Photoisomerization action spectrum of retinal protonated Schiff base in the gas phase

Mode-Specific Photoresponse of Retinal Protonated Schiff Base Isomers in the Reversible Photochromic Reactions of Microbial and Animal Rhodopsins

The primary photoisomerization reactions of the all-trans to 13-cis and 11-cis to all-trans retinal protonated Schiff base (RPSB) in microbial and animal rhodopsins, respectively, occur on a subpicosecond time scale with high quantum yields. At the same time, the isolated RPSB exhibits slower excited-state decay, in particular, in its all-trans form, and hence the interaction with the protein environment is capable of changing the time scale as well as the specificity of the reaction. Here, by using the high-level QM/MM calculations, we provide a comparative study of the primary photoresponse of cis and trans RPSB isomers in both the initial forms and first photoproducts of microbial Krokinobacter eikastus rhodopsin 2 (KR2) and Halobacterium salinarum bacteriorhodopsin (BR), and animal Bos taurus visual rhodopsin (Rho). By simulating photoabsorption band shapes of RPSB inside the proteins, we show that its photoresponse is highly mode-specific for the forward reactions, resulting in excitation of those vibrational modes that facilitate particular double-bond isomerization. The reverse reaction shows specificity only for 13-cis isomers in microbial rhodopsins, whereas the specificity is lost for all-trans RPSB in visual rhodopsin. This indicates evolutionary highly tuned 11-cis chromophore-protein interactions in visual rhodopsin. We also highlight the differences in the photoresponse of RPSB in two microbial rhodopsins and discuss the implications to their excited-state dynamics.

UV-vis spectroscopy of gas-phase ions

Photodissociation action spectroscopy has made a great progress in expanding investigations of gas-phase ion structures. This review deals with aspects of gas-phase ion electronic excitations that result in wavelength-dependent dissociation and light emission via fluorescence, chiefly covering the ultraviolet and visible regions of the spectrum. The principles are briefly outlined and a few examples of instrumentation are presented. The main thrust of the review is to collect and selectively present applications of UV-vis action spectroscopy to studies of stable gas-phase ion structures and combinations of spectroscopy with ion mobility, collision-induced dissociation, and ion-ion reactions leading to the generation of reactive intermediates and electronic energy transfer.

Photo-induced 6π-electrocyclisation and cycloreversion of isolated dithienylethene anions

The diarylethene chromophore is commonly used in light-triggered molecular switches. The chromophore undergoes reversible 6π-electrocyclisation (ring closing) and cycloreversion (ring opening) reactions upon exposure to UV and visible light, respectively, providing bidirectional photoswitching. Here, we investigate the gas-phase photoisomerisation of meta- (m) and para- (p) substituted dithienylethene carboxylate anions (DTE^-) using tandem ion mobility mass spectrometry coupled with laser excitation. The ring-closed forms of p-DTE^- and m-DTE^- are found to undergo cycloreversion in the gas phase with maximum responses associated with bands in the visible (λ_max ≈ 600 nm) and the ultraviolet (λ_max ≈ 360 nm). The ring-open p-DTE^- isomer undergoes 6π-electrocyclisation in the ultraviolet region at wavelengths shorter than 350 nm, whereas no evidence is found for the corresponding electrocyclisation of ring-open m-DTE^-, a situation attributed to the fact that the antiparallel geometry required for electrocyclisation of m-DTE^- is energetically disfavoured. This highlights the influence of the carboxylate substitution position on the photochemical properties of DTE molecules. We find no evidence for the formation in the gas phase of the undesirable cyclic byproduct, which causes fatigue of DTE photoswitches in solution.

Light Driven Ultrafast Bioinspired Molecular Motors: Steering and Accelerating Photoisomerization Dynamics of Retinal

Photoisomerization of retinal protonated Schiff base in microbial and animal rhodopsins are strikingly ultrafast and highly specific. Both protein environments provide conditions for fine-tuning the photochemistry of their chromophores. Here, by combining time-resolved action absorption spectroscopy and high-level electronic structure theory, we show that similar control can be gained in a synthetically engineered retinal chromophore. By locking the dimethylated retinal Schiff base at the C11═C12 double bond in its trans configuration (L-RSB), the excited-state decay is rendered from a slow picosecond to an ultrafast subpicosecond regime in the gas phase. Steric hindrance and pretwisting of L-RSB are found to be important for a significant reduction in the excited-state energy barriers, where isomerization of the locked chromophore proceeds along C9═C10 rather than the preferred C11═C12 isomerization path. Remarkably, the accelerated excited-state dynamics also becomes steered. We show that L-RSB is capable of unidirectional 360° rotation from all-trans to 9-cis and from 9-cis to all-trans in only two distinct steps induced by consecutive absorption of two 600 nm photons. This opens a way for the rational design of red-light-driven ultrafast molecular rotary motors based on locked retinal chromophores.

