Direct Measurement of the Isomerization Barrier of the Isolated Retinal Chromophore.

摘要

Ion mobility spectroscopy (IMS) allows one to differentiate between different isomers of a given molecular ion according to their collisional cross-section. Using two stages of IMS (IMS-IMS) one can select a specific isomer, collisionally heat it and follow its isomerization pathways. Recently it has been shown that this technique allows one to determine internal energy barrier for isomerization. Here we apply the technique to the important case of the retinal protonated Schiff base (RPSB). Photoisomerization of the RPSB is the primary in animal vision. We find that the energy barrier for a single cis-trans isomerization is 0.64-0.05 eV, which is significantly lower than that observed for the reaction within opsin proteins. Thus the protein has a significant role in increasing the barrier energy for thermal isomerization relative to the gas phase which lacks interaction with the RPSB counterion and steric constraints. High barrier energy is mandatory for efficient vision processes, otherwise thermal noise would overwhelm the signal originating from the photochemical isomerization.