Amide Spectral Fingerprints are Hydrogen Bonding-Mediated

The origin of the peculiar amide spectral features of proteins in aqueous solution is investigated, by exploiting a combined theoretical and experimental approach to UVRR spectra are recorded by tuning Synchrotron Radiation at several excitation wavelengths and modeled by using a recently developed multiscale protocol based on a polarizable QM/ MM approach. Thanks to the unparalleled agreement between theory and experiment, we demonstrate that specific hydrogen bond interactions, which dominate hydration dynamics around these solutes, play a crucial role in the selective enhancement of amide signals. These results further argue the capability of vibrational spectroscopy methods as valuable tools for refined structural analysis of peptides and proteins in aqueous solution

Threshold voltage instability by charge trapping effects in the gate region of p-GaN HEMTs

In this work, the threshold voltage instability of normally-off p-GaN high electron mobility transistors (HEMTs) has been investigated by monitoring the gate current density during device on-state. The origin of the gate current variations under stress has been ascribed to charge trapping occurring at the different interfaces in the metal/p-GaN/AlGaN/GaN system. In particular, depending on the stress bias level, electrons (VG 6 V) are trapped, causing a positive or negative threshold voltage shift {DVTH, respectively. By monitoring the gate current variations at different temperatures, the activation energies associated to the electrons and holes trapping could be determined and correlated with the presence of nitrogen (electron traps) or gallium (hole traps) vacancies. Moreover, the electrical measurements suggested the generation of a new electron-trap upon long-time bias stress, associated to the creation of crystallographic dislocation-like defects extending across the different interfaces (p-GaN/AlGaN/GaN) of the gate stack

Solvation effects of ionic liquid/water mixtures on biomolecules

onic liquids (ILs) belong to a broad class of ionic compounds that, differently from conventional salts, are usually liquid at T < 100\ub0C. They are characterized by vanishing vapour pressure, good thermal stability, high ion density and ionic conductivity. Thanks to the large variety of available ions, the physico-chemical properties of ILs can be modulated by careful selection of both cation and anion with specic characteristics for tailored applications. A more convenient strategy for an ecient tuning of the performances of ILs consists in mixing ILs with other ionic or molecular liquids, such as e.g. water. It is observed that addition of water to ILs allows to improve some of their properties and performances, especially for applications in biological eld. For instance, recent studies reported on the capability of IL/water solutions to enhance the structural stability of proteins, enzymes and deoxyribonucleic acid (DNA) also at high temperatures. This PhD thesis aims to show the usefulness of synchrotron-based UV Resonance Raman (SR-UVRR) spectroscopy for investigating i) the structural dynamics of IL/water solutions and ii) the solvation effects of these IL-based solvents on bio-molecules, such as peptides and DNA. UVRR exhibits several advantages with respect to conventional spontaneous Raman spectroscopy, as the signicant increment of the detection limit that allows to study the samples in very high diluted conditions and the selective enhancement of the Raman cross section of vibrations associated to specic molecular groups of the same system. Thanks to the unique tunability of the synchrotron emission, the UVRR spectra of aqueous solutions of imidazolium-based ILs have been excited at different wavelengths nely matching with the resonance transitions occurring in the system. These spectra showed to have good sensitivity to the modications induced on the local structure of solutions of ILs by i) the change of the anion and ii) the substitution on the imidazolium ring of progressively longer alkyl-chains. Additionally, some UVRR signals are specically informative on the effect induced by addition of water on the strength of hydrogen bonds (H-bonds) in IL-water solutions. The molecular view provided by SR-UVRR experiments has been further complemented by the structural parameters extracted by Small Angle Neutron Scattering (SANS) measurements performed on the same IL/water mixtures. The investigation of the structure-dynamic relationship in IL/water solutions is the preliminary step for the deep comprehension of the effects of these mixtures on the solvation dynamics of molecules of biological interest, such as peptides and DNA. This is an issue of special interest by considering that the solute-solvent interactions are strongly related to the biological activity of bio-macromolecules. Some results will be presented in this thesis, concerning the case of two dierent type of bio-systems: i) small peptides dissolved in IL/water solutions: the UVRR spectra of peptides contain several spectroscopic markers of the structural rearrangement induced by the hydration shell on peptides, such as the Amide bands that are usually not well detectable in spontaneous Raman spectra. The analysis of these spectral features can provide insights on the peculiar effect induced on the hydration dynamics of peptides by dfferent ILs; ii) DNA dissolved in IL/water solutions: a suitable choice of the exciting radiation allows to collect UVRR spectra of DNA where the vibrational signals associated to the different nitrogenous bases are selectively enhanced. This gives the unique opportunity to disentangle specic bands in the Raman spectra of DNA that appear usually very complex. Such approach can be conveniently used to obtain insights on the molecular mechanism responsible of the dierent thermal stability exhibited by DNA structure in the presence of different IL/water solutions.Ionic liquids (ILs) belong to a broad class of ionic compounds that, differently from conventional salts, are usually liquid at T < 100\ub0C. They are characterized by vanishing vapour pressure, good thermal stability, high ion density and ionic conductivity. Thanks to the large variety of available ions, the physico-chemical properties of ILs can be modulated by careful selection of both cation and anion with specic characteristics for tailored applications. A more convenient strategy for an ecient tuning of the performances of ILs consists in mixing ILs with other ionic or molecular liquids, such as e.g. water. It is observed that addition of water to ILs allows to improve some of their properties and performances, especially for applications in biological eld. For instance, recent studies reported on the capability of IL/water solutions to enhance the structural stability of proteins, enzymes and deoxyribonucleic acid (DNA) also at high temperatures. This PhD thesis aims to show the usefulness of synchrotron-based UV Resonance Raman (SR-UVRR) spectroscopy for investigating i) the structural dynamics of IL/water solutions and ii) the solvation efects of these IL-based solvents on bio-molecules, such as peptides and DNA. UVRR exhibits several advantages with respect to conventional spontaneous Raman spectroscopy, as the signicant increment of the detection limit that allows to study the samples in very high diluted conditions and the selective enhancement of the Raman cross section of vibrations associated to specic molecular groups of the same system. Thanks to the unique tunability of the synchrotron emission, the UVRR spectra of aqueous solutions of imidazolium-based ILs have been excited at diferent wavelengths nely matching with the resonance transitions occurring in the system. These spectra showed to have good sensitivity to the modications induced on the local structure of solutions of ILs by i) the change of the anion and ii) the substitution on the imidazolium ring of progressively longer alkyl-chains. Additionally, some UVRR signals are specically informative on the effect induced by addition of water on the strength of hydrogen bonds (H-bonds) in IL-water solutions. The molecular view provided by SR-UVRR experiments has been further complemented by the structural parameters extracted by Small Angle Neutron Scattering (SANS) measurements performed on the same IL/water mixtures. The investigation of the structure-dynamic relationship in IL/water solutions is the preliminary step for the deep comprehension of the effects of these mixtures on the solvation dynamics of molecules of biological interest, such as peptides and DNA. This is an issue of special interest by considering that the solute-solvent interactions are strongly related to the biological activity of bio-macromolecules. Some results will be presented in this thesis, concerning the case of two different type of bio-systems: i) small peptides dissolved in IL/water solutions: the UVRR spectra of peptides contain several spectroscopic markers of the structural rearrangement induced by the hydration shell on peptides, such as the Amide bands that are usually not well detectable in spontaneous Raman spectra. The analysis of these spectral features can provide insights on the peculiar effect induced on the hydration dynamics of peptides by different ILs; ii) DNA dissolved in IL/water solutions: a suitable choice of the exciting radiation allows to collect UVRR spectra of DNA where the vibrational signals associated to the different nitrogenous bases are selectively enhanced. This gives the unique opportunity to disentangle specic bands in the Raman spectra of DNA that appear usually very complex. Such approach can be conveniently used to obtain insights on the molecular mechanism responsible of the different thermal stability exhibited by DNA structure in the presence of different IL/water solutions

