Sara Mangialardo

Unravelling the structure of protic ionic liquids with theoretical and experimental methods: ethyl-, propyl- and butylammonium nitrate explored by Raman spectroscopy and DFT calculations.

We present an analysis of gas-phase structures of small clusters of n-alkylammonium nitrates (ethyl, propyl, and butyl) together with vibrational Raman spectroscopy of their respective liquid phases. The assignment and interpretation of the resonant frequencies have been performed by comparison with high-quality ab initio (DFT) computations. The theoretical spectra are in excellent agreement with the measured ones and allow the interpretation and assignment of almost all the spectral features. A careful analysis of the vibrational frequencies and of the electronic structure of the compounds has provided additional information on various structural features and on the rather complex hydrogen bonding network that exists in such compounds. A geometric structure of the short-range local arrangement in the bulk phases is also proposed.

Structure of the Molten Salt Methyl Ammonium Nitrate Explored by Experiments and Theory

We present an analysis of the structure of the monomethylammonium nitrate (MMAN) compound. Vibrational Raman spectroscopy and X-ray powder diffraction have been used to characterize the bulk phases of MMAN, and assignment of the resonant frequencies has been performed by ab initio (DFT) computations on small clusters of the compound. The theoretical spectra are in excellent agreement with the experimental ones and provide a means by which an interpretation of the hydrogen-bonding network that exists in such compound can be analyzed. In particular, we found that the spectrum of one of the solid phases is structurally very similar to that of the liquid. We present experimental evidence for the existence of such phase both from X-ray data and Raman spectra which, in turn, is easily interpreted with a one-to-one correspondence with the ab initio simulation of the small clusters. A geometric structure of the short-range local arrangement in these two bulk phases is therefore proposed.

Pathological biominerals: Raman and infrared studies of bioapatite deposits in human heart valves

We studied pathological bioapatite from patients undergoing valvular replacement due to severe aortic and mitral stenosis. Three different types of mineralized human cardiac valves were analyzed. We used infrared and Raman spectroscopy to infer the presence of the carbonate group and evaluate the carbonate substitution in bioapatite structure. The Raman spectra showed that the pathological bioapatite is a B-type “carbonateapatite” (CO32- for PO43-) similar to the major mineralized products derived from normal biomineralization processes occurring in the human body. Fourier transform infrared spectra (FT-IR) confirmed the B-type carbonate substitution (CO32- for PO43-) and showed evidence for the partial replacement of [OH] by [CO3] (A-type substitution). The carbonate content of the samples inferred by the spectroscopic measurements is in good agreement with the range of values estimated for biological apatite. On the contrary, the crystal size of the pathological apatite estimated using the percentage area of the component at 1059 cm−1 of the infrared spectrum is in the nanometer range and it is significantly smaller than the crystal size of normal mineralized tissues.

Interaction of a long alkyl chain protic ionic liquid and water.

A combined experimental/theoretical approach has been used to investigate the role of water in modifying the microscopic interactions characterizing the optical response of 1-butyl-ammonium nitrate (BAN) water solutions. Raman spectra, dominated by the signal from the protic ionic liquid, were collected as a function of the water content, and the corresponding spatial organization of the ionic couples, as well as their local arrangement with water molecules, was studied exploiting classical molecular dynamics calculations. High quality spectroscopic data, combined with a careful analysis, revealed that water affects the vibrational spectrum BAN in solution: as the water concentration is increased, peaks assigned to stretching modes show a frequency hardening together with a shape narrowing, whereas the opposite behavior is observed for peaks assigned to bending modes. Calculation results clearly show a nanometric spatial organization of the ionic couples that is not destroyed on increasing the water content at least within an intermediate range. Our combined results show indeed that small water concentrations even increase the local order. Water molecules are located among ionic couples and are closer to the anion than the cation, as confirmed by the computation of the number of H-bonds which is greater for water-anion than for water-cation. The whole results set thus clarifies the microscopic scenario of the BAN-water interaction and underlines the main role of the extended hydrogen bond network among water molecules and nitrate anions.

Control of Structural, Electronic, and Optical Properties of Eumelanin Films by Electrospray Deposition

The capability to monitor finely the physical properties of eumelanin, an important class of biopolymers, involved in melanoma cancer pathologies, whose function and intrinsic disorder still collects the interest of many investigators, was achieved by means of electrospray deposition (ESD). By alleviating the problem of the solubility of melanin through the realization of high-quality films it was possible to spread light on the unknown biopolymer supramolecular organization. In fact, on the basis of scanning probe microscopies, electron spectroscopies, and transport properties, it was possible to delineate peculiar features of the melanin organization varying from heteropolymeric to oligomeric in character and eventually turning in a cross-linked secondary molecular structure.

