41 research outputs found
A Synchrotron Radiation Study of Nitroimidazoles and their Derivatives
No full textNitroimidazole derived molecules are used in radiotherapy thanks to their capability to sensitize hypoxic tumor cells to radiation by ‘mimicking’ the effects of the presence of oxygen as a damaging agent. Inthis work we present the results of a bottom-up approach, which goes from the model molecule to the real drugs used in therapy. Mass spectrometry and several spectroscopic techniques (XPS, PES, NEXAFS, PEPICO) basedon the use of synchrotron radiation have been combined with computational methods to link the electronic and geometric structure of the molecule to their functions.The investigation of the fragmentation patterns of the nitroimidazole isomers [1,2] has allowed to understand their capacity to produce reactive molecular species like nitric oxide, carbon monoxide or hydrogencyanide and their potential impact on the biological system. Guided by these results, the fragmentation mechanisms of metronidazole and misonidazole, the two radiosensitisers built on the 5-nitroimidazole and 2-nitroimidazole compounds used in therapy, as well as the 1-Methyl-5-nitroimidazole have been investigated. The results on these more complex systems suggest that different mechanisms are active. The release of nitric oxideis hampered by the efficient formation of nitrous acid or nitrogen dioxide and the long and branched tails attached to the imidazole ring increase the ring stability, providing an efficient channel for excess energy dissipation
Gas Phase Oxidation of Carbon Monoxide by Sulfur Dioxide Radical Cation: Reaction Dynamics and Kinetic Trend With the Temperature
No full textGas phase ion chemistry has fundamental and applicative purposes since it allows the study of the chemical processes in a solvent free environment and represents models for reactions occurring in the space at low and high temperatures. In this work the ion-molecule reaction of sulfur dioxide ion SO2.+ with carbon monoxide CO is investigated in a joint experimental and theoretical study. The reaction is a fast and exothermic chemical oxidation of CO into more stable CO2 by a metal free species, as SO2.+, excited into ro-vibrational levels of the electronic ground state by synchrotron radiation. The results show that the reaction is hampered by the enhancement of internal energy of sulfur dioxide ion and the only ionic product is SO.+. The theoretical approach of variational transition state theory (VTST) based on density functional electronic structure calculations, shows an interesting and peculiar reaction dynamics of the interacting system along the reaction path. Two energy minima corresponding to [SO2–CO].+ and [OS–OCO].+ complexes are identified. These minima are separated by an intersystem crossing barrier which couples the bent 3B2 state of CO2 with C2v symmetry and the 1A1 state with linear D∞h symmetry. The spin and charge reorganization along the minimum energy path (MEP) are analyzed and eventually the charge and spin remain allocated to the SO.+ moiety and the stable CO2 molecule is easily produced. There is no bottleneck that slows down the reaction and the values of the rate coefficient k at different temperatures are calculated with capture theory. A value of 2.95 × 10−10 cm3s−1molecule−1 is obtained at 300 K in agreement with the literature experimental measurement of 3.00 × 10−10 ± 20% cm3s−1molecule−1, and a negative trend with temperature is predicted consistently with the experimental observations
Inner shell photofragmentation of 2Cl-pyrimidine studied by mass spectrometry and electron–ion coincidence experiments
No full textPhotoelectron spectroscopy, mass spectrometry and electron–ion coincidence experiments combined with tunable synchrotron radiation have been used to study the decay and fragmentation of 2Cl-pyrimidine after Cl(2p), C(1s) and N(1s) excitations. The goal is to investigate how the state- and site-selected excitation and the chemical environment affect the fragmentation paths of the molecule and to make a comparison with fragmentation induced by direct valence ionization. It has been found that the site-selective inner shell excitation affects the branching ratio of the fragments, while the particular fragmentation channels of the cation are determined by the final state populated in the resonant decay of the core excited states. Effects of nuclear motion in the core excited states and the possible ultrafast molecular dissociation following the Cl(2p → σ*) core excitation are discussed
Ultrafast dynamics of adenine following XUV ionization
No full textThe dynamics of biologically relevant molecules exposed to ionizing radiation contains many facets and spans several orders of magnitude in time and energy. In the extreme ultraviolet (XUV) spectral range, multi-electronic phenomena and bands of correlated states with inner-valence holes must be accounted for in addition to a plethora of vibrational modes and available dissociation channels. The ability to track changes in charge density and bond length during ultrafast reactions is an important endeavor toward more general abilities to simulate and control photochemical processes, possibly inspired by those that have evolved biologically. By using attosecond XUV pulses extending up to 35 eV and few-femtosecond near-infrared pulses, we have previously time-resolved correlated electronic dynamics and charge migration occurring in the biologically relevant molecule adenine after XUV-induced sudden ionization. Here, using additional experimental data, we comprehensively report on both electronic and vibrational dynamics of this nucleobase in an energy range little explored to date with high temporal resolution. The time-dependent yields of parent and fragment ions in the mass spectra are analyzed to extract exponential time constants and oscillation periods. Together with time-dependent density functional theory and ab-initio Green’s function methods, we identify different vibrational and electronic processes. Beyond providing further insights into the XUV-induced dynamics of an important nucleobase, our work demonstrates that yields of specific dissociation outcomes can be influenced by sufficiently well-timed ultrashort pulses, therefore providing a new route for the control of the multi-electronic and dissociative dynamics of a DNA building block