Trajectory surface hopping molecular dynamics simulations for retinal protonated Schiff-base photoisomerization

Global switching trajectory surface hopping molecular dynamics simulations are performed using accurate on-the-fly (TD)CAM-B3LYP/6-31G potential energy surfaces to study retinal protonated Schiff-base photoisomerization up to S₁ excitation. The simulations detected two-layer conical intersection networks: one is at an energy as high as 8 eV and the other is in the energy range around 3-4 eV. Six conical intersections within the low-layer energy region that correspond to active conical intersections under experimental conditions are found via the use of pairwise isomers, within which nonadiabatic molecular dynamics simulations are performed. Eight isomer products are populated with simulated sampling trajectories from which the simulated quantum yield in the gas phase is estimated to be 0.11 (0.08) moving from the all-trans isomer to the 11-cis (11-cis to all-trans) isomer in comparison with an experimental value of 0.09 (0.2) in the solution phase. Each conical intersection is related to one specific twist angle accompanying a related CC double bond motion during photoisomerization. Nonplanar distortion of the entire dynamic process has a significant role in the formation of the relevant photoisomerization products. The present simulation indicates that all hopping points show well-behaved potential energy surface topology, as calculated via the conventional TDDFT method, at conical intersections between S₁ and S₀ states. Therefore, the present nonadiabatic dynamics simulations with the TDDFT method are very encouraging for simulating various large systems related to retinal Schiff-base photoisomerization in the real world.

Ultrafast photoisomerisation of an isolated retinoid

The photoinduced excited state dynamics of gas-phase trans-retinoate (deprotonated trans-retinoic acid, trans-RA^-) are studied using tandem ion mobility spectrometry coupled with laser spectroscopy, and frequency-, angle- and time-resolved photoelectron imaging. Photoexcitation of the bright S₃(ππ*) ← S₀ transition leads to internal conversion to the S₁(ππ*) state on a ≈80 fs timescale followed by recovery of S₀ and concomitant isomerisation to give the 13-cis (major) and 9-cis (minor) photoisomers on a ≈180 fs timescale. The sub-200 fs stereoselective photoisomerisation parallels that for the retinal protonated Schiff base chromophore in bacteriorhodopsin. Measurements on trans-RA^- in methanol using the solution photoisomerisation action spectroscopy technique show that 13-cis-RA^- is also the principal photoisomer, although the 13-cis and 9-cis photoisomers are formed with an inverted branching ratio with photon energy in methanol when compared with the gas phase, presumably due to solvent-induced modification of potential energy surfaces and inhibition of electron detachment processes. Comparison of the gas-phase time-resolved data with transient absorption spectroscopy measurements on retinoic acid in methanol suggest that photoisomerisation is roughly six times slower in solution. This work provides clear evidence that solvation significantly affects the photoisomerisation dynamics of retinoid molecules.

Intrinsic photoisomerization dynamics of protonated Schiff-base retinal

The retinal protonated Schiff-base (RPSB) in its all-trans form is found in bacterial rhodopsins, whereas visual rhodopsin proteins host 11-cis RPSB. In both cases, photoexcitation initiates fast isomerization of the retinal chromophore, leading to proton transport, storage of chemical energy or signaling. It is an unsolved problem, to which degree this is due to protein interactions or intrinsic RPSB quantum properties. Here, we report on time-resolved action-spectroscopy studies, which show, that upon photoexcitation, cis isomers of RPSB have an almost barrierless fast 400 fs decay, whereas all-trans isomers exhibit a barrier-controlled slow 3 ps decay. Moreover, formation of the 11-cis isomer is greatly favored for all-trans RPSB when isolated. The very fast photoresponse of visual photoreceptors is thus directly related to intrinsic retinal properties, whereas bacterial rhodopsins tune the excited state potential-energy surface to lower the barrier for particular double-bond isomerization, thus changing both the timescale and specificity of the photoisomerization.

The primary photoresponse of protonated Schiff-base retinal in visual and bacterial rhodopsins is fast sub-ps isomerisation. Here, the authors show that the fast photoisomerization of rhodopsin is related to an intrinsic retinal property, whereas bacterial rhodopsins tune the excited-state potential-energy surface and improve the isomerization timescale and specificity.

Action and Ion Mobility Spectroscopy of a Shortened Retinal Derivative

The development of tandem ion mobility spectroscopy (IMS) known as IMS-IMS has led to extensive research into isomerizations of isolated molecules. Many recent works have focused on the retinal chromophore which is the optical switch used in animal vision. Here, we study a shortened derivative of the chromophore, which exhibits a rich IM spectrum allowing for a detailed analysis of its isomerization pathways, and show that the longer the chromophore is, the lower the barrier energies for isomerization are. Graphical Abstract.

