Search

The VUV photoionisation and photofragmentation of cyclo-alanine-alanine (cAA) has been studied in a joint experimental and theoretical work. The photoelectron spectrum and the photoelectronphotoion coincidence (PEPICO) measurements, which enable control of the energy being deposited, combined with quantum chemistry calculations, provide direct insight into the cAA molecular stability after photoionisation. The analysis of the ion-neutral coincidence experiments with the molecular dynamics simulations and the exploration of the potential energy surface allows a complete identification of the fragmentation pathways. It has been found that the fragmentation always start with the ring opening through the C–C bond cleavage, followed by release of neutral moieties CO or HNCO.

The photoionisation and photofragmentation of the two cyclic dipetides cyclo(alanyl-glycine) cGA and cyclo(glycyl-glycine) cGG, have been studied combining experiments and simulations.

The valence ionization of uracil and mixed water-uracil clusters has been studied experimentally and by ab initio calculations. In both measurements, the spectrum onset shows a red shift with respect to the uracil molecule, with the mixed cluster characterized by peculiar features unexplained by the sum of independent contributions of the water or uracil aggregation. To interpret and assign all the contributions, we performed a series of multi-level calculations, starting from an exploration of several cluster structures using automated conformer-search algorithms based on a tight-binding approach. Ionization energies have been assessed on smaller clusters via a comparison between accurate wavefunction-based approaches and cost-effective DFT-based simulations, the latter of which were applied to clusters up to 12 uracil and 36 water molecules. The results confirm that (i) the bottom-up approach based on a multilevel method [Mattioli et al. Phys. Chem. Chem. Phys. 23, 1859 (2021)] to the structure of neutral clusters of unknown experimental composition converges to precise structure-property relationships and (ii) the coexistence of pure and mixed clusters in the water-uracil samples. A natural bond orbital (NBO) analysis performed on a subset of clusters highlighted the special role of H-bonds in the formation of the aggregates. The NBO analysis yields second-order perturbative energy between the H-bond donor and acceptor orbitals correlated with the calculated ionization energies. This sheds light on the role of the oxygen lone-pairs of the uracil CO group in the formation of strong H-bonds, with a stronger directionality in mixed clusters, giving a quantitative explanation for the formation of core-shell structures. This article is based upon work from COST Action CA18212—Molecular Dynamics in the GAS phase (MD-GAS) supported by COST (European Cooperation in Science and Technology), and it was partially supported by the CNRS PICS Project BIFACE, the International Associated Laboratory “DYNAMO,” and the PRIN 20173B72NB “Predicting and controlling the fate of bio-molecules driven by extreme-ultraviolet radiation”. C.N. is co-funded by Région Normandie and Synchrotron SOLEIL. SOLEIL support is acknowledged under Project No. 20171346. A.C. acknowledges the access to the CINECA supercomputer facility via the ISCRA C HP10C652JX Grant. F.M. and T.V.C. acknowledge the VSC (Flemish Supercomputer Center), funded by the Research Foundation-Flanders (FWO) and the Flemish Government.

Nitroimidazoles are a class of chemicals with a remarkable broad spectrum of applications from the production of explosives to the use as radiosensitizers in radiotherapy. The understanding of thedynamics of their fragmentation induced by ionizing sources is of fundamental interest. The goal of this work is to theoretically investigate the kinetic competition between the two most important decomposition channels of 2, 4 and 5-Nitroimidazole cations: the NO and NO2 losses. The calculated rate constants of the two processes are in very good agreement with the experimental Photoelectron-Photoion Coincidence (PEPICO) branching ratio. This study solves the intriguing and theoretically unexplained experimental observation that 2-Nitroimidazole, at variance with the other two regio-isomers is a source for only NO at low energies (

Dipeptides, the prototype peptides, exist in both linear (l-) and cyclo (c-) structures. Since the first mass spectrometry experiments, it has been observed that some l-structures may turn into the cyclo ones, likely via a temperature-induced process. In this work, combining several different experimental techniques (mass spectrometry, infrared and Raman spectroscopy, and thermogravimetric analysis) with tight-binding and ab initio simulations, we provide evidence that, in the case of L -phenylalanyl- L -alanine, an irreversible cyclization mechanism, catalyzed by water and driven by temperature, occurs in the condensed phase. This process can be considered as a very efficient strategy to improve dipeptide stability by turning the comparatively fragile linear structure into the robust and more stable cyclic one. This mechanism may have played a role in prebiotic chemistry and can be further exploited in the preparation of nanomaterials and drugs.

