Absorption wavelength along chromophore low-barrier hydrogen bonds

Masaki Tsujimura; Hiroyuki Tamura; Keisuke Saito; Hiroshi Ishikita

doi:10.1016/j.isci.2022.104247

Absorption wavelength along chromophore low-barrier hydrogen bonds

iScience. 2022 Apr 13;25(5):104247. doi: 10.1016/j.isci.2022.104247. eCollection 2022 May 20.

Authors

Masaki Tsujimura¹, Hiroyuki Tamura^{2

3}, Keisuke Saito^{2

3}, Hiroshi Ishikita^{2

3}

Affiliations

¹ Department of Advanced Interdisciplinary Studies, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan.
² Department of Applied Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8654, Japan.
³ Research Center for Advanced Science and Technology, Graduate School of Engineering, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8904, Japan.

Abstract

In low-barrier hydrogen bonds (H-bonds), the pK _a values for the H-bond donor and acceptor moieties are nearly equal, whereas the redox potential values depend on the H⁺ position. Spectroscopic details of low-barrier H-bonds remain unclear. Here, we report the absorption wavelength along low-barrier H-bonds in protein environments, using a quantum mechanical/molecular mechanical approach. Low-barrier H-bonds form between Glu46 and p-coumaric acid (pCA) in the intermediate pR_CW state of photoactive yellow protein and between Asp116 and the retinal Schiff base in the intermediate M-state of the sodium-pumping rhodopsin KR2. The H⁺ displacement of only ∼0.4 Å, which does not easily occur without low-barrier H-bonds, is responsible for the ∼50 nm-shift in the absorption wavelength. This may be a basis of how photoreceptor proteins have evolved to proceed photocycles using abundant protons.

Keywords: chemistry; computational chemistry; optical materials.