Electronic Spectrum of the Retinal Protonated Schiff Base in the Gas Phase

Abstract

Retinal and its anologues are among the most important photoactive molecules in biology, playing key roles in visual receptors and photosynthesis in bacteria. Disturbingly, despite considerable experimental and theoretical efforts, no consensus has been reached concerning the absorption wavelength of the RPSB in the gas phase and the influence of the molecular conformation on the absorption spectrum. Here, the isolated retinal protonated n-butylamine Schiff base (RPSB) is investigated in the gas phase using a combination of ion mobility spectrometry and laser spectroscopy. The RPSB cations are introduced into an ion mobility mass spectrometer where they are exposed to tuneable laser radiation over the 410-700 nm range. RPSB ions that absorb laser radiation undergo photoisomerization, leading to a detectable change in their speed. By monitoring the photo-isomer signal as a function of laser wavelength an action spectra is obtained that is directly related to the absorption spectrum of isolated retinal molecules. The new spectrum should serve as a benchmark for future electronic structure calculations of the isolated RPSB molecule.