Absorption Wavelength Along Chromophore Low-Barrier Hydrogen Bonds

In low-barrier hydrogen bonds (H-bonds), the pKa 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 Schiff base in the intermediate M-state of the sodium-pumping rhodopsin KR2. The absorption wavelength remains constant at ~460 nm until H+ arrives at the pCA moiety. As H+ moves ~0.4 A and arrives at the pCA moiety, the absorption wavelength is discontinuously shortened to ~420 nm. The step-function-like switching mechanism may be advantageous in the adoption of the specific absorption wavelength of the signaling state prior to time-consuming large structural changes during the photoreceptor photocycle. This may be a basis of how photoreceptor proteins have evolved to proceed photocycles using abundant protons.