Schiff base deprotonation and structural changes in a mouse UV-sensitive cone visual pigment revealed by FTIR spectroscopy at 77 K

Abstract

UV-sensitive cone visual pigments are widespread among vertebrates, including birds, fish, and rodents such as mice, and play essential roles in non-primate vision. Unlike visible-light pigments, which contain a protonated retinal Schiff base and absorb at 400-700 nm, UV pigments maintain a deprotonated Schiff base, but how this state is stabilized has remained unresolved.

Here, we applied low-temperature FTIR spectroscopy to the mouse UV-sensitive cone visual pigment (MUV), capturing structural changes associated with Batho intermediate formation. Spectral features characteristic of a deprotonated Schiff base were observed in both the initial and Batho states. Importantly, analysis of the Glu113 mutant demonstrated that Glu113 is protonated under these condition. Combined with the analysis of protein-bound water signals, these results indicate that the hydroxyl group of Glu113 serves as a direct hydrogen-bond donor to the deprotonated Schiff base. Moreover, the Glu113 C=O stretching vibration appeared at an unusually low frequency, revealing the presence of an exceptionally strong hydrogen bond with its surrounding protein environment. Comparison with bovine rhodopsin and cone pigments further revealed that MUV binds fewer water molecules near the retinal. This reduction in hydration suggests that a more hydrophobic environment contributes to lowering the Schiff base pKa and stabilizing its deprotonated state. Sequence analyses across species support this view, highlighting conserved nonpolar residues in transmembrane helix 2 (TM2) that likely disrupt hydrogen-bonding networks and promote UV sensitivity. Together, these findings establish MUV as a model for understanding how specific amino acid environments and hydration patterns enable UV vision in vertebrates.

Caption of Graphical Abstract

Low-temperature FTIR spectroscopy of the mouse UV-sensitive cone visual pigment (MUV) revealed vibrational features of a deprotonated Schiff base stabilized by an exceptionally strong hydrogen bond involving Glu113. Structural modeling and spectral analysis indicate that MUV possesses fewer internal water molecules than visible-light pigments such as MB and bovine rhodopsin (bRh), resulting in a more hydrophobic environment around the retinal. Sequence comparison across vertebrate UV pigments shows conserved nonpolar residues in transmembrane helix 2 that likely disrupt hydrogen-bonding networks, thereby lowering the Schiff base pKa and enabling ultraviolet light absorption.