Absorption wavelength along chromophore low-barrier hydrogen bonds.

In low-barrier hydrogen bonds (H-bonds), the p 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 -coumaric acid (CA) in the intermediate pR 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.