Charge migration induced by attosecond pulses in bio-relevant molecules
No full textAfter sudden ionization of a large molecule, the positive charge can migrate throughout the system on a sub-femtosecond time scale, purely guided by electronic coherences. The possibility to actively explore the role of the electron dynamics in the photo-chemistry of bio-relevant molecules is of fundamental interest for understanding, and perhaps ultimately controlling, the processes leading to damage, mutation and, more generally, to the alteration of the biological functions of the macromolecule. Attosecond laser sources can provide the extreme time resolution required to follow this ultrafast charge flow. In this review we will present recent advances in attosecond molecular science: after a brief description of the results obtained for small molecules, recent experimental and theoretical findings on charge migration in bio-relevant molecules will be discussed.</p
Core Shell Investigation of 2-nitroimidazole
No full textTunability and selectivity of synchrotron radiation have been used to study the excitation and ionization of 2-nitroimidazole at the C, N, and O K-edges. The combination of a set of different measurements (X-ray photoelectron spectroscopy, near-edge photoabsorption spectroscopy, Resonant Auger electron spectroscopy, and mass spectrometry) and computational modeling have successfully disclosed local effects due to the chemical environment on both excitation/ionization and fragmentation of the molecule
The Reaction of Sulfur Dioxide Radical Cation with Hydrogen and its Relevance in Solar Geoengineering Models
No full textSO2 has been proposed in solar geoengineering as a precursor of H2SO4 aerosol, a cooling agent active in the stratosphere to contrast climate change. Atmospheric ionization sources can ionize SO2 into excited states of (Formula presented.), which quickly reacts with trace gases in the stratosphere. In this work we explore the reaction of (Formula presented.) with (Formula presented.) excited by tunable synchrotron radiation, leading to (Formula presented.) ((Formula presented.)), where H contributes to O3 depletion and OH formation. Density Functional Theory and Variational Transition State Theory have been used to investigate the dynamics of the title barrierless and exothermic reaction. The present results suggest that solar geoengineering models should test the reactivity of (Formula presented.) with major trace gases in the stratosphere, such as H2 since this is a relevant channel for the OH formation during the nighttime when there is not OH production by sunlight. OH oxides SO2, triggering the chemical reactions leading to H2SO4 aerosol
Carbon and Nitrogen K-Edge NEXAFS Spectra of Indole, 2,3-Dihydro-7-azaindole, and 3-Formylindole
No full textThe near-edge X-ray absorption fine structure (NEXAFS) spectra of indole, 2,3-dihydro-7-azaindole, and 3-formylindole in the gas phase have been measured at the carbon and nitrogen K-edges. The spectral features have been interpreted based on density functional theory (DFT) calculations within the transition potential (TP) scheme, which is accurate enough for a general description of the measured C 1s NEXAFS spectra as well as for the assignment of the most relevant features. For the nitrogen K-edge, the agreement between experimental data and theoretical spectra calculated with TP-DFT was not quite satisfactory. This discrepancy was mainly attributed to the many-body effects associated with the excitation of the core electron, which are better described using the time-dependent density functional theory (TDDFT) with the range-separated hybrid functional CAM-B3LYP. An assignment of the measured N 1s NEXAFS spectral features has been proposed together with a complete description of the observed resonances. Intense transitions from core levels to unoccupied antibonding π* states as well as several transitions with mixed-valence/Rydberg or pure Rydberg character have been observed in the C and N K-edge spectra of all investigated indoles
Ultrafast Dynamics in the DNA Building Blocks Thymidine and Thymine Initiated By Ionizing Radiation
No full textUnderstanding of how energetic charged particles damage DNA is crucial for improving radiotherapy techniques such as hadron therapy and for the development of new radiosensitizer drugs. In the present study, the damage caused by energetic particles was simulated by measuring the action of extreme ultraviolet (XUV) attosecond pulses on the DNA building blocks thymine and thymidine. This allowed the ultrafast processes triggered by direct ionization to be probed with an optical pulse with a time resolution of a few femtoseconds. By measuring the yields of fragment ions as a function of the delay between the XUV pulse and the probe pulse, a number of transient processes typically lasting 100 femtoseconds or less were observed. These were particularly strong in thymidine which consists of the thymine base attached to a deoxyribose sugar. This dynamics was interpreted as excited states of the cation, formed by the XUV pulse, rapidly decaying via non-adiabatic coupling between electronic states. This provides the first experimental insight into the mechanisms which immediately proceed from the action of ionizing radiation on DNA and provides a basis on which further theoretical and experimental studies can be conducted
Electrospray deposition as a smart technique for laccase immobilisation on carbon black-nanomodified screen-printed electrodes
No full textEnzymes 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
