An ultrafast phototrigger of the Trp5CN–Trp motif in a β-hairpin peptide

Abstract

Phototriggers are useful molecular tools to initiate reactions in enzymes by light for the purpose of photoenzymatic design and mechanistic investigations. Here, we incorporated the non-natural amino acid 5-cyanotryptophan (W5CN) in a polypeptide scaffold and resolved the photochemical reaction of the W5CN–W motif using femtosecond transient UV/Vis and mid-IR spectroscopy. We identified a marker band of ∼2037 cm−1 from the CN stretch of the electron transfer intermediate W5CN·− in the transient IR measurement and found UV/Vis spectroscopic evidence for the W·+ radical at 580 nm. Through kinetic analysis, we characterized that the charge separation between the excited W5CN and W occurs in 253 ps, with a charge-recombination lifetime of 862 ps. Our study highlights the potential use of the W5CN–W pair as an ultrafast phototrigger to initiate reactions in enzymes that are not light-sensitive, making downstream reactions accessible to femtosecond spectroscopic detection.

Triple-Bond Vibrations: Emerging Applications in Energy and Biological Sciences

“…Because of the aforementioned reasons, the CN stretching vibration is ideally suited to interrogating the dynamics of photoinduced charge transfer or separation via ultrafast optical-pump–IR-probe spectroscopic techniques. For example, it has been extensively used to investigate the dynamics of the locally excited (LE) and the intramolecular charge-transfer (ICT) states of 4-dimethylaminobenzonitrile (DMABN) (Figure , (5)), − as well as other systems. − More recently, the CN stretching vibration has also been used to assess various charge generation and separation steps in OPV devices, as demonstrated in several studies. , For A-D-A type of acceptors, an excited-state symmetry breaking (ES-SB) process, which leads the initially symmetrically distributed electronic excitation to become localized on one side (i.e., on one A) of the molecule, can occur under appropriate solvent conditions. ,− An example is the recent study of Vauthey and co-workers, who investigated the ES-SB properties of two A-D-A molecules (Figure , (6) and (7)), where D is a pyrrolo-pyrrole moiety and A is either a cyanophenyl or a dicyanovinyl group, using time-resolved vibrational spectroscopy. By comparing the transient absorption spectra of the CN group as well as other results obtained on these two systems, they concluded that although dicyanovinyl is a stronger EW moiety, ES-SB is not observed with the dicyanovinyl-based molecule even in highly polar media due to the large electronic coupling between the corresponding D and A branches.…”

“…Finally, findings showing that the ε CN of aromatic nitrile can be significantly enhanced upon photoexcitation , suggest new applications where this vibrational mode of an appropriate cyanotryptophan can be used to study various protein conformational dynamics and processes at relatively low protein concentrations via 2D IR spectroscopy in conjunction with an ultrafast optical action pulse. Taking 4-cyanotryptophan as an example, its electronic transition molar extinction coefficient is ca.…”