The fabrication of enzyme-based biosensors has received much attention for their selectivity and sensitivity. In particular, laccase-based biosensors have attracted a lot of interest for their capacity to detect highly toxic molecules in the environment, becoming essential tools in the fields of white biotechnology and green chemistry. The manufacturing of a new, metal-free, laccase-based biosensor with unprecedented reuse and storage capabilities has been achieved in this work through the application of the electrospray deposition (ESD) methodology as the enzyme immobilization technique. Electrospray ionization (ESI) has been used for ambient soft-landing of laccase enzymes on a carbon substrate, employing sustainable chemistry. This study shows how the ESD technique can be successfully exploited for the fabrication of a new promising environment-friendly electrochemical amperometric laccase-based biosensor, with storage capability up to two months without any particular care and reuse performance up to 63 measurements on the same electrode just prepared and 20 measurements on the one-year-old electrode subjected to redeposition. The laccase-based biosensor has been tested for catechol detection in the linear range 2-100 μM, with a limit of detection of 1.7 μM, without interference from chrome, cadmium, arsenic, and zinc and without any memory effects.

Nitroimidazoles are a class of chemicals with a remarkable broad spectrum of applications from the production of explosives to the use as radiosensitizers in radiotherapy. The understanding of thedynamics of their fragmentation induced by ionizing sources is of fundamental interest. The goal of this work is to theoretically investigate the kinetic competition between the two most important decomposition channels of 2, 4 and 5-Nitroimidazole cations: the NO and NO

The properties of mixed water-uracil nanoaggregates have been probed by core electron-photoemission measurements to investigate supramolecular assembly in the gas phase driven by weak interactions. The interpretation of the measurements has been assisted by multilevel atomistic simulations, based on semi-empirical tight-binding and DFT-based methods. Our protocol established a positive-feedback loop between experimental and computational techniques, which has enabled a sound and detailed atomistic description of such complex heterogeneous molecular aggregates. Among biomolecules, uracil offers interesting and generalized skeletal features; its structure encompasses an alternation of hydrophilic H-bond donor and acceptor sites and hydrophobic moieties, typical in biomolecular systems, that induces a supramolecular core-shell-like organization of the mixed clusters with a water core and an uracil shell. This structure is far from typical models of both solid-state hydration, with water molecules in defined positions, or liquid solvation, where disconnected uracil molecules are completely surrounded by water.

International audience; We propose to combine quantum chemical calculations, statistical mechanical methods, and photoionization and particle collision experiments to unravel the redistribution of internal energy of the furan cation and its dissociation pathways. This approach successfully reproduces the relative intensity of the different fragments as a function of the internal energy of the system in photoelectronphotoion coincidence experiments and the different mass spectra obtained when ions ranging from Ar + to Xe 25+ or electrons are used in collision experiments. It provides deep insights into the redistribution of the internal energy in the ionized molecule and its influence on the dissociation pathways and resulting charged fragments. The present pilot study demonstrates the efficiency of a statistical exchange of excitation energy among various degrees of freedom of the molecule and proves that the proposed approach is mature to be extended to more complex systems.

Superhydrogenated polycyclic aromatic hydrocarbons (PAHs) have been suggested to catalyze the formation of H2 in certain regions of space, but it remains unclear under which circumstances this mechanism is viable given the reduced carbon backbone stability of superhydrogenated PAHs. We report a laboratory study on the stability of the smallest pericondensed PAH, pyrene (C16H10+N , with N = 4, 6, and 16 additional H atoms), against photodestruction by single vacuum ultraviolet photons using the photoelectron–photoion coincidence technique. For N = 4, we observe a protective effect of hydrogenation against the loss of native hydrogens, in the form of an increase in the appearance energies of the and C16H8 + daughter ions compared to those reported for pristine pyrene (C16H10). No such effect is seen for N = 6 or 16, where the weakening effect of replacing aromatic bonds with aliphatic ones outweighs the buffering effect of the additional hydrogen atoms. The onset of fragmentation occurs at similar internal energies for N = 4 and 6, but is significantly lower for N = 16. In all three cases, H-loss and C m H n -loss (m ≥ 1, carbon backbone fragmentation) channels open at approximately the same energy. The branching fractions of the primary channels favor H-loss for N = 4, C m H n -loss for N = 16, and are roughly equal for the intermediate N = 6. We conclude that superhydrogenated pyrene is probably too small to support catalytic H2-formation, while trends in the current and previously reported data suggest that larger PAHs may serve as catalysts up to a certain level of hydrogenation.