Tautomerism and electronic spectroscopy of protonated 1- and 2-aminonaphthalene

Protonation sites can be controlled by the electrospray source as written in the figure.

Ion Mobility Collision Cross Section Compendium

In this review, we focus on an important aspect of ion mobility (IM) research, namely the reporting of quantitative ion mobility measurements in the form of the gas-phase collision cross section (CCS), which has provided a common basis for comparison across different instrument platforms and offers a unique form of structural information, namely size and shape preferences of analytes in the absence of bulk solvent. This review surveys the over 24,000 CCS values reported from IM methods spanning the era between 1975 to 2015, which provides both a historical and analytical context for the contributions made thus far, as well as insight into the future directions that quantitative ion mobility measurements will have in the analytical sciences. The analysis was conducted in 2016, so CCS values reported in that year are purposely omitted. In another few years, a review of this scope will be intractable, as the number of CCS values which will be reported in the next three to five years is expected to exceed the total amount currently published in the literature.

Isomerisation of an intramolecular hydrogen-bonded photoswitch: protonated azobis(2-imidazole)

Reversible E–Z photoswitching of a protonated azoheteroarene is demonstrated using ion mobility mass spectrometry.

Measurements of the stabilities of isolated retinal chromophores

The barrier energies for isomerization and fragmentation were measured for a series of retinal chromophore derivatives using a tandem ion mobility spectrometry approach. These measurements allow us to quantify the effect of charge delocalization on the rigidity of chromophores. We find that the role of the methyl group on the C13 position is pivotal regarding the ground state dynamics of the chromophore. Additionally, a correlation between quasi-equilibrium isomer distribution and fragmentation pathways is observed.

Charged Particle Imaging of the Deprotonated Octatrienoic Acid Anion: Evidence for a Photoinduced Cyclization Reaction

Photoelectron spectroscopy of the deprotonated octatrienoic acid anion, [C₇H₉-CO₂]^-, shows the formation of [C₇H₉]^- and loss of H^- at hν = 4.13 eV. Using velocity map imaging, the H^- fragment was characterized to have a Boltzmann-like kinetic energy distribution consistent with dissociation on a ground electronic state. Similar dynamics were not observed at hν = 4.66 eV even though there is clear evidence for recovery of the ground electronic state of [C₇H₉-CO₂]^-. In accord with supporting electronic structure calculations, the production of H^- at hν = 4.13 eV is explained by excited-state dissociation of CO₂ to form [C₇H₉]^-, which subsequently undergoes a ring-closure isomerization reaction to yield toluene and H^-. These data represent the first evidence for a photoinduced ring-closing isomerization reaction in an anionic polyene and provides an interesting example of the rich anion dynamics that can occur in the detachment continuum and that can influence photochemistry.

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 chain's 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.

Infrared Spectroscopy of Mobility-Selected H+-Gly-Pro-Gly-Gly (GPGG)

We report the first results from a new instrument capable of acquiring infrared spectra of mobility-selected ions. This demonstration involves using ion mobility to first separate the protonated peptide Gly-Pro-Gly-Gly (GPGG) into two conformational families with collisional cross-sections of 93.8 and 96.8 Å(2). After separation, each family is independently analyzed by acquiring the infrared predissociation spectrum of the H(2)-tagged molecules. The ion mobility and spectroscopic data combined with density functional theory (DFT) based molecular dynamics simulations confirm the presence of one major conformer per family, which arises from cis/trans isomerization about the proline residue. We induce isomerization between the two conformers by using collisional activation in the drift tube and monitor the evolution of the ion distribution with ion mobility and infrared spectroscopy. While the cis-proline species is the preferred gas-phase structure, its relative population is smaller than that of the trans-proline species in the initial ion mobility drift distribution. This suggests that a portion of the trans-proline ion population is kinetically trapped as a higher energy conformer and may retain structural elements from solution.

Direct measurement of the isomerization barrier of the isolated retinal chromophore

Isomerizations of the retinal chromophore were investigated using the IMS-IMS technique. Four different structural features of the chromophore were observed, isolated, excited collisionally, and the resulting isomer and fragment distributions were measured. By establishing the threshold activation voltages for isomerization for each of the reaction pathways, and by measuring the threshold activation voltage for fragmentation, the relative energies of the isomers as well as the energy barriers for isomerization were determined. 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.

Photoisomerization action spectroscopy: flicking the protonated merocyanine–spiropyran switch in the gas phase

Molecular ions drifting through buffer gas are exposed to tunable laser light causing a detectable change in their collision cross section and arrival time at the ion detector. The effect can be exploited to obtain action electronic spectra for selected molecular isomers in the gas phase.

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

Isomers of the retinal protonated Schiff base are separated and probed using laser radiation in a tandem ion mobility spectrometer yielding isomer-specific electronic spectra.

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.