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Photofragmentation spectra of protonated vanillin produced under electrospray ionisation (ESI) conditions have been recorded in the 3000-3700 cm-1 (vibrational) and 225-460 nm (electronic) ranges, using room temperature IRMPD (infrared multiphoton dissociation) and cryogenic UVPD (ultraviolet photodissociation) spectroscopies, respectively. The cold (â¼50 K) electronic UVPD spectrum exhibits very well resolved vibrational structure for the S1 â S0 and S3 â S0 transitions, suggesting long excited state dynamics, similar to its simplest analogue, protonated benzaldehyde. The experimental data were combined with theoretical calculations to determine the protonation site and configurational isomer observed in the experiments.
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CO2 is a major component of the icy mantles surrounding dust grains in planet and star formation regions. Understanding its photodesorption is crucial for explaining gas phase abundances in the coldest environments of the interstellar medium irradiated by vacuum-UV (VUV) photons. Photodesorption yields determined experimentally from CO2 samples grown at low temperatures (T = 15 K) have been found to be very sensitive to experimental methods and conditions. Several mechanisms have been suggested for explaining the desorption of CO2, O2 and CO from CO2 ices. In the present study, the cross-sections characterizing the dynamics of photodesorption as a function of photon fluence (determined from released molecules in the gas phase) and of ice composition modification (determined in situ in the solid phase) are compared for the first time for different photon flux conditions (from 7.3 × 1012 photon per s cm-2 to 2.2× 1014 photon per s cm-2) using monochromatic synchrotron radiation in the VUV range (on the DESIRS beamline at SOLEIL). This approach reveals that CO and O2 desorptions are decorrelated from that of CO2. CO and O2 photodesorption yields depend on photon flux conditions and can be linked to surface chemistry. In contrast, the photodesorption yield of CO2 is independent of the photon flux conditions and can be linked to bulk ice chemical modification, consistently with indirect desorption induced by an electronic transition (DIET) process.
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Dipole-bound states in anions exist when a polar neutral core binds an electron in a diffuse orbital through charge-dipole interaction. Electronically excited polar neutral cores can also bind an electron in a diffuse orbital to form Core-Excited Dipole-Bound States (CE-DBSs), which are difficult to observe because they usually lie above the electron detachment threshold, leading to very short lifetimes and, thus, unstructured transitions. We report here the photodetachment spectroscopy of cryogenically cooled acetylacetonate anion (C5H7O2-) recorded by detecting the neutral radical produced upon photodetachment and the infrared spectroscopy in He-nanodroplets. Two DBSs were identified in this anion. One of them lies close to the electron detachment threshold (â¼2.74 eV) and is associated with the ground state of the radical (D0-DBS). Surprisingly, the other DBS appears as resonant transitions at 3.69 eV and is assigned to the CE-DBS associated with the first excited state of the radical (D1-DBS). It is proposed that the resonant transitions of the D1-DBS are observed â¼1 eV above the detachment threshold because its lifetime is determined by the internal conversion to the D0-DBS, after which the fast electron detachment takes place.
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A signaling complex comprising members of the LORELEI (LRE)-LIKE GPI-anchored protein (LLG) and Catharanthus roseus RECEPTOR-LIKE KINASE 1-LIKE (CrRLK1L) families perceive RAPID ALKALINIZATION FACTOR (RALF) peptides and regulate growth, reproduction, immunity, and stress responses in Arabidopsis (Arabidopsis thaliana). Genes encoding these proteins are members of multigene families in most angiosperms and could generate thousands of signaling complex variants. However, the links between expansion of these gene families and the functional diversification of this critical signaling complex as well as the evolutionary factors underlying the maintenance of gene duplicates remain unknown. Here, we investigated LLG gene family evolution by sampling land plant genomes and explored the function and expression of angiosperm LLGs. We found that LLG diversity within major land plant lineages is primarily due to lineage-specific duplication events, and that these duplications occurred both early in the history of these lineages and more recently. Our complementation and expression analyses showed that expression divergence (i.e. regulatory subfunctionalization), rather than functional divergence, explains the retention of LLG paralogs. Interestingly, all but one monocot and all eudicot species examined had an LLG copy with preferential expression in male reproductive tissues, while the other duplicate copies showed highest levels of expression in female or vegetative tissues. The single LLG copy in Amborella trichopoda is expressed vastly higher in male compared to in female reproductive or vegetative tissues. We propose that expression divergence plays an important role in retention of LLG duplicates in angiosperms.
