Paola Sassi - Academia.edu (original) (raw)

Papers by Paola Sassi

The Journal of Physical Chemistry B, 2015

The thermal aggregation of lysozyme has been analyzed in water/ethanol solutions at low pH to ind... more The thermal aggregation of lysozyme has been analyzed in water/ethanol solutions at low pH to induce the specific protein aggregation pathway which leads to fibrillar structures in a few hours. In this solvating medium, the protein undergoes a conformational rearrangement promoting the formation of fibrils that are structurally similar to amyloid ones. As the process evolves with different steps, a multitechnique approach has been used by means of analytical probes that can be selectively sensitive in the detection of the different stages of protein association. Fourier transform infrared spectroscopy, intrinsic fluorescence, stationary fluorescence anisotropy, transmission electron microscopy (TEM), and atomic force microscopy (AFM) measurements have been carried out at different times to access and characterize the whole aggregation pathway. The data recorded with different experimental setups revealed different sensitivity to different stages of protein assembling. The whole set of data together with the direct visualization of different aggregate structures by use of TEM and AFM imaging enable to discuss a possible mechanism of fibrillation.

Biophysical Chemistry, 2015

In this paper we test the ability of Raman micro-spectroscopy and Raman mapping to investigate th... more In this paper we test the ability of Raman micro-spectroscopy and Raman mapping to investigate the status of cells grown in adhesion on different substrates. The spectra of immortalized SH-SY5Y cells, grown on silicon and on metallic substrates are compared with those obtained for the same type of cells adhering on organic polyaniline (PANI), a memristive substrate chosen to achieve a living bio-hybrid system. Raman spectra give information on the status of the single cell, its local biochemical composition, and on the modifications induced by the substrate interaction. The good agreement between Raman spectra collected from cells adhering on different substrates confirms that the PANI, besides allowing the cell growth, doesn't strongly affect the general biochemical properties of the cell. The investigation of the cellular state in a label free condition is challenging and the obtained results confirm the Raman ability to achieve this information.

Phys. Chem. Chem. Phys., 2015

Protein low-frequency vibrational modes are an important portion of a proteins' dynamical reperto... more Protein low-frequency vibrational modes are an important portion of a proteins' dynamical repertoire. Yet, it is notoriously difficult to isolate specific vibrational features in the spectra of proteins. Given an appropriately chosen model peptide, and using different experimental conditions, we can simplify the system and gain useful insights into the protein vibrational properties. Combining neutron scattering, depolarized light scattering, and molecular dynamics simulations, we analyse the low frequency vibrations of biological molecules, comparing the results from a small globular protein, lysozyme, and an amphiphilic peptide, NALMA, both in solution and in powder states. Lysozyme and NALMA present similar spectral features in the frequency range between 1 and 10 THz. With the aid of MD simulations, we assign the spectral features to methyl groups' librations (1-5 THz) and hindered torsions (5-10 THz) in NALMA. Our data also show that, while proteins display boson peak vibrations in both powder and solution forms, NALMA exhibits boson peak vibrations in powder form only. This provides insight into the nature of this feature, suggesting a connection of BP collective motions to a characteristic length scale of heterogeneities present in the system. These results provide context for the use of model peptide systems to study protein dynamics; demonstrating both their utility, and the great care that has to be used in extrapolating results observed in powder to solutions. † Electronic supplementary information (ESI) available: Additional details on MD simulations; integrated intensity of SF DLS spectra of lysozyme; graphical summary of methyl groups' dynamics of hydrated NALMA powders from MD simulations. See

Physical Review Letters, 2003

The phase behavior of a biomimetic monolayer consisting of diphospholipid molecules on water is i... more The phase behavior of a biomimetic monolayer consisting of diphospholipid molecules on water is investigated using vibrational sum-frequency generation and fluorescence microscopy. In addition to the transition from the -molecularly disordered -liquid phase to the highly ordered and oriented condensed phase, a novel, extremely sharp transition is observed at low compression, which is attributed to the uncurling of the hydrophobic alkane chains upon compression.

