Article Absorption wavelength along chromophore low-barrier hydrogen bonds

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 pRCW state of photoactive yellow protein and between Asp116 and the retinal Schiff base in the intermediate M-state of the sodiumpumping rhodopsin KR2. The H+ displacement of only similar to 0.4 A degrees, which does not easily occur without low-barrier H-bonds, is responsible for the similar to 50 nm-shift in the absorption wavelength. This may be a basis of how photoreceptor proteins have evolved to proceed photocycles using abundant protons.

Automated summary

In protein environments, the formation of low-barrier H-bonds can lead to a shift in absorption wavelength, providing a basis for how photoreceptor proteins use protons in photocycles.