for proteins with flavin as redoxactive cofactor.

Flavin-induced charge separation in transmembrane model peptides

“…These observations align well with the TA spectra of flavin derivatives reported 26 and our previous investigations of flavin-modified amino acids with a different peptide secondary structure as a reference (polyproline helix instead of α-helix). 23 Global analysis identifies the primary feature as the lifetime of the first singlet excited state, exceeding 1.2 ns – the maximum delay achievable with our setup. This time constant is also evident for the Trp-modified peptides EGFR Fl 2–EGFR Fl 5 (for EGFR Fl 2 see Fig.…”

“…Even more quenching is observed for EGFR Fl 4 and EGFR Fl 5, with a quantum yield of 0.03 in MeCN : H 2 O (1 : 1) and 0.03-0.04 in POPC vesicles, due to the additional The absorption spectrum of the peptides is typical for flavin systems, with two characteristic band peaks at 360 and 445 nm. 23 Introducing additional Trp units into the peptide sequence does not significantly change the position or the shape of this moiety. Time-resolved absorption measurements were carried out for EGFR Fl and the Trp modified systems EGFR Fl 2-EGFR Fl 5.…”