Journal of Raman …, Jan 1, 2009

The collective properties of liquid tert-butyl alcohol (TBA) were analysed by low-wavenumber Rama... more The collective properties of liquid tert-butyl alcohol (TBA) were analysed by low-wavenumber Raman (LWR) scattering spectroscopy. Vibrational and relaxation phenomena of this H-bonding system were assessed in pure liquid phase at different temperatures in the 15-70 • C range, and in solution with 2,2 -dimethyl butane (2,2 -DMB) and water as a function of composition in the 0.7 ≤ x TBA ≤ 0.9 range at constant temperature (T = 25 • C). The LWR spectrum of pure TBA (below 150 cm −1 ) was expressed by the dynamical (or Raman) susceptibility χ (ν) and reproduced by curve fitting using three functional forms. The high-wavenumber band, whose intensity is poorly dependent on the temperature, was assigned to the ultrafast librational mode; the remaining components with an intensity that increased with temperature were attributed to relaxation dynamics in the range of picosecond ('slow' 3 ps) and sub-picosecond ('fast', 0.4 ps) timescales. Adding 2,2 -DMB to TBA had no significant effect on the intermolecular interactions in alcohol-rich solutions, with almost unchanged LWR scattering features. On the other hand, water added to TBA determined an increase in interactions, similar to the effect of a temperature decrease in pure liquid alcohol; this was clearly depicted by the LWR profiles. Moreover, through the analysis of the OH stretching bands of water in solution, the confinement of aqueous pools in the hydrophilic spaces of alcohol-rich solutions was confirmed.

Journal of Molecular Liquids, 2002

ABSTRACT The vibrational and orientational dynamics of the pseudo-symmetric top molecule CD3NO2 i... more ABSTRACT The vibrational and orientational dynamics of the pseudo-symmetric top molecule CD3NO2 is investigated through the bandshape analysis of the totally symmetric ν3 (CD3 bending), ν4 (CN stretching) and ν5 (NO2 bending) motions. Temperature and concentration effects have also been examined. The stochastic Kubo theory and the hydrodynamic model have been utilised for the interpretation of the dephasing and of the rotational relaxation processes, respectively. Similarities and differences with the analogues dynamical processes in the light isotopomer CH3NO2 are evidenced.

The Journal of Physical Chemistry B, 2009

Journal of Non-Crystalline Solids, 2015

Extended frequency range depolarized light scattering is a spectroscopic technique operating in t... more Extended frequency range depolarized light scattering is a spectroscopic technique operating in the GHz-THz range that, applied to aqueous solutions of biomolecules, is able to disentangle the dynamics of the solute from that of water, and relaxation processes of bulk from those of hydration water. Experiments performed on aqueous solution of a variety of biological systems of different nature, such as small hydrophobic and hydrophilic molecules, amino acids, dipeptides, and proteins, have shown that a significant increase in the dynamical retardation and in the extent of the perturbation of water surrounding solute molecules occurs at increasing chemical complexity of the solute. The behavior of aqueous solutions of lysozyme is here analyzed in detail, as a function of solute concentration. Our results provide evidence of a dynamical perturbation extending over more than three water layers in diluted solutions. We find a strong reduction in the average hydration number at increasing solute concentration that cannot be explained by the random superposition of hydration layers among lysozyme molecules in close proximity. This behavior is consistent with the formation of clusters in solution.

Journal of Raman Spectroscopy, 2015

In this study, Fourier transform infrared, Raman and Brillouin spectroscopy have been used to stu... more In this study, Fourier transform infrared, Raman and Brillouin spectroscopy have been used to study lipid phase behavior of hydrated as well as dried multilamellar L-α-phosphatidylcholine assemblies, in order to compare limitations and potentials of the different techniques. Dried lipid samples have been studied in the presence and absence of trehalose, which is known to affect the phase behavior of these systems. The methylene C-H stretching (2800-3000 cm À1 ) region in infrared (IR) and Raman spectra provided mutually consistent information on the rearrangement of lipid acyl chains occurring at the lipid melting temperature. IR spectra have a higher signal-to-noise ratio, thus permitting a more precise evaluation of the melting temperature. In the hydrated lipid samples, the C-H stretching region in the Raman spectra is less affected by the contribution of water compared with that in the IR spectra. Raman spectra are particularly suitable to simultaneously study both lipid and water contributions allowing to distinguish ice from non-frozen water below 0°C. Brillouin light scattering was used to probe the collective dynamics, i.e. the propagation velocity and the attenuation of longitudinal acoustic modes in the lipid samples. Lipid phase transitions are evident from a change in the temperature behavior of the acoustic velocity. Moreover, a strong relaxation process with a characteristic time of 14 ps was observed in the sample dried without trehalose with a maximum in acoustic attenuation at about 45°C, which likely reflects the rearrangement of acyl chains.

