School of Chemistry - Research Publications

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Author: Bieske, Evan × Author: Catani, Katherine × Author: Wille, Uta × Start date: 2014 × Type: Journal Article ×

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    Photoisomerization action spectrum of retinal protonated Schiff base in the gas phase
    Coughlan, NJA ; Catani, KJ ; Adamson, BD ; Wille, U ; Bieske, EJ (AMER INST PHYSICS, 2014-04-28)
    The photophysical behaviour of 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 by electrospray ionisation into an ion mobility mass spectrometer where they are exposed to tunable laser radiation in the region of the S1 ← S0 transition (420-680 nm range). Four peaks are observed in the arrival time distribution of the RPSB ions. On the basis of predicted collision cross sections with nitrogen gas, the dominant peak is assigned to the all-trans isomer, whereas the subsidiary peaks are assigned to various single, double and triple cis geometric isomers. RPSB ions that absorb laser radiation undergo photoisomerization, leading to a detectable change in their drift speed. By monitoring the photoisomer signal as a function of laser wavelength an action spectrum, extending from 480 to 660 nm with a clear peak at 615 ± 5 nm, is obtained. The photoisomerization action spectrum is related to the absorption spectrum of isolated retinal RPSB molecules and should help benchmark future electronic structure calculations.
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    Ion Mobility Unlocks the Photofragmentation Mechanism of Retinal Protonated Schiff Base
    Coughlan, NJA ; Adamson, BD ; Catani, KJ ; Wille, U ; Bieske, EJ (AMER CHEMICAL SOC, 2014-09-18)
    Retinal protonated Schiff base (RPSB) is a key molecular component of biological photoreceptors and bacterial photosynthetic structures, where its action involves photoisomerization around bonds in the polyene chain. In a vacuum environment, collisional activation or exposure to visible light causes the RPSB molecule to disintegrate, producing charged molecular fragments with m/z = 248 Da that cannot be formed by simple cleavage of the polyene chain. Photofragments resulting from laser excitation of RPSB at a wavelength of 532 nm are analyzed in an ion mobility mass spectrometer (IMMS) and found to be the protonated Schiff base of β-ionone. Density functional theory calculations at the M06-2X/cc-pVDZ level support a fragmentation mechanism in which RPSB undergoes an electrocyclization/fragmentation cascade with the production of protonated Schiff base of β-ionone and toluene.