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Arabidopsis , Embriófitas , Magnoliopsida , Arabidopsis/metabolismo , Família Multigênica , Fosfotransferases/genética , Sementes/metabolismo , Embriófitas/genética , Magnoliopsida/genética , Magnoliopsida/metabolismo , Proteínas/genética , Duplicação Gênica , Evolução Molecular , FilogeniaRESUMO
The photodetachment and stability of R-Mandelate, the deprotonated form of the R-Mandelic acid, was investigated by observing the neutral species issued from either simple photodetachment or dissociative photodetachment in a cold anions set-up. R-Mandalate has the possibility to form an intramolecular ionic hydrogen-bond between adjacent hydroxyl and carboxylate groups. The potential energy surface along the proton transfer (PT) coordinate between both groups (O- H+ - OCO) features a single local minima, with the proton localized on the O- group (OH - OCO). However, the structure with the proton localized on the - OCO group (O- HOCO) is also observed because it falls within the extremity of the vibrational wavefunction of the OH - OCO isomer along the PT coordinate. The stability of the corresponding radicals, produced upon photodetachment, is strongly dependent on the position of the proton in the anion: the radicals produced from the OH - OCO isomer decarboxylate without barrier, while the radicals produced from the O- HOCO isomer are stable.
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Hidrogênio , Prótons , Isomerismo , Ácidos Mandélicos , Ânions/químicaRESUMO
The infrared excitation and photodesorption of carbon monoxide (CO) and water-containing ices have been investigated using the FEL-2 free-electron laser light source at the FELIX laboratory, Radboud University, The Netherlands. CO-water mixed ices grown on a gold-coated copper substrate at 18 K were investigated. No CO photodesorption was observed, within our detection limits, following irradiation with light resonant with the C-O vibration (4.67 µm). CO photodesorption was seen as a result of irradiation with infrared light resonant with water vibrational modes at 2.9 µm and 12 µm. Changes to the structure of the water ice, which modifies the environment of the CO in the mixed ice, were also seen subsequent to irradiation at these wavelengths. No water desorption was observed at any wavelength of irradiation. Photodesorption at both wavelengths is due to a single-photon process. Photodesorption arises due to a combination of fast and slow processes of indirect resonant photodesorption (fast), and photon-induced desorption resulting from energy accumulation in the librational heat bath of the solid water (slow) and metal-substrate-mediated laser-induced thermal desorption (slow). Estimated cross-sections for the slow processes at 2.9 µm and 12 µm were found to be â¼7.5 × 10-18 cm2 and â¼4.5 × 10-19 cm2, respectively.
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The photophysics of biochromophore ions often depends on the isomeric or protomeric distribution, yet this distribution, and the individual isomer contributions to an action spectrum, can be difficult to quantify. Here, we use two separate photodissociation action spectroscopy instruments to record electronic spectra for protonated forms of the green (pHBDI+) and cyan (Cyan+) fluorescent protein chromophores. One instrument allows for cryogenic (T = 40 ± 10 K) cooling of the ions, while the other offers the ability to perform protomer-selective photodissociation spectroscopy. We show that both chromophores are generated as two protomers when using electrospray ionisation, and that the protomers have partially overlapping absorption profiles associated with the S1 â S0 transition. The action spectra for both species span the 340-460 nm range, although the spectral onset for the pHBDI+ protomer with the proton residing on the carbonyl oxygen is red-shifted by ≈40 nm relative to the lower-energy imine protomer. Similarly, the imine and carbonyl protomers are the lowest energy forms of Cyan+, with the main band for the carbonyl protomer red-shifted by ≈60 nm relative to the lower-energy imine protomer. The present strategy for investigating protomers can be applied to a wide range of other biochromophore ions.
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Subunidades Proteicas , Subunidades Proteicas/química , Análise Espectral , Proteínas de Fluorescência Verde/química , Íons/químicaRESUMO
Recent experimental work revealed that the lifetime of the S3 state of protonated 7-azaindole is about ten times longer than that of protonated 6-azaindole. We simulated the nonradiative decay pathways of these molecules using trajectory surface hopping dynamics after photoexcitation into S3 to elucidate the reason for this difference. Both isomers mainly follow a common ππ* relaxation pathway involving multiple state crossings while coming down from S3 to S1 in the subpicosecond time scale. However, the simulations reveal that the excited-state topographies are such that while the 6-isomer can easily access the region of nonadiabatic transitions, the internal conversion of the 7-isomer is delayed by a pre-Dewar bond formation with a boat conformation.