Journal of the Chemical Society, Faraday Transactions, 1994

The phase behavior of a biomimetic monolayer consisting of diphospholipid molecules on water is i... more The phase behavior of a biomimetic monolayer consisting of diphospholipid molecules on water is investigated using vibrational sum-frequency generation and fluorescence microscopy. In addition to the transition from the -molecularly disordered -liquid phase to the highly ordered and oriented condensed phase, a novel, extremely sharp transition is observed at low compression, which is attributed to the uncurling of the hydrophobic alkane chains upon compression.

The phase behavior of a biomimetic monolayer consisting of diphospholipid molecules on water is i... more The phase behavior of a biomimetic monolayer consisting of diphospholipid molecules on water is investigated using vibrational sum-frequency generation and fluorescence microscopy. In addition to the transition from the -molecularly disordered -liquid phase to the highly ordered and oriented condensed phase, a novel, extremely sharp transition is observed at low compression, which is attributed to the uncurling of the hydrophobic alkane chains upon compression.

ABSTRACT DRS spectra of liquid 1-octanol and n-octane have been measured at different temperature... more ABSTRACT DRS spectra of liquid 1-octanol and n-octane have been measured at different temperatures. Relevant spectral features are compared in order to evaluate the effect of specific H-bonding interactions on spectral profiles. A central sharp lorentzian component (FWHH of about 1 cm-1) is clearly detected for both systems. This component is interpreted in terms of torsional dynamics of the alkylic chain and appears sensitive to the presence of H-bonding interactions. By contrast at high wavenumbers the low-frequency distribution, represented in the reduced form, results essentially related to more local non-specific interactions.

Physical Chemistry Chemical Physics, 2000

Ab initio study of the PtC molecule. A new assignment of the red bands to the transitions 1 3P X ... more Ab initio study of the PtC molecule. A new assignment of the red bands to the transitions 1 3P X (X = 1,0')-X 1R'

The Journal of Physical Chemistry B, 2014

Molecular dynamics of acetone-alkali metal halide (LiBr, LiI) solutions were investigated using d... more Molecular dynamics of acetone-alkali metal halide (LiBr, LiI) solutions were investigated using depolarized Rayleigh scattering (DRS) and low-frequency Raman spectroscopy in the frequency range from ~0.5 to 200 cm(-1) (~20 GHz to 6 THz). These experiments probe fast dynamical fluctuations of the polarizability anisotropy at picosecond and sub-picosecond time scales that are mainly driven by acetone orientational dynamics. Two distinct contributions were revealed: a fast process (units of picosecond, ps) related to the essentially unperturbed bulk solvent and a slow one (tens of ps) assigned to acetone molecules forming Li(+) solvation shells, decelerated by the motional constraint imposed by the cation. The increase of LiBr and LiI concentration significantly slows down the overall solvent relaxation as a consequence of the increased fraction of acetone molecules involved in the ion solvation shells. The global retardation is larger in LiI than LiBr solutions consistently with viscosity trends. This is explained in terms of ion association (at least ion pairing) more favorably promoted by Br(-) than I(-), with reduced Li(+)-acetone interactions in LiBr than LiI solutions. Anion-induced modulation of the Li(+)···O═C contacts, largely responsible for electrostriction phenomena, also affects the reduced THz-Raman spectral density, ascribed to ultrafast librational motions of acetone molecules. Overall, these findings enlighten the interplay between ion-dipole and ion-ion interactions on the fast solvation dynamics in electrolyte solutions of a typical polar aprotic solvent.