A combined experimental and theoretical study shows how the interaction of VUV radiation with cyclo-(alanine-alanine), one of the 2,5-diketopiperazines (DKPs), produces reactive oxazolidinone intermediates. The theoretical simulations reveal that the interaction of these intermediates with other neutral and charged fragments, released in the molecular decomposition, leads either to the reconstruction of the cyclic dipeptide or to the formation of longer linear peptide chains. These results may explain how DKPs could have, on one hand, survived hostile chemical environments and, on the other, provided the seed for amino acid polymerization. Shedding light on the mechanisms of production of such prebiotic building blocks is of paramount importance to understanding the abiotic synthesis of relevant biologically active compounds, This article is based upon work from COST action CA18212 - Molecular Dynamics in the GAS phase (MD-GAS), supported by COST (European Cooperation in Science and Technology). The authors acknowledge the generous allocation of computer time at the Centro de Computación Cientıfí ca at the Universidad Autonoma de Madrid (CCC-UAM). This work ́ was partially supported by MICINN (Spanish Ministry of Science and Innovation) project PID2019-110091GB-I00, the “Marıa de Maeztu ́ ” (CEX2018-000805-M) Program for Centers of Excellence in R&D, MAECI Italy-Sweden project “Novel molecular tools for the exploration of the nanoworld”, and PRIN 20173B72NB project “Predicting and controlling the fate of biomolecules driven by extreme-ultraviolet radiation”. D.B.-L. acknowledges the FPI grant associated with MICINN project CTQ2016-76061-P. H.Z. acknowledges the Swedish Research Council for the individual project grant with contract no. 2020- 03437

The C, N and O 1s XPS spectra of uracil clusters in the gas phase have been measured. A new bottom-up approach, which relies on computational simulations starting from the crystallographic structure of uracil, has been adopted to interpret the measured spectra. This approach sheds light on the different molecular interactions (H-bond, π-stacking, dispersion interactions) at work in the cluster and provides a good understanding of the observed XPS chemical shifts with respect to the isolated molecule in terms of intramolecular and intermolecular screening occurring after the core–hole ionization. The proposed bottom-up approach, reasonably expensive in terms of computational resources, has been validated by finite-temperature molecular dynamics simulations of clusters composed of up to fifty molecules.

Enzymes immobilisation represents a critical issue in the design of biosensors to achieve standardization as well as suitable analytical performances in terms of sensitivity, selectivity, and stability. In this work electrospray deposition (ESD) has been exploited as a novel technique for the immobilisation of laccase enzyme on carbon black modified screen-printed electrodes. The aim is to fabricate an amperometric biosensor for phenolic compound detection. The electrodes produced by ESD have been analysed by scanning electron microscopy and characterised electrochemically to prove that this immobilisation technique is suited to manufacture high performance biosensors. The results show that the laccase enzyme maintains its activity after undergoing the electrospray ionisation process and deposition and the fabricated biosensor has improved performances in terms of storage (up to 3 months at room temperature) and working (up to 25 measurements on the same electrode) stability. The laccase-based biosensor has been tested for phenolic compound detection, with catechol as target analyte, in the linear range 2.5–50 μM, with 2.0 μM limit of detection, without interference from lead, cadmium, atrazine, and paraoxon, and without matrix effect in drinking, surface, and wastewater.

Dihalomethanes XCH2Y (X and Y = F, Cl, Br, and I) are a class of compounds involved in several processes leading to the release of halogen atoms, ozone consumption, and aerosol particle formation. Neutral dihalomethanes have been largely studied, but chemical physics properties and processes involving their radical ions, like the pathways of their decomposition, have not been completely investigated. In this work the photodissociation dynamics of the ClCH2I molecule has been explored in the photon energy range 9-21 eV using both VUV rare gas discharge lamps and synchrotron radiation. The experiments show that, among the different fragment ions, CH2I+ and CH2Cl+, which correspond to the Cl- and I-losses, respectively, play a dominant role. The experimental ionization energy of ClCH2I and the appearance energies of the CH2I+ and CH2Cl+ ions are in agreement with the theoretical results obtained at the MP2/CCSD(T) level of theory. Computational investigations have been also performed to study the isomerization of geminal [ClCH2I]o+ into the iso-chloroiodomethane isomers: [CH2I-Cl]o+ and [CH2Cl-I]o+.

