Spectroscopy and photoisomerization of protonated Schiff-base retinal derivatives in vacuo.

The protonated Schiff-base retinal acts as the chromophore in bacteriorhodopsin as well as in rhodopsin. In both cases, photoexcitation initializes fast isomerization which eventually results in storage of chemical energy or signaling. The details of the photophysics for this important chromophore is still not fully understood. In this study, action-absorption spectra and photoisomerization dynamics of three retinal derivatives are measured in the gas phase and compared to that of the protonated Schiff-base retinal. The retinal derivatives include C9C10 trans-locked, C13C14 trans-locked and a retinal derivative without the β-ionone ring. The spectroscopy as well as the isomerization speed of the chromophores are altered significantly as a consequence of the steric constraints.