Peptide nanostructures compose a new class of materials that have gained attention due to their interesting properties. Among them, nanotubes of diphenylalanine (FF) and its analogues have been one of the most studied structures in the last few years. Their importance originates from the need to better understand the formation of β-amyloid fibrils which are associated with Alzheimer's disease. In this work, the FF self-assembly process was probed using time-resolved Raman microscopy. The changes in the Raman spectra are followed over time after injecting water into a FF-film until micro/nanotubes (MNTs) are formed. Specific features of the Raman spectra clearly suggest that FF-molecules after water injection form an intermediate species before forming FF-MNTs. The broad Raman bands observed for the intermediate species suggest the presence of very heterogeneous structures based on FF. The FF-MNTs appear almost instantaneously (detected via the rise of the typical Raman bands of FF-MNTs at 761, 1249 and 1426 cm) after the intermediate structures are formed. This delayed formation of FF-MNTs supports a nucleation process. The formation via nucleation of FF-MNTs is further corroborated by a simulation of the Raman spectra based on a 2-step kinetic model and the respective vibrational Raman modes are identified using Density Functional Theory vibrational calculations. Our results indicate that the driving force for the FF-MNT patterning process is the electric dipole re-orientation originating from the FF dipeptide unit connectivity over time.
The temperature dependence (10 − 290 K) of the low-frequency (20 − 150 cm −1 ) Raman-active phonon modes of supercooled confined water in L,L-diphenylalanine micro/nanotubes was analysed. The isolated dynamics of a specific geometry of water cluster (pentamer) in supercooled confined regime was studied in detail. A particular mode concerning water-nanotube interaction was also probed. A fragile-to-strong transition at 204 K was observed and related to the crossing of the Widom line. The critical exponent analyses of the relaxation rate data based on mode-coupling theory indicated perfect agreement among experimental data and theory. Our results are consistent with the existence of a second critical point of water.The distinction between gas and liquid disappears above its critical point. At pressure and temperature above this point, the system is said to be in a fluid state (supercritical fluid) [1]. Supercritical fluids are recognized as possessing unique solvation properties that make them important technological materials [1]. Of particular interest is the behavior of water in confined spaces since it plays a key role in protein hydration since nanoscale fluctuations associated with the so-called Widom line can influence biological processes [2,3].Poole et al.[4] presented a thermodynamically consistent molecular dynamical simulation study view regarding the global phase behavior of supercooled water. According to these authors, in the supercooled region just below the line of homogeneous ice nucleation, a critical point of liquid-liquid coexistence (LLCP) could exist that would eliminate the first-order transition line between low-density liquid (LDL) and high-density liquid (HDL) aqueous phases. Thus, liquid-liquid phase separation and the existence of the LLCP in water remains as a plausible hypothesis and requires further verification [5]. The Widom line temperature T W corresponds to the loci of maxima of thermodynamic response function in the one-phase region beyond the LLCP proposed to exist in supercooled liquid water [4].Molecular dynamics simulations of the TIP4P/2005 model of water performed by Kumar et al. [6] indicated that the onset of the Boson peak in supercooled bulk water coincides with the crossover to a predominantly LDL-like below T W . Gallo, Corradini and Roveri [7] studied the dynamical properties of aqueous solution of NaCl upon supercooling by molecular simulations. They found a crossover from a fragile (super-Arrhenius) to a strong (Arrhenius) behavior upon crossing the T W by both ionic solution and bulk water.Experiments in the supercooling region are extremely difficult due to crystal nucleation processes. Thus exper- * herculano.martinho@ufabc.edu.br imental pieces of evidence concerning the different hypotheses supporting the existence of LLCP are hard to test [8].In confinement water can be more easily supercooled and studied in region of phase space where crystallization of bulk water cannot be avoided. Confined water in nanoporous silica have been extensively studied [9][10]...
It has been recognized in the literature that some physical properties of hydrated biomolecules, e.g., the occurrence of Boson peak , resembles of those of glassy state. In the present work is shown that quantum fluctuations play a fundamental role on describing the glassy state of biomolecules, specially at lower hydration levels. It is reported a remarkable linear dependence on the quantumness and the slope of the Boson peak frequency temperature dependence which would be used to classify de degree of quantum contributions to the glassy state by glasses in general. Finally, it is shown that the Boson peak two-bands spectral structure observed in some cases could be direct linked to the anisotropy of the material elastic properties.
A detailed quantitative analysis of the specific heat in the 0.5- to 200-K temperature range for almost dry L-cysteine and its dimer, L-cystine, amino acids is presented. We report the occurrence of a sharp first-order transition at ∼76 K for L-cysteine associated with the thiol group ordering which was successfully modeled with the two-dimensional Ising model. We demonstrated that quantum rotors, two-level systems (TLS), Einstein oscillators, and acoustic phonons (the Debye model) are essential ingredients to correctly describe the overall experimental data. Our analysis pointed out the absence of the TLS contribution to the low temperature specific heat of L-cysteine. This result was similar to that found in other noncrystalline amorphous materials, e.g., amorphous silicon, low density amorphous water, and ultrastable glasses. L-cystine presented an unusual nonlinear acoustic dispersion relation ω(q)=vq0.95 and a Maxwell-Boltzmann-type distribution of tunneling barriers. The presence of Einstein oscillators with ΘE∼70 K was common in both systems and adequately modeled the boson peak contributions.
Investigating the molecular mechanism underlying the aggregation process of amyloid fibers is of great importance both for its implications in several degenerative diseases and for design of new materials based...
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