Synopsis The electronic structure and the fragmentation dynamics of a series of isolated cyclic-dipeptides (cyclo-Ala-Ala; cyclo-Gly-Ala; cyclo-Gly-Gly) have been studied by mass spectrometry, photoemission and photoelectron-photoion coincidence spectroscopy.

Synopsis The main objective of this study is to determine the energy transfer occuring in ion-molecule collisions. In order to solve this problem, we followed two approaches; the first one by validating a purely experimental method and the second one by testing a new theoretical model M3C (Microcanonical Metropolis Monte Carlo).

We present a synopsis of 90 references on the last trends in electrochemical laccase-based biosensors and highlight how forefront technologies have prompted the development of smarter biosensors boosting their applications in agro-environmental and biomedical fields. This review updates a previous one published by Rodriguez-Delgado and co-workers in Trends in Analytical Chemistry 74 (2015) 21–45 and covers the research advances over the period between 2015 and 2019. In particular, a general overview on laccase structure and function is reported emphasizing its electrochemistry features as suitable bioreceptor for electrochemical devices. Then, we describe how crosscutting technologies like nanotechnology, material science, and immobilisation strategies can help to enhance the analytical performances of laccase-based biosensors.

The fragmentation of glycine (NH2CH2COOH) has been studied by photoelectron-photoion coincidence, PEPICO, experiments at 60 eV photon energy. Glycine practically fragments at the ionization threshold, with the charge being on the H2NCH2+ moiety, due to ejection of an electron from the nitrogen lone pair of the highest occupied molecular orbital. To observe the formation of the complementary cation COOH+ further energy is needed. The flexibility with respect to rotation about the C-C, C-N and C-O bonds makes glycine exist in the gas phase in several conformers of both Cs and C1 point group symmetry in the neutral as well as ion states. The ionization can lead to stabilization of some conformations, rearrangements and, last but not least, H migration between the two moieties. The results of these experiments prove the sensitivity of PEPICO to pin point all these processes.

Nitromidazoles are relevant compounds of multidisciplinary interest, and knowledge of their physical chemical parameters as well as their decomposition under photon irradiation is needed. Here we report an experimental and theoretical study of the mechanisms of VUV photo fragmentation of 2- and 4(5)-nitromidazoles, compounds used as radiosensitizers in conjunction with radiotherapy as well as high-energy density materials. Photoelectron photoion coincidence experiments, measurements of the appearance energies of the most important ionic fragments, density functional theory, and single-point coupled cluster calculations have been used to provide an overall insight into the energetics and structure of the different ionic/neutral products of the fragmentation processes. The results show that these compounds can be an efficient source of relevant CO, HCN, NO, and NO2 molecules and produce ions of particular astrophysical interest, like the isomers of azirinyl cation (m/z 40), predicted to exist in the interstellar medium, and protonated hydrogen cyanide (m/z 28).

Synopsis This work relies on complementary theoretical and experimental studies of the processes induced by ion-furan collisions. Results of the Molecular Dynamics simulations and exploration of the energy profiles combined with coincidence mass spectrometry provide complete picture of the fragmentation of furan dication.

The electronic structures of nicotine, nicotinic acid and nicotinamide have been studied by valence photoemission spectroscopy (PES), core X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorp- tion fine structure (NEXAFS) and interpreted with the aid of quantum chemical calculations. Nicotinamide and nicotinic acid are closely related and show correspondingly similar spectral features, while nicotine is both struc- turally and spectroscopically diverse

A combined experimental and theoretical approach has been used to disentangle the fundamental mechanisms of the fragmentation of the three isomers of nitroimidazole induced by vacuum ultra-violet (VUV) radiation, namely, 4-, 5-, and 2-nitroimidazole. The results of mass spectrometry as well as photoelectron-photoion coincidence spectroscopy display striking differences in the radiation-induced decomposition of the different nitroimidazole radical cations. Based on density functional theory (DFT) calculations, a model is proposed which fully explains such differences, and reveals the subtle fragmentation mechanisms leading to the release of neutral species like NO, CO, and HCN. Such species have a profound impact in biological media and may play a fundamental role in radiosensitising mechanisms during radiotherapy.