Neville J. A. Coughlan

An ion mobility mass spectrometer for investigating photoisomerization and photodissociation of molecular ions.

An ion mobility mass spectrometry apparatus for investigating the photoisomerization and photodissociation of electrosprayed molecular ions in the gas phase is described. The device consists of a drift tube mobility spectrometer, with access for a laser beam that intercepts the drifting ion packet either coaxially or transversely, followed by a quadrupole mass filter. An ion gate halfway along the drift region allows the instrument to be used as a tandem ion mobility spectrometer, enabling mobility selection of ions prior to irradiation, with the photoisomer ions being separated over the second half of the drift tube. The utility of the device is illustrated with photoisomerization and photodissociation action spectra of carbocyanine molecular cations. The mobility resolution of the device for singly charged ions is typically 80 and it has a mass range of 100-440 Da, with the lower limit determined by the drive frequency for the ion funnels, and the upper limit by the quadrupole mass filter.

Ion Mobility Unlocks the Photofragmentation Mechanism of Retinal Protonated Schiff Base

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.

Retinal shows its true colours: photoisomerization action spectra of mobility-selected isomers of the retinal protonated Schiff base.

Retinal is one of Natures most important and widespread chromophores, exhibiting remarkable versatility in its function and spectral response, depending on its protein environment. Reliable spectroscopic and photochemical data for the isolated retinal molecule are essential for calibrating theoretical approaches that seek to model retinals behaviour in complex protein environments. However, due to low densities and possible co-existence of multiple isomers, retinal is a challenging target for gas-phase investigations. Here, the photoisomerization behaviour of the trans isomer of the retinal protonated Schiff base (RPSB) is investigated in the gas phase by irradiating mobility-selected RPSB ions with tunable light in a tandem ion mobility spectrometer. trans RPSB ions are converted to single cis isomers and also more compact isomers through irradiation with visible light. The S1← S0 photoisomerization action spectrum of trans RPSB, obtained by monitoring production of cis isomers as a function of wavelength, exhibits a single well-defined peak with a maximum at 618 ± 5 nm. Corresponding action spectra of cis RPSB isomers exhibit broader peaks, conclusively demonstrating an isomeric dependence for the RPSB spectrum in the gas phase.

Monitoring Isomerization of Molecules in Solution Using Ion Mobility Mass Spectrometry

An ion mobility spectrometer (IMS) with an electrospray ion source is used to investigate photo and thermal isomerization of photoactive molecules in the electrospray syringe. A light emitting diode adjacent to the syringe establishes a photostationary state that relaxes thermally toward the more stable isomer once illumination ceases. The arrangement is demonstrated by measuring Z-E thermal isomerization rates for several azoheteroarene compounds. The IMS technique has a distinct advantage over UV-vis spectrophotometry for measuring isomer populations in situations where there are multiple isomers with overlapping absorption profiles. In another development, an LED array adjacent to the silica capillary connecting the syringe to the electrospray ion source, is used to activate photochromic molecules, and investigate sequential photoswitching events.

Photoisomerization of Protonated Azobenzenes in the Gas Phase

Because of their high photoisomerization efficiencies, azobenzenes and their functionalized derivatives are used in a broad range of molecular photoswitches. Here, the photochemical properties of the trans isomers of protonated azobenzene (ABH+) and protonated 4-aminoazobenzene (NH2ABH+) cations are investigated in the gas phase using a tandem ion mobility spectrometer. Both cations display a strong photoisomerization response across their S1 ← S0 bands, with peaks in their photoisomerization yields at 435 and 525 nm, respectively, red-shifted with respect to the electronic absorption bands of the unprotonated AB and NH2AB molecules. The experimental results are interpreted with the aid of supporting electronic structure calculations considering the relative stabilities and geometries of the possible isomers and protomers and vertical electronic excitation energies.

Protomer-Specific Photochemistry Investigated Using Ion Mobility Mass Spectrometry

The utility of tandem ion mobility mass spectrometry coupled with electronic spectroscopy to investigate protomer-specific photochemistry is demonstrated by measuring the photoisomerization response for protomers of protonated 4-dicyanomethylene-2-methyl-6-para-dimethylaminostyryl-4H-pyran (DCM) molecules. The target DCMH+ species has three protomers that are distinguished by their different collision cross sections with He, N2, and CO2 buffer gases, trends in abundance with ion source conditions, and from their photoisomerization responses. The trans-DCMH+ protomers with the proton located either on the tertiary amine N atom or on a cyano group N atom exhibit distinct S1← S0 photoisomerization responses, with the maxima in their photoisomerization action spectra occurring at 420 and 625 nm, respectively, consistent with predictions from accompanying electronic structure calculations. The cis-DCMH+ protomers are not distinguishable from one another through ion mobility separation and give no discernible photoisomerization or photodissociation response, suggesting the dominance of other deactivation pathways such as fluorescence. The study demonstrates that isobaric protomers and isomers generated by an electrospray ion source can possess quite different photochemical behaviors and emphasizes the utility of isomer and protomer selective techniques for exploring the spectroscopic and photochemical properties of protonated molecules in the gas phase.