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Conformação Molecular , IsomerismoRESUMO
Interstellar and cometary ices play an important role in the formation of planetary systems around young stars. Their main constituent is amorphous solid water (ASW). Although ASW is widely studied, vibrational energy dissipation and structural changes due to vibrational excitation are less well understood. The hydrogen-bonding network is likely a crucial component in this. Here, we present experimental results on hydrogen-bonding changes in ASW induced by the intense, nearly monochromatic mid-IR free-electron laser (FEL) radiation of the FELIX-2 beamline at the HFML-FELIX facility at the Radboud University in Nijmegen, The Netherlands. Structural changes in ASW are monitored by reflection-absorption infrared spectroscopy and depend on the irradiation history of the ice. The experiments show that FEL irradiation can induce changes in the local neighborhood of the excited molecules due to energy transfer. Molecular dynamics simulations confirm this picture: vibrationally excited molecules can reorient for a more optimal tetrahedral surrounding without breaking existing hydrogen bonds. The vibrational energy can transfer through the hydrogen-bonding network to water molecules that have the same vibrational frequency. We hence expect a reduced energy dissipation in amorphous material with respect to crystalline material due to the inhomogeneity in vibrational frequencies as well as the presence of specific hydrogen-bonding defect sites, which can also hamper the energy transfer.
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Amorphous solid water (ASW) is one of the most widely studied solid phase systems. A better understanding of the nature of inter- and intramolecular forces in ASW is, however, still required to correctly interpret the catalytic role of ASW in the formation and preservation of molecular species in environments such as the icy surfaces of Solar System objects, on interstellar icy dust grains, and potentially even in the upper layers of the Earth's atmosphere. In this work, we have systematically exposed porous ASW (pASW) to mid-infrared radiation generated by a free-electron laser at the HFML-FELIX facility in The Netherlands to study the effect of vibrational energy injection into the surface and bulk modes of pASW. During multiple sequential irradiations on the same ice spot, we observed selective effects both at the surface and in the bulk of the ice. Although the density of states in pASW should allow for a fast vibrational relaxation through the H-bonded network, part of the injected energy is converted into structural ice changes as illustrated by the observation of spectral modifications when performing Fourier transform infrared spectroscopy in reflection-absorption mode. Future studies will include the quantification of such effects by systematically investigating ice thickness, ice morphology, and ice composition.
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The decarboxylation (CO2 loss) mechanism of cold monodeprotonated phthalic acid was studied in a photodissociation action spectrometer by quantifying mass-selected product anions and neutral particles as a function of the excitation energy. The analysis proceeded by interpreting the translational energy distribution of the generated uncharged products, and with the help of quantum calculations. In particular, this study reveals different fragmentation pathways in the deprotonated anion and in the radical generated upon electron photodetachment. Unlike the behavior found in other deprotonated aryl carboxylic acids, which do not fragment in the anion excited state, a double loss of CO2 molecules takes place in the phthalic monoanion. Moreover, at higher excitation energies the phthalic monoanion experiences decarboxylative photodetachment with a statistical distribution of product translational energies, which contrasts with the impulsive dissociation reactions characteristic of other aryl carboxylic anions.
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Electron photodetachment of cold deprotonated indole and azaindole anions has been studied by use of a mass-selective photofragmentation spectrometer capable of negative ion and neutral particle detection. The electron affinities of the indolyl radical and the 5-, 6- and 7-azaindolyl radicals have been measured with an uncertainty of less than 0.002 eV. The presence of the nitrogen atom in the six-membered ring of the azaindolide anions stabilises the electron by 0.3 to 0.4 eV, i.e. about 10-15%, compared to the indolide anion. No fragmentation was observed in either the anionic or radical forms of the species studied. The appearance of dipole-bound states in the spectra of deprotonated 6- and 7-azaindole anions allowed us to analyse the vibrational structure of the neutral 6- and 7-azaindolyl radicals produced following photodetachment. Although no dipole-bound states were clearly identified for deprotonated indole or 5-azaindole, the shape of the photodetachment threshold suggests the presence of a very weakly dipole-bound state or dipole resonance, which cannot be resolved with our laser resolution.