The Journal of Physical Chemistry B, 2014

Molecular dynamics and structural properties of water−tert-butyl alcohol (TBA) mixtures are studi... more Molecular dynamics and structural properties of water−tert-butyl alcohol (TBA) mixtures are studied as a function of concentration by extended depolarized light scattering (EDLS) experiments. The wide frequency range, going from fraction to several thousand GHz, explored by EDLS allows distinguishing TBA rotational dynamics from structural relaxation of water and intermolecular vibrational and librational modes of the solution. Contributions to the water relaxation originating from two distinct populations, i.e. hydration and bulk water, are clearly identified. The dynamic retardation factor of hydration water with respect to the bulk, ξ ≈ 4, almost concentration independent, is one of the smallest found by EDLS among a variety of systems of different nature and complexity. This result, together with the small number of water molecules perturbed by the presence of TBA, supports the idea that hydrophobic simple molecules are less effective than hydrophilic and more complex molecules in perturbing the H-bond network of liquid water. At increasing TBA concentrations the average number of perturbed water molecules shows a pronounced decrease and the characteristic frequency of librational motions reduces significantly, both of which are results consistent with the occurrence of self-aggregation of TBA molecules.

Physical Chemistry Chemical Physics, 2014

The molecular dynamics of aqueous solutions of a model amphiphilic peptide is studied as a functi... more The molecular dynamics of aqueous solutions of a model amphiphilic peptide is studied as a function of concentration by broad-band light scattering experiments. Similarly to protein aqueous solutions, a considerable retardation, of about a factor 6-8, of hydration water dynamics with respect to bulk water is found, showing a slight dependence on solute concentration. Conversely, the average number of water molecules perturbed by the presence of peptide, i.e. the hydration number, appears to be strongly modified by adding solute. Its behaviour, decreasing upon increasing concentration, can be interpreted considering the random close-to-contact condition experienced by solute particles. Overall, the present findings support the view of a "long range" effect of peptides on the surrounding water, extending beyond the first two hydration shells.

The Journal of Chemical Physics, 2014

Extended frequency range depolarized light scattering measurements of water-levoglucosan solution... more Extended frequency range depolarized light scattering measurements of water-levoglucosan solutions are reported at different concentrations and temperatures to assess the effect of the presence and distribution of hydroxyl groups on the dynamics of hydration water. The anhydro bridge, reducing from five to three the number of hydroxyl groups with respect to glucose, considerably affects the hydration properties of levoglucosan with respect to those of mono and disaccharides. In particular, we find that the average retardation of water dynamics is ≈3-4, that is lower than ≈5-6 previously found in glucose, fructose, trehalose, and sucrose. Conversely, the average number of retarded water molecules around levoglucosan is 24, almost double that found in water-glucose mixtures. These results suggest that the ability of sugar molecules to form H-bonds through hydroxyl groups with surrounding water, while producing a more effective retardation, it drastically reduces the spatial extent of the perturbation on the H-bond network. In addition, the analysis of the concentration dependence of the hydration number reveals the aptitude of levoglucosan to produce large aggregates in solution. The analysis of shear viscosity and rotational diffusion time suggests a very short lifetime for these aggregates, typically faster than ≈20 ps. © 2014 AIP Publishing LLC.

Food Biophysics, 2013

The concentration dependence of hydration numbers of molecules modelled as nearly spherical parti... more The concentration dependence of hydration numbers of molecules modelled as nearly spherical particles is studied by simple analytic and numerical approaches, in the ideal limit of absence of intermolecular interactions. It is shown that the random close-to-contact condition achieved by solute molecules, noticeably affects the average hydration numbers. Comparison with experimental results obtained by light scattering in glucose and trehalose water solutions shows a reduction of the hydration number that is twice faster than that calculated in absence of interactions, suggesting important aggregation phenomena to occur in both systems, even at relatively low solute concentration. The effect of concentration on shear viscosity is also reported, suggesting that the leading contribution to the increase of viscosity arises from hydration water.