E-1(A) electronic band gap in Wurtzite InAs nanowires studied by resonant raman scattering

We report on resonant Raman experiments carried out on wurtzite InAs nanowires. Resonant conditions have been obtained by tuning either the excitation energy or the band gap through external high pressure at fixed excitation energy. A complete azimuthal study of the Raman spectra with two laser excitation lines (2.41 and 1.92 eV) has also been performed on a single wire. The measured E2(H) mode resonance indicates that the E1(A) gap is about 2.4 eV, which is considerably reduced with respect to the zinc-blende InAs E1 gap. These findings confirm recent theoretical calculations of crystal phase induced bandstructure modifications.

Role of ionic liquids in protein refolding: Native/fibrillar versus treated lysozyme

Several ionic liquids (ILs) are known to revert aggregation processes and to improve the in vitro refolding of denatured/fibrillar proteins. Here, Raman spectroscopy is exploited to verify the refolding capability of several ammonium-based ILs and to identify the microscopic signatures of the structural rearrangements induced by the interaction of ILs with fibrillar lysozyme. We collected and carefully analyzed spectra from native, fibrillar and ILs-treated fibrillar lysozyme to follow the microscopic process induced by ILs. These allowed us to identify different mechanisms of interaction depending on the length of the cation alkyl chain. A clear refolding effect was observed with EAN, as well as a tendency of the longer alkyl chain (PAN and BAN) of dissolving the fibril packing. A specific interaction mainly affecting the aromatic residues was identified for MEOAN (a long chain ILs with an ether group). The whole of the results, thus, provides new and detailed information on the ILs–protein interaction and shows Raman spectroscopy as a simple, reliable and effective diagnostic technique in this field.

Sequential dissociation of insulin amyloids probed by high pressure Fourier transform infrared spectroscopy

High pressure (HP) Fourier transform infrared (FTIR) spectroscopy has been here employed to investigate the thermodynamic stability of bovine pancreatic insulin (BPI) amyloids. Once the aggregation reaction has started, the backbone arrangement of the proteins forming the amyloid is known to reach a stationary phase in a few hours; after this time the infrared absorption of fibrils becomes stable. It is here shown how further stabilization of the structure during the stationary phase can be probed via FTIR spectroscopy, through the observation of the high pressure behaviour of fibrils formed at different maturation stages. We report on the high pressure fragmentation of insulin amyloids, probed on fibrils formed in the early stages of the stationary phase. Moreover, we noticed a sequentiality high pressure dissociation that seems to respect a pre-existing hierarchy of structures: the stabilization of a protofibrillar state is observed at pressures in the order of a few kbar and our results suggest the possible occurrence of a partial refolding, induced by pressures up to 11.4 kbar. Our findings remark the importance of high pressure in stabilizing intermediate structures and in evaluating the driving forces of fibrillation, demonstrating how the control of electrostatic interactions and hydrophobic effects can be used to characterize the factors that modulate amyloids stability.

Local disorder investigation in NiS2−xSex using Raman and Ni K-edge x-ray absorption spectroscopies

We report on Raman and Ni K-edge x-ray absorption investigations of a NiS(2-x)Se(x) (with x = 0.00, 0.50/0.55, 0.60, and 1.20) pyrite family. The Ni K-edge absorption edge shows a systematic shift going from an insulating phase (x = 0.00 and 0.50) to a metallic phase (x = 0.60 and 1.20). The near-edge absorption features show a clear evolution with Se doping. The extended x-ray absorption fine structure data reveal the evolution of the local structure with Se doping which mainly governs the local disorder. We also describe the decomposition of the NiS(2-x)Se(x) Raman spectra and investigate the weights of various phonon modes using Gaussian and Lorentzian profiles. The effectiveness of the fitting models in describing the data is evaluated by means of Bayes factor estimation. The Raman analysis clearly demonstrates the disorder effects due to Se alloying in describing the phonon spectra of NiS(2-x)Se(x) pyrites.

Infrared Microspectroscopy study of insulin crystals at high pressure

During the last years the coupling of high pressure techniques and infrared spectroscopy has proven to be a very powerful tool in the study of conformational changes of proteins. Protein unfolding and monomerization are events that are expected to take place at high pressure due to the peculiarity of pressure to shift the system towards the state that occupies the minimum volume. We observed the growth of apparently cubic crystals at a pressure of about 4 kbar, subjecting to high pressure a solution of misfolded insulin. Even if high pressure is commonly used to tune the growth rate of crystals, protein crystallization at high pressure is not a well known process and no evidences of the particular case of insulin are present in literature.