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We present a study of the photofragmentation of three protonated azaindole molecules - 7-azaindole, 6-azaindole, and 5-azaindole - consisting of fused pyrrole-pyridine bicyclic aromatic systems, in which the pyridinic (protonated) nitrogen heteroatom is located at the 7, 6, and 5 positions, respectively. Photofragmentation electronic spectra of the isolated aforementioned azaindolinium cations reveal that their photodynamics extends over timescales covering nine orders of magnitude and provide evidence about the resultant fragmentation pathways. Moreover, we show how the position of the heteroatom in the aromatic skeleton influences the excited state energetics, fragmentation pathways, and fragmentation timescales. Computed ab initio adiabatic transition energies are used to assist the assignation of the spectra, while geometry optimisation in the excited electronic states as well as ab initio calculations along the potential surfaces demonstrate the role of ππ*/πσ* coupling and/or large geometry changes in the dynamics of these species. Evidence supporting the formation of Dewar valence isomers as intermediates involved in sub-picosecond relaxation processes is discussed.
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The photocatalytic oxidation of water with molecular or polymeric N-heterocyclic chromophores is a topic of high current interest in the context of artificial photosynthesis, that is, the conversion of solar energy to clean fuels. Hydrogen-bonded clusters of N-heterocycles with water molecules in a molecular beam are simple model systems for which the basic mechanisms of photochemical water oxidation can be studied under well-defined conditions. In this work, we explored the photoinduced H-atom transfer reaction in pyrimidine-water clusters yielding pyrimidinyl and hydroxyl radicals with laser spectroscopy, mass spectrometry and trajectory-based ab initio molecular dynamics simulations. The oxidation of water by photoexcited pyrimidine is unequivocally confirmed by the detection of the pyrimidinyl radical. The dynamics simulations provide information on the time scales and branching ratios of the reaction. While relaxation to local minima of the S1 potential-energy surface is the dominant reaction channel, the H-atom transfer reaction occurs on ultrafast time scales (faster than about 100 fs) with a branching ratio of a few percent.
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The ultrafast relaxation occurring in pyrene upon excitation at 4.68 eV was studied in a supersonic gas-jet fs pump-probe experiment. Mass spectrometry and velocity map imaging of photoelectrons produced by probing via multiphoton ionisation at 800 nm reveal that the initially prepared wave packet exhibits a fast relaxation (<80 fs), followed by a slower one of 200 fs. By comparing the propensity rules of photoionisation observed at one color with ab initio calculations, we tentatively assign these two timescales to a first internal conversion to the dark bB3g state followed by a second one to the long lived aB2u first excited state. Vertical excitation energies determined using ab initio Multi-State Complete Active Space 2nd order Perturbation Theory (MS-CASPT2), as well as oscillator strengths between several electronic states, are reported.
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The infrared gas-phase absorption spectrum of methane was used to determine its Clapeyron solid-gas equilibrium curve in the 40-77 K temperature range. For comparative purposes and to obtain more reliable results, two different optical experimental setups were used. At higher temperatures (53-77 K), a single pass cryogenically cooled cell was coupled to a standard low-resolution Fourier transform infrared spectrometer. The second system was a state-of-the-art vertical-external-cavity surface-emitting laser tunable source operating at around 2.3 µm, combined with a 7 m path Herriott cell, to record methane absorption features down to 40 K. From the measurements, the vapor pressure curve ln( p/Pa) = -(1191.92 ± 8.92)/( T/K) + (22.49 ± 0.16) was derived in the range 40-77 K. This corresponds to a value of 9910 ± 75 J mol-1 for the sublimation enthalpy. The relation was validated down to 40 K, increasing our knowledge of the saturation pressure by 2 orders of magnitude. Data were compared with available pressure measurements from the literature, obtained by manometric or mass spectrometry techniques, and the sublimation enthalpy was compared with a thermodynamic approach based on heat capacity measurements in the solid and gas phases.