Photo and Collision Induced Isomerization of a Cyclic Retinal Derivative: An Ion Mobility Study

A cationic degradation product, formed in solution from retinal Schiff base (RSB), is examined in the gas phase using ion mobility spectrometry, photoisomerization action spectroscopy, and collision induced dissociation (CID). The degradation product is found to be N-n-butyl-2-(β-ionylidene)-4-methylpyridinium (BIP) produced through 6π electrocyclization of RSB followed by protonation and loss of dihydrogen. Ion mobility measurements show that BIP exists as trans and cis isomers that can be interconverted through buffer gas collisions and by exposure to light, with a maximum response at λ = 420 nm.Graphical Abstract

Photoisomerization of β-Ionone Protonated Schiff Base in the Gas Phase

The photoisomerization of β-ionone protonated Schiff base (BIPSB) is investigated in the gas phase by irradiating mobility-selected ions in a tandem ion mobility spectrometer with tunable radiation. Four distinguishable isomers are produced by electrospray ionization whose structures are deduced from their collision cross sections and photoisomerization behavior along with density functional theory calculations. They include two geometric isomers of BIPSB with trans or cis configurations about the polyene chains terminal C═N double bond, a bicyclic structure formed through electrocyclization of the polyene chain, and a Z-retro-γ-ionone isomer. Although trans-BIPSB and 9-cis-BIPSB have similar photoisomerization action spectra, with a maximum response at 375 nm, they photoconvert to different isomers. The trans-BIPSB isomer transforms to the bicyclic form upon exposure to light over the 320-400 nm range, whereas the cis-BIPSB isomer is prevented by steric hindrance from forming the bicyclic BIPSB isomer following irradiation and is proposed instead to form the 7,9-di-cis isomer. Neither the bicyclic isomer nor the Z-retro-γ-ionone isomer respond strongly to near-UV light.

Reversible Photoisomerization of the Isolated Green Fluorescent Protein Chromophore

Fluorescent proteins have revolutionized the visualization of biological processes, prompting efforts to understand and control their intrinsic photophysics. Here we investigate the photoisomerization of deprotonated p-hydroxybenzylidene-2,3-dimethylimidazolinone anion (HBDI-), the chromophore in green fluorescent protein and in Dronpa protein, where it plays a role in switching between fluorescent and nonfluorescent states. In the present work, isolated HBDI- molecules are switched between the Z and E forms in the gas phase in a tandem ion mobility mass spectrometer outfitted for selecting the initial and final isomers. Excitation of the S1 ← S0 transition provokes both Z → E and E → Z photoisomerization, with a maximum response for both processes at 480 nm. Photodetachment is a minor channel at low light intensity. At higher light intensities, absorption of several photons in the drift region drives photofragmentation, through channels involving CH3 loss and concerted CO and CH3CN loss, although isomerization remains the dominant process.

Seleniranium ions undergo π-ligand exchange via an associative mechanism in the gas phase

Collision-induced dissociation mass spectrometry of the ammonium ions 4a and 4b results in the formation of the seleniranium ion 5, the structure and purity of which were verified using gas-phase infrared spectroscopy coupled to mass spectrometry and gas-phase ion-mobility measurements. Ion-molecule reactions between the ion 5 (m/z = 261) and cyclopentene, cyclohexene, cycloheptene, and cyclooctene resulted in the formation of the seleniranium ions 7 (m/z = 225), 6 (m/z = 239), 8 (m/z = 253), and 9 (m/z = 267), respectively. Further reaction of seleniranium 6 with cyclopentene resulted in further π-ligand exchange giving seleniranium ion 7, confirming that direct π-ligand exchange between seleniranium ion 5 and cycloalkenes occurs in the gas phase. Pseudo-first-order kinetics established relative reaction efficiencies for π-ligand exchange for cyclopentene, cyclohexene, cycloheptene. and cyclooctene as 0.20, 0.07, 0.43, and 4.32. respectively. DFT calculations at the M06/6-31+G(d) level of theory provide the following insights into the mechanism of the π-ligand exchange reactions; the cycloalkene forms a complex with the seleniranium ion 5 with binding energies of 57 and 62 kJ/mol for cyclopentene and cyclohexene, respectively, with transition states for π-ligand exchange having barriers of 17.8 and 19.3 kJ/mol for cyclopentene and cyclohexene, respectively.