Transient electronic and vibrational signatures during reversible photoswitching of a cyanobacteriochrome photoreceptor

Sean R Tachibana; Longteng Tang; Cheng Chen; Liangdong Zhu; Yuka Takeda; Keiji Fushimi; Travis K Seevers; Rei Narikawa; Moritoshi Sato; Chong Fang

doi:10.1016/j.saa.2020.119379

Transient electronic and vibrational signatures during reversible photoswitching of a cyanobacteriochrome photoreceptor

Spectrochim Acta A Mol Biomol Spectrosc. 2021 Apr 5:250:119379. doi: 10.1016/j.saa.2020.119379. Epub 2020 Dec 23.

Authors

Affiliations

¹ Department of Chemistry, Oregon State University, 153 Gilbert Hall, Corvallis, OR 97331-4003, United States.
² Graduate School of Integrated Science and Technology, Shizuoka University, 422-8529 Shizuoka, Japan.
³ Graduate School of Integrated Science and Technology, Shizuoka University, 422-8529 Shizuoka, Japan; Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency, 332-0012 Saitama, Japan.
⁴ Graduate School of Arts and Sciences, University of Tokyo, 153-8902 Tokyo, Japan; Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Agency, 332-0012 Saitama, Japan.
⁵ Department of Chemistry, Oregon State University, 153 Gilbert Hall, Corvallis, OR 97331-4003, United States. Electronic address: Chong.Fang@oregonstate.edu.

PMID: 33401182
DOI: 10.1016/j.saa.2020.119379

Abstract

Cyanobacteriochromes (CBCRs) are an emerging class of photoreceptors that are distant relatives of the phytochromes family. Unlike phytochromes, CBCRs have gained popularity in optogenetics due to their highly diverse spectral properties spanning the UV to near-IR region and only needing a single compact binding domain. AnPixJg2 is a CBCR that can reversibly photoswitch between its red-absorbing (^15ZP_r) and green-absorbing (^15EP_g) forms of the phycocyanobilin (PCB) cofactor. To reveal primary events of photoconversion, we implemented femtosecond transient absorption spectroscopy with a homemade LED box and a miniature peristaltic pump flow cell to track transient electronic responses of the photoexcited AnPixJg2 on molecular time scales. The 525 nm laser-induced P_g-to-P_r reverse conversion exhibits a ~3 ps excited-state lifetime before reaching the conical intersection (CI) and undergoing further relaxation on the 30 ps time scale to generate a long-lived Lumi-G ground state intermediate en route to P_r. The 650 nm laser-induced P_r-to-P_g forward conversion is less efficient than reverse conversion, showing a longer-lived excited state which requires two steps with ~13 and 217 ps time constants to enter the CI region. Furthermore, using a tunable ps Raman pump with broadband Raman probe on both the Stokes and anti-Stokes sides, we collected the pre-resonance ground-state femtosecond stimulated Raman spectroscopy (GS-FSRS) data with mode assignments aided by quantum calculations. Key vibrational marker bands at ~850, 1050, 1615, and 1649 cm^-1 of the P_r conformer exhibit a notable blueshift to those of the P_g conformer inside AnPixJg2, reflecting the PCB chromophore terminal D (major) and A (minor) ring twist along the primary photoswitching reaction coordinate. This integrated ultrafast spectroscopy and computational platform has the potential to elucidate photochemistry and photophysics of more CBCRs and photoactive proteins in general, providing the highly desirable mechanistic insights to facilitate the rational design of functional molecular sensors and devices.

Keywords: Cyanobacteriochromes; Excited-state reaction coordinate; Femtosecond stimulated Raman; Photoconversion; Protein cofactor structural dynamics; Time-resolved laser spectroscopy.

MeSH terms

Bacterial Proteins
Electronics
Light
Photoreceptors, Microbial*
Phytochrome*

Substances

Bacterial Proteins
Photoreceptors, Microbial
Phytochrome