The Journal of Physical Chemistry B, 2015

The thermal aggregation of lysozyme has been analyzed in water/ethanol solutions at low pH to ind... more The thermal aggregation of lysozyme has been analyzed in water/ethanol solutions at low pH to induce the specific protein aggregation pathway which leads to fibrillar structures in a few hours. In this solvating medium, the protein undergoes a conformational rearrangement promoting the formation of fibrils that are structurally similar to amyloid ones. As the process evolves with different steps, a multitechnique approach has been used by means of analytical probes that can be selectively sensitive in the detection of the different stages of protein association. Fourier transform infrared spectroscopy, intrinsic fluorescence, stationary fluorescence anisotropy, transmission electron microscopy (TEM), and atomic force microscopy (AFM) measurements have been carried out at different times to access and characterize the whole aggregation pathway. The data recorded with different experimental setups revealed different sensitivity to different stages of protein assembling. The whole set of data together with the direct visualization of different aggregate structures by use of TEM and AFM imaging enable to discuss a possible mechanism of fibrillation.

Biophysical Chemistry, 2015

In this paper we test the ability of Raman micro-spectroscopy and Raman mapping to investigate th... more In this paper we test the ability of Raman micro-spectroscopy and Raman mapping to investigate the status of cells grown in adhesion on different substrates. The spectra of immortalized SH-SY5Y cells, grown on silicon and on metallic substrates are compared with those obtained for the same type of cells adhering on organic polyaniline (PANI), a memristive substrate chosen to achieve a living bio-hybrid system. Raman spectra give information on the status of the single cell, its local biochemical composition, and on the modifications induced by the substrate interaction. The good agreement between Raman spectra collected from cells adhering on different substrates confirms that the PANI, besides allowing the cell growth, doesn't strongly affect the general biochemical properties of the cell. The investigation of the cellular state in a label free condition is challenging and the obtained results confirm the Raman ability to achieve this information.

Phys. Chem. Chem. Phys., 2015

Protein low-frequency vibrational modes are an important portion of a proteins' dynamical reperto... more Protein low-frequency vibrational modes are an important portion of a proteins' dynamical repertoire. Yet, it is notoriously difficult to isolate specific vibrational features in the spectra of proteins. Given an appropriately chosen model peptide, and using different experimental conditions, we can simplify the system and gain useful insights into the protein vibrational properties. Combining neutron scattering, depolarized light scattering, and molecular dynamics simulations, we analyse the low frequency vibrations of biological molecules, comparing the results from a small globular protein, lysozyme, and an amphiphilic peptide, NALMA, both in solution and in powder states. Lysozyme and NALMA present similar spectral features in the frequency range between 1 and 10 THz. With the aid of MD simulations, we assign the spectral features to methyl groups' librations (1-5 THz) and hindered torsions (5-10 THz) in NALMA. Our data also show that, while proteins display boson peak vibrations in both powder and solution forms, NALMA exhibits boson peak vibrations in powder form only. This provides insight into the nature of this feature, suggesting a connection of BP collective motions to a characteristic length scale of heterogeneities present in the system. These results provide context for the use of model peptide systems to study protein dynamics; demonstrating both their utility, and the great care that has to be used in extrapolating results observed in powder to solutions. † Electronic supplementary information (ESI) available: Additional details on MD simulations; integrated intensity of SF DLS spectra of lysozyme; graphical summary of methyl groups' dynamics of hydrated NALMA powders from MD simulations. See

Physical Review Letters, 2003

The phase behavior of a biomimetic monolayer consisting of diphospholipid molecules on water is i... more The phase behavior of a biomimetic monolayer consisting of diphospholipid molecules on water is investigated using vibrational sum-frequency generation and fluorescence microscopy. In addition to the transition from the -molecularly disordered -liquid phase to the highly ordered and oriented condensed phase, a novel, extremely sharp transition is observed at low compression, which is attributed to the uncurling of the hydrophobic alkane chains upon compression.

