Inelastic X-ray Scattering of Sub- and Supercritical Methanol

Abstract

Inelastic X-ray scattering experiments have been performed on methanol as a function of density from the ambient condition to the supercritical state by using third-generation synchrotron radiation. The positive dispersion of the sound velocity as compared to the hydrodynamic values was observed over ~4 nm -1 of the momentum transfer (Q) for ambient methanol. This finding in methanol is similar to that in water; however, the positive dispersion in methanol is smaller (~50 %) than that in water (~100 %). With decreasing density of methanol, the positive dispersion first decreased in the range 0.791 < < 0.60 gcm -3 at Q = 1-10 nm -1 and then increased over the range 0.50 < < 0.20 gcm -3 again only at higher Q (> ~4 nm -1 ) in supercritical methanol. These results have suggested that small clusters of a few methanol molecules are formed in the supercritical region as observed for supercritical water at low densities. It is consistent with the Raman spectra and neutron diffraction results.

Vibrational Collective Dynamics of Dry Proteins in the Terahertz Region

“…In addition, a weak bump can be identified at 0.6 Å –1 , likely due to the center-to-center α helix packing distance . These features were already observed in few other proteins investigated by neutron and X-ray − scattering and by numerical simulations, which corroborates the quality of our data and the overall reliability of the applied correction procedure.…”

“…In addition, in this lower-frequency range, the modes get delocalized and involve the motion of larger groups of atoms. The coherent inelastic contribution observed herewith was not revealed by previous X-ray scattering measurements of dry lysozyme, probably due to the long Lorentzian tails of the energy resolution function. On the contrary, an inelastic coherent signal had already been observed in dry perdeuterated C-phycocyanin by neutron triple-axis spectroscopy, although the double spectral feature was not resolved, likely due to the limited statistics at that time …”

“…Indeed, the absence of sharper inelastic peaks is due to intrinsic properties of the system, as is wellknown from few other Brillouin investigations on similar disordered samples. [11][12][13][14]16,24,28 In fact, this is further confirmed by several other experimental techniques, such as incoherent neutron scattering, 29−31 far-infrared, 32,33 Raman, 34 and terahertz 35−37 spectroscopies, which have shown that no sharp inelastic feature can be found in the protein vibrational density of states below 40 meV. This happens because, at variance with small molecular systems where the vibrational resonances are widely spaced and relatively narrow, the collective vibrational modes of complex macromolecules become dense, and individual modes overlap with each other.…”

“…With a protein mass density of 1.4 g/cm, , the longitudinal modulus of the protein alone turns out to be 20 GPa, definitely higher than bulk water (9 GPa). We can compare the elastic properties of the dry protein with those of cytochrome c hydrated at 0.2 h ( h = grams of water/grams of protein) and of β-lactoglobulin at 0.5 h and 1.0 h , where the longitudinal velocities, as measured by inelastic X-ray scattering, result to be 3458 ± 42 m/s, 2900 ± 160 m/s, and 2860 ± 110 m/s, respectively. If we suppose these proteins to have elastic average behavior and mass density similar to MBP, we find that the 20 GPa longitudinal modulus of the dry system decreases to 16, 11, and 11 GPa when the hydration level increases from 0 h to 0.2 h , 0.5 h , and 1.0 h , respectively.…”

“…Inelastic neutron and X-ray − scattering experiments have shown that hydrated proteins support collective density fluctuations similar to those existing in liquids and disordered matter. Yet, the intimate nature of this short-wavelength collective dynamics is still far from being clear: neither are known if coherent excitations can propagate through the dry protein system nor if the role of the solvent has been completely elucidated, , although the propagation of collective excitations in protein hydration water was recently observed …”