Interfacial Water and Micro-heterogeneity in Aqueous Solutions of Ionic Liquids

In this work, aqueous solutions of two prototypical ionic liquids (ILs), [BMIM][BF4] and [BMIM][TfO], were investigated by UV Raman spectroscopy and small-angle neutron scattering (SANS) in the water-rich domain, where strong heterogeneities at mesoscopic length scales (microheterogeneity) were expected. Analyzing Raman data by a differential method, the solute-correlated (SC) spectrum was extracted from the OH stretching profiles, emphasizing specific hydration features of the anions. SC-UV Raman spectra pointed out the molecular structuring of the interfacial water in these microheterogeneous IL/water mixtures, in which IL aggregates coexist with bulk water domains. The organization of the interfacial water differs for the [BMIM][BF4] and [BMIM][TfO] solutions, being affected by specific anion−water interactions. In particular, in the case of [BMIM][BF4], which forms weaker H-bonds with water, the aggregation properties clearly depend on concentration, as reflected by local changes in the interfacial water. On the other hand, stronger water−anion hydrogen bonds and more persistent hydration layers were observed for [BMIM][TfO], which likely prevent changes in IL aggregates. The modeling of SANS profiles, extended to [BPy][BF4] and [BPy][TfO], evidences the occurrence of significant concentration fluctuations for all of the systems: this appears as a rather general phenomenon that can be ascribed to the presence of IL aggregation, mainly induced by (cation-driven) hydrophobic interactions. Nevertheless, larger concentration fluctuations were observed for [BMIM][BF4], suggesting that anion−water interactions are relevant in modulating the microheterogeneity of the mixture

Crowding and conformation interplay on human DNA G-quadruplex by ultraviolet resonant Raman scattering

The G-quadruplex-forming telomeric sequence (TTAGGG)4TT was investigated by polarized Ultraviolet Resonance Raman Scattering (UVRR) at 266 nm. The presence of 40% poly(ethylene glycol) and the so-called \u2018\u2018self-crowding\u2019\u2019 condition were used to induce the hybrid-to-parallel topology transition. Analysis of frequency shifts with temperature showed the role of several functional groups in the topological transitions and provides structural dynamical information. Circular dichroism under similar conditions was used as a reference. UVRR shed light on the effect of intramolecular interactions and of local and environmental dynamics in promoting different G-quadruplex topologies, induced by solution conditions or by temperature changes. Overall, these findings showed the enormous potential of this spectroscopy for G-quadruplex conformational studies

A combined SR-based Raman and InfraRed investigation of pigmenting matter used in wall paintings: The San Gennaro and San Gaudioso Catacombs (Naples, Italy) case

In the present paper, we report on a recent ultraviolet (UV) Raman study in conjunction with an FTIR absorbance investigation by using synchrotron radiation techniques, performed on wall paintings samples of frescoes, stored in the charming sites of San Gennaro and San Gaudioso Catacombs (Naples, Italy). The main goal was the analysis of the pigmenting materials in order to characterize the pigmenting agents and the binders used by the artists. In this frame, the combined use of both complementary SRbased techniques played a fundamental role. In addition, in principle the characterization of the pigmenting matter could be useful for reproducing similar materials in accordance with the ancient recipes to use in any restoration process

Hard X-ray transient grating spectroscopy on bismuth germanate

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