Journal of Raman …, Jan 1, 2009

The collective properties of liquid tert-butyl alcohol (TBA) were analysed by low-wavenumber Rama... more The collective properties of liquid tert-butyl alcohol (TBA) were analysed by low-wavenumber Raman (LWR) scattering spectroscopy. Vibrational and relaxation phenomena of this H-bonding system were assessed in pure liquid phase at different temperatures in the 15-70 • C range, and in solution with 2,2 -dimethyl butane (2,2 -DMB) and water as a function of composition in the 0.7 ≤ x TBA ≤ 0.9 range at constant temperature (T = 25 • C). The LWR spectrum of pure TBA (below 150 cm −1 ) was expressed by the dynamical (or Raman) susceptibility χ (ν) and reproduced by curve fitting using three functional forms. The high-wavenumber band, whose intensity is poorly dependent on the temperature, was assigned to the ultrafast librational mode; the remaining components with an intensity that increased with temperature were attributed to relaxation dynamics in the range of picosecond ('slow' 3 ps) and sub-picosecond ('fast', 0.4 ps) timescales. Adding 2,2 -DMB to TBA had no significant effect on the intermolecular interactions in alcohol-rich solutions, with almost unchanged LWR scattering features. On the other hand, water added to TBA determined an increase in interactions, similar to the effect of a temperature decrease in pure liquid alcohol; this was clearly depicted by the LWR profiles. Moreover, through the analysis of the OH stretching bands of water in solution, the confinement of aqueous pools in the hydrophilic spaces of alcohol-rich solutions was confirmed.

Journal of Molecular Liquids, 2002

ABSTRACT The vibrational and orientational dynamics of the pseudo-symmetric top molecule CD3NO2 i... more ABSTRACT The vibrational and orientational dynamics of the pseudo-symmetric top molecule CD3NO2 is investigated through the bandshape analysis of the totally symmetric ν3 (CD3 bending), ν4 (CN stretching) and ν5 (NO2 bending) motions. Temperature and concentration effects have also been examined. The stochastic Kubo theory and the hydrodynamic model have been utilised for the interpretation of the dephasing and of the rotational relaxation processes, respectively. Similarities and differences with the analogues dynamical processes in the light isotopomer CH3NO2 are evidenced.

The Journal of Physical Chemistry B, 2009

Journal of Non-Crystalline Solids, 2015

Extended frequency range depolarized light scattering is a spectroscopic technique operating in t... more Extended frequency range depolarized light scattering is a spectroscopic technique operating in the GHz-THz range that, applied to aqueous solutions of biomolecules, is able to disentangle the dynamics of the solute from that of water, and relaxation processes of bulk from those of hydration water. Experiments performed on aqueous solution of a variety of biological systems of different nature, such as small hydrophobic and hydrophilic molecules, amino acids, dipeptides, and proteins, have shown that a significant increase in the dynamical retardation and in the extent of the perturbation of water surrounding solute molecules occurs at increasing chemical complexity of the solute. The behavior of aqueous solutions of lysozyme is here analyzed in detail, as a function of solute concentration. Our results provide evidence of a dynamical perturbation extending over more than three water layers in diluted solutions. We find a strong reduction in the average hydration number at increasing solute concentration that cannot be explained by the random superposition of hydration layers among lysozyme molecules in close proximity. This behavior is consistent with the formation of clusters in solution.

Journal of Raman Spectroscopy, 2015

In this study, Fourier transform infrared, Raman and Brillouin spectroscopy have been used to stu... more In this study, Fourier transform infrared, Raman and Brillouin spectroscopy have been used to study lipid phase behavior of hydrated as well as dried multilamellar L-α-phosphatidylcholine assemblies, in order to compare limitations and potentials of the different techniques. Dried lipid samples have been studied in the presence and absence of trehalose, which is known to affect the phase behavior of these systems. The methylene C-H stretching (2800-3000 cm À1 ) region in infrared (IR) and Raman spectra provided mutually consistent information on the rearrangement of lipid acyl chains occurring at the lipid melting temperature. IR spectra have a higher signal-to-noise ratio, thus permitting a more precise evaluation of the melting temperature. In the hydrated lipid samples, the C-H stretching region in the Raman spectra is less affected by the contribution of water compared with that in the IR spectra. Raman spectra are particularly suitable to simultaneously study both lipid and water contributions allowing to distinguish ice from non-frozen water below 0°C. Brillouin light scattering was used to probe the collective dynamics, i.e. the propagation velocity and the attenuation of longitudinal acoustic modes in the lipid samples. Lipid phase transitions are evident from a change in the temperature behavior of the acoustic velocity. Moreover, a strong relaxation process with a characteristic time of 14 ps was observed in the sample dried without trehalose with a maximum in acoustic attenuation at about 45°C, which likely reflects the rearrangement of acyl chains.

