Spectroscopy of cryogenic protonated Schiff-base retinal derivatives

Retinal protonated Schiff base (RPSB) is the active chromophore in opsin proteins, including rhodopsin for vision. Yet, the spectral consequences of geometric constraints imposed by the protein environment remain insufficiently characterised. We report on gas-phase action-absorption spectra of six retinal analogues with defined steric modifications, recorded in an electrostatic ion-storage ring after cooling in a cryogenic ion trap. Analogues bearing out-of-plane distortions or a shortened π-conjugated polyene chain exhibit pronounced blue-shifts in their absorption maxima. We further present the spectrum of a cryogenically cooled RPSB photofragment of mass 248 amu, whose absorption band near 370 nm matches that of a synthesised β-ionone protonated Schiff base, consistent with substantial truncation of the polyene system. These results isolate the intrinsic spectral signatures of constrained RPSB geometries and provide a framework for understanding protein-induced tuning in opsins.