Excited-state barrier controls E → Z photoisomerization in p-hydroxycinnamate biochromophores

Molecules based on the deprotonated p-hydroxycinnamate moiety are widespread in nature, including serving as UV filters in the leaves of plants and as the biochromophore in photoactive yellow protein. The photophysical behavior of these chromophores is centered around a rapid E → Z photoisomerization by passage through a conical intersection seam. Here, we use photoisomerization and photodissociation action spectroscopies with deprotonated 4-hydroxybenzal acetone (pCK–) to characterize a wavelength-dependent bifurcation between electron autodetachment (spontaneous ejection of an electron from the S1 state because it is situated in the detachment continuum) and E → Z photoisomerization. While autodetachment occurs across the entire S1(ππ*) band (370–480 nm), E → Z photoisomerization occurs only over a blue portion of the band (370–430 nm). No E → Z photoisomerization is observed when the ketone functional group in pCK– is replaced with an ester or carboxylic acid. The wavelength-dependent bifurcation is consistent with potential energy surface calculations showing that a barrier separates the Franck–Condon region from the E → Z isomerizing conical intersection. The barrier height, which is substantially higher in the gas phase than in solution, depends on the functional group and governs whether E → Z photoisomerization occurs more rapidly than autodetachment

Action spectroscopy of the isolated red Kaede fluorescent protein chromophore

Incorporation of fluorescent proteins into biochemical systems has revolutionized the field of bioimaging. In a bottom-up approach, understanding the photophysics of fluorescent proteins requires detailed investigations of the light-absorbing chromophore, which can be achieved by studying the chromophore in isolation. This paper reports a photodissociation action spectroscopy study on the deprotonated anion of the red Kaede fluorescent protein chromophore, demonstrating that at least three isomers–assigned to deprotomers–are generated in the gas phase. Deprotomer-selected action spectra are recorded over the S1 ← S0 band using an instrument with differential mobility spectrometry coupled with photodissociation spectroscopy. The spectrum for the principal phenoxide deprotomer spans the 480–660 nm range with a maximum response at ≈610 nm. The imidazolate deprotomer has a blue-shifted action spectrum with a maximum response at ≈545 nm. The action spectra are consistent with excited state coupled-cluster calculations of excitation wavelengths for the deprotomers. A third gas-phase species with a distinct action spectrum is tentatively assigned to an imidazole tautomer of the principal phenoxide deprotomer. This study highlights the need for isomer-selective methods when studying the photophysics of biochromophores possessing several deprotonation sites

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

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 <i>trans</i> or <i>cis</i> configurations about the polyene chain’s terminal CN double bond, a bicyclic structure formed through electrocyclization of the polyene chain, and a <i>Z</i>-retro-γ-ionone isomer. Although <i>trans</i>-BIPSB and 9-<i>cis</i>-BIPSB have similar photoisomerization action spectra, with a maximum response at 375 nm, they photoconvert to different isomers. The <i>trans</i>-BIPSB isomer transforms to the bicyclic form upon exposure to light over the 320–400 nm range, whereas the <i>cis</i>-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-<i>cis</i> isomer. Neither the bicyclic isomer nor the <i>Z</i>-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

Excited-State Barrier Controls <i>E</i> → <i>Z</i> Photoisomerization in <i>p</i>‑Hydroxycinnamate Biochromophores

Molecules based on the deprotonated p-hydroxycinnamate moiety are widespread in nature, including serving as UV filters in the leaves of plants and as the biochromophore in photoactive yellow protein. The photophysical behavior of these chromophores is centered around a rapid E → Z photoisomerization by passage through a conical intersection seam. Here, we use photoisomerization and photodissociation action spectroscopies with deprotonated 4-hydroxybenzal acetone (pCK–) to characterize a wavelength-dependent bifurcation between electron autodetachment (spontaneous ejection of an electron from the S1 state because it is situated in the detachment continuum) and E → Z photoisomerization. While autodetachment occurs across the entire S1(ππ*) band (370–480 nm), E → Z photoisomerization occurs only over a blue portion of the band (370–430 nm). No E → Z photoisomerization is observed when the ketone functional group in pCK– is replaced with an ester or carboxylic acid. The wavelength-dependent bifurcation is consistent with potential energy surface calculations showing that a barrier separates the Franck–Condon region from the E → Z isomerizing conical intersection. The barrier height, which is substantially higher in the gas phase than in solution, depends on the functional group and governs whether E → Z photoisomerization occurs more rapidly than autodetachment

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 <i>trans</i> 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

Seleniranium Ions Undergo π‑Ligand Exchange via an Associative Mechanism in the Gas Phase

Collision-induced dissociation mass spectrometry of the ammonium ions <b>4a</b> and <b>4b</b> results in the formation of the seleniranium ion <b>5</b>, 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 <b>5</b> (<i>m</i>/<i>z</i> = 261) and cyclopentene, cyclohexene, cycloheptene, and cyclooctene resulted in the formation of the seleniranium ions <b>7</b> (<i>m</i>/<i>z</i> = 225), <b>6</b> (<i>m</i>/<i>z</i> = 239), <b>8</b> (<i>m</i>/<i>z</i> = 253), and <b>9</b> (<i>m</i>/<i>z</i> = 267), respectively. Further reaction of seleniranium <b>6</b> with cyclopentene resulted in further π-ligand exchange giving seleniranium ion <b>7</b>, confirming that direct π-ligand exchange between seleniranium ion <b>5</b> 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 <b>5</b> 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