Ultrafast proton coupled electron transfer between tryptophan and tyrosine in peptides Trp-Pro_n-Tyr

A series of model peptides (Trp-Pro_n-Tyr, WP_nY, n=O, 1, 2, 3, 5), which contain tryptophan (Trp, W), tyrosine (Tyr, Y), and proline (Pro, P), have been studied under three typical pH conditions (3, 7, and 10) by steady-state absorption and fluorescence spectroscopy, nanosecond time-resolved fluorescence and femtosecond time-resolved transient absorption spectroscopy. When the peptide’s chain length is increased, Trp fluorescence quenching is expected to be gradually weakened. However, Trp fluorescence in WPY is strongly quenched and reveals even stronger quenching with increasing pH values, whose hypochromicity is clearly different from other model peptides. Transient absorption spectra also demonstrate that the excited state decay of WPY is much faster than that of other model peptides, especially at pH = 10. It is attributed to the efficient proton coupled electron transfer (PCET) between Trp and Tyr. Moreover, due to the very short distance between Trp and Tyr in WPY, this PCET process could be achieved by “direct transfer”, contrasted with the slow and long-range PCET process in other model peptides. Our results of the dipeptides WY and WP further suggest that Trp may also have more complex interactions with the peptide backbone or proline in those peptides. This work provides an experimental evidence for the electron transfer mechanism in WY dyads, which can help ones to understand how to reduce Trp radicals in proteins.