Journal of the Chemical Society, Faraday Transactions, 1994

The phase behavior of a biomimetic monolayer consisting of diphospholipid molecules on water is i... more The phase behavior of a biomimetic monolayer consisting of diphospholipid molecules on water is investigated using vibrational sum-frequency generation and fluorescence microscopy. In addition to the transition from the -molecularly disordered -liquid phase to the highly ordered and oriented condensed phase, a novel, extremely sharp transition is observed at low compression, which is attributed to the uncurling of the hydrophobic alkane chains upon compression.

The phase behavior of a biomimetic monolayer consisting of diphospholipid molecules on water is i... more The phase behavior of a biomimetic monolayer consisting of diphospholipid molecules on water is investigated using vibrational sum-frequency generation and fluorescence microscopy. In addition to the transition from the -molecularly disordered -liquid phase to the highly ordered and oriented condensed phase, a novel, extremely sharp transition is observed at low compression, which is attributed to the uncurling of the hydrophobic alkane chains upon compression.

ABSTRACT DRS spectra of liquid 1-octanol and n-octane have been measured at different temperature... more ABSTRACT DRS spectra of liquid 1-octanol and n-octane have been measured at different temperatures. Relevant spectral features are compared in order to evaluate the effect of specific H-bonding interactions on spectral profiles. A central sharp lorentzian component (FWHH of about 1 cm-1) is clearly detected for both systems. This component is interpreted in terms of torsional dynamics of the alkylic chain and appears sensitive to the presence of H-bonding interactions. By contrast at high wavenumbers the low-frequency distribution, represented in the reduced form, results essentially related to more local non-specific interactions.

Physical Chemistry Chemical Physics, 2000

Ab initio study of the PtC molecule. A new assignment of the red bands to the transitions 1 3P X ... more Ab initio study of the PtC molecule. A new assignment of the red bands to the transitions 1 3P X (X = 1,0')-X 1R'

The Journal of Physical Chemistry B, 2014

Molecular dynamics of acetone-alkali metal halide (LiBr, LiI) solutions were investigated using d... more Molecular dynamics of acetone-alkali metal halide (LiBr, LiI) solutions were investigated using depolarized Rayleigh scattering (DRS) and low-frequency Raman spectroscopy in the frequency range from ~0.5 to 200 cm(-1) (~20 GHz to 6 THz). These experiments probe fast dynamical fluctuations of the polarizability anisotropy at picosecond and sub-picosecond time scales that are mainly driven by acetone orientational dynamics. Two distinct contributions were revealed: a fast process (units of picosecond, ps) related to the essentially unperturbed bulk solvent and a slow one (tens of ps) assigned to acetone molecules forming Li(+) solvation shells, decelerated by the motional constraint imposed by the cation. The increase of LiBr and LiI concentration significantly slows down the overall solvent relaxation as a consequence of the increased fraction of acetone molecules involved in the ion solvation shells. The global retardation is larger in LiI than LiBr solutions consistently with viscosity trends. This is explained in terms of ion association (at least ion pairing) more favorably promoted by Br(-) than I(-), with reduced Li(+)-acetone interactions in LiBr than LiI solutions. Anion-induced modulation of the Li(+)···O═C contacts, largely responsible for electrostriction phenomena, also affects the reduced THz-Raman spectral density, ascribed to ultrafast librational motions of acetone molecules. Overall, these findings enlighten the interplay between ion-dipole and ion-ion interactions on the fast solvation dynamics in electrolyte solutions of a typical polar aprotic solvent.