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In flowering plants, the female gametophyte controls pollen tube reception immediately before fertilization and regulates seed development immediately after fertilization, although the controlling mechanisms remain poorly understood. Previously, we showed that LORELEI (LRE), which encodes a putative glycosylphosphatidylinositol-anchored membrane protein, is critical for pollen tube reception by the female gametophyte before fertilization and the initiation of seed development after fertilization. Here, we show that LRE is expressed in the synergid, egg, and central cells of the female gametophyte and in the zygote and proliferating endosperm of the Arabidopsis (Arabidopsis thaliana) seed. Interestingly, LRE expression in the developing seeds was primarily from the matrigenic LRE allele, indicating that LRE expression is imprinted. However, LRE was biallelically expressed in 8-d-old seedlings, indicating that the patrigenic allele does not remain silenced throughout the sporophytic generation. Regulation of imprinted LRE expression is likely novel, as LRE was not expressed in pollen or pollen tubes of mutants defective for MET1, DDM1, RNA-dependent DNA methylation, or MSI-dependent histone methylation. Additionally, the patrigenic LRE allele inherited from these mutants was not expressed in seeds. Surprisingly, and contrary to the predictions of the parental conflict hypothesis, LRE promotes growth in seeds, as loss of the matrigenic but not the patrigenic LRE allele caused delayed initiation of seed development. Our results showed that LRE is a rare imprinted gene that functions immediately after double fertilization and supported the model that a passage through the female gametophyte establishes monoalleleic expression of LRE in seeds and controls early seed development.
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Proteínas de Arabidopsis/metabolismo , Arabidopsis/genética , Glicoproteínas de Membrana/metabolismo , Arabidopsis/citologia , Arabidopsis/crescimento & desenvolvimento , Proteínas de Arabidopsis/genética , Endosperma/citologia , Endosperma/genética , Endosperma/crescimento & desenvolvimento , Fertilização , Glicoproteínas de Membrana/genética , Mutação , Especificidade de Órgãos , Óvulo Vegetal/citologia , Óvulo Vegetal/genética , Óvulo Vegetal/crescimento & desenvolvimento , Pólen/citologia , Pólen/genética , Pólen/crescimento & desenvolvimento , Tubo Polínico/citologia , Tubo Polínico/genética , Tubo Polínico/crescimento & desenvolvimento , Polinização , Plântula/citologia , Plântula/genética , Plântula/crescimento & desenvolvimento , Sementes/citologia , Sementes/genética , Sementes/crescimento & desenvolvimento , ZigotoRESUMO
Correction for 'Adsorption of PAHs on interstellar ice viewed by classical molecular dynamics' by Eric Michoulier et al., Phys. Chem. Chem. Phys., 2018, DOI: 10.1039/c8cp00593a.
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Polycyclic Aromatic Hydrocarbons (PAHs) are a family of molecules which represent the best candidates to explain the observation of one set of features in the Interstellar Medium (ISM): the Aromatic Interstellar Bands (AIBs). They could also contribute to the Diffuse Interstellar Bands (DIBs). In dense molecular clouds, PAHs may condense onto interstellar grains, contributing to the complex chemistry occurring in their icy mantles, composed essentially of water. In this context, the adsorption of various aromatic molecules, from benzene to ovalene, on different ices - both amorphous and crystalline - is investigated by means of classical molecular dynamics simulations. Initially, a systematic parametrization of the electronic charges on the chosen PAHs in these environments is carried out, and benchmarked with reference to free energies of solvation in liquid water. Then we propose a new, rigorous methodology, transferable to any other PAH or molecular species, to evaluate the charges to be applied to the molecule in the gas phase, at interfaces, or in liquid water. Ultimately, the adsorption energies calculated for the various PAHs are used to derive a function predicting the adsorption energy of any PAH on a given ice surface as a function of the number of C and H atoms it contains. For all PAHs studied, the largest adsorption energies are found on the crystalline hexagonal ice surface (Ih). Binding energy maps constructed for each PAH-ice pair give valuable insight into adsorption site densities and the barriers to surface diffusion. One key result is that the amorphous surface offers a smaller number of adsorption sites compared to that of hexagonal ice. A direct correlation between the location of energetically favourable adsorption sites and the presence of dangling H-bonds is also demonstrated using these maps, showing that PAHs adsorb preferentially on sites offering dangling H-bonds. The present work represents a complete description of PAH-ice interaction in the ground electronic state and at low temperature, providing the binding energies and barrier heights necessary to the ongoing improvement of astrochemical models.
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Experimental and theoretical investigations of the excited states of protonated 1- and 2-aminonaphthalene are presented. The electronic spectra are obtained by laser induced photofragmentation of the ions captured in a cold ion trap. Using ab initio calculations, the electronic spectra can be assigned to different tautomers which have the proton on the amino group or on the naphthalene moiety. It is shown that the tautomer distribution can be varied by changing the electrospray source conditions, favoring either the most stable form in solution (amino protonation) or that in the gas phase (aromatic ring protonation). Calculations for larger amino-polyaromatics predict that these systems should behave as "proton sponges" i.e. have a proton affinity larger than 11 eV.