The Journal of Physical Chemistry B, 2014

Molecular dynamics and structural properties of water−tert-butyl alcohol (TBA) mixtures are studi... more Molecular dynamics and structural properties of water−tert-butyl alcohol (TBA) mixtures are studied as a function of concentration by extended depolarized light scattering (EDLS) experiments. The wide frequency range, going from fraction to several thousand GHz, explored by EDLS allows distinguishing TBA rotational dynamics from structural relaxation of water and intermolecular vibrational and librational modes of the solution. Contributions to the water relaxation originating from two distinct populations, i.e. hydration and bulk water, are clearly identified. The dynamic retardation factor of hydration water with respect to the bulk, ξ ≈ 4, almost concentration independent, is one of the smallest found by EDLS among a variety of systems of different nature and complexity. This result, together with the small number of water molecules perturbed by the presence of TBA, supports the idea that hydrophobic simple molecules are less effective than hydrophilic and more complex molecules in perturbing the H-bond network of liquid water. At increasing TBA concentrations the average number of perturbed water molecules shows a pronounced decrease and the characteristic frequency of librational motions reduces significantly, both of which are results consistent with the occurrence of self-aggregation of TBA molecules.

Physical Chemistry Chemical Physics, 2014

The molecular dynamics of aqueous solutions of a model amphiphilic peptide is studied as a functi... more The molecular dynamics of aqueous solutions of a model amphiphilic peptide is studied as a function of concentration by broad-band light scattering experiments. Similarly to protein aqueous solutions, a considerable retardation, of about a factor 6-8, of hydration water dynamics with respect to bulk water is found, showing a slight dependence on solute concentration. Conversely, the average number of water molecules perturbed by the presence of peptide, i.e. the hydration number, appears to be strongly modified by adding solute. Its behaviour, decreasing upon increasing concentration, can be interpreted considering the random close-to-contact condition experienced by solute particles. Overall, the present findings support the view of a "long range" effect of peptides on the surrounding water, extending beyond the first two hydration shells.

The Journal of Chemical Physics, 2014

Extended frequency range depolarized light scattering measurements of water-levoglucosan solution... more Extended frequency range depolarized light scattering measurements of water-levoglucosan solutions are reported at different concentrations and temperatures to assess the effect of the presence and distribution of hydroxyl groups on the dynamics of hydration water. The anhydro bridge, reducing from five to three the number of hydroxyl groups with respect to glucose, considerably affects the hydration properties of levoglucosan with respect to those of mono and disaccharides. In particular, we find that the average retardation of water dynamics is ≈3-4, that is lower than ≈5-6 previously found in glucose, fructose, trehalose, and sucrose. Conversely, the average number of retarded water molecules around levoglucosan is 24, almost double that found in water-glucose mixtures. These results suggest that the ability of sugar molecules to form H-bonds through hydroxyl groups with surrounding water, while producing a more effective retardation, it drastically reduces the spatial extent of the perturbation on the H-bond network. In addition, the analysis of the concentration dependence of the hydration number reveals the aptitude of levoglucosan to produce large aggregates in solution. The analysis of shear viscosity and rotational diffusion time suggests a very short lifetime for these aggregates, typically faster than ≈20 ps. © 2014 AIP Publishing LLC.

Food Biophysics, 2013

The concentration dependence of hydration numbers of molecules modelled as nearly spherical parti... more The concentration dependence of hydration numbers of molecules modelled as nearly spherical particles is studied by simple analytic and numerical approaches, in the ideal limit of absence of intermolecular interactions. It is shown that the random close-to-contact condition achieved by solute molecules, noticeably affects the average hydration numbers. Comparison with experimental results obtained by light scattering in glucose and trehalose water solutions shows a reduction of the hydration number that is twice faster than that calculated in absence of interactions, suggesting important aggregation phenomena to occur in both systems, even at relatively low solute concentration. The effect of concentration on shear viscosity is also reported, suggesting that the leading contribution to the increase of viscosity arises from hydration water.