Evidence of coexistence of change of caged dynamics at Tg and the dynamic transition at Td in solvated proteins

Mossbauer spectroscopy and neutron scattering measurements on proteins embedded in solvents including water and aqueous mixtures have emphasized the observation of the distinctive temperature dependence of the atomic mean square displacements, , commonly referred to as the dynamic transition at some temperature Td. At low temperatures, increases slowly, but it assume stronger temperature dependence after crossing Td, which depends on the time/frequency resolution of the spectrometer. Various authors have made connection of the dynamics of solvated proteins including the dynamic transition to that of glass-forming substances. Notwithstanding, no connection is made to the similar change of temperature dependence of obtained by quasielastic neutron scattering when crossing the glass transition temperature Tg, generally observed in inorganic, organic and polymeric glass-formers. Evidences are presented to show that such change of the temperature dependence of from neutron scattering at Tg is present in hydrated or solvated proteins, as well as in the solvents used unsurprisingly since the latter is just another organic glass-formers. The obtained by neutron scattering at not so low temperatures has contributions from the dissipation of molecules while caged by the anharmonic intermolecular potential at times before dissolution of cages by the onset of the Johari-Goldstein beta-relaxation. The universal change of at Tg of glass-formers had been rationalized by sensitivity to change in volume and entropy of the beta-relaxation, which is passed onto the dissipation of the caged molecules and its contribution to . The same rationalization applies to hydrated and solvated proteins for the observed change of at Tg.Comment: 28 pages, 10 figures, 1 Tabl

Excess wing and Johari-Goldstein relaxation in binary mixtures of glass formers

Dielectric loss Spectra Of pure quinaldine and tert-butylpyridine and their mixtures With tristyrene are presented. The pure systems present lit excess wing and no secondary peaks in the temperature interval from above to well below the glass transition. However, when mixed in low concentration with tristyrene the excess wing is replaced by a distinct secondary peak. This distinct process can be identified as a Johari-Goldstein relaxation within the Coupling mode interpretation. lit the frame Of the Coupling Model the transition from the relaxation scenario with the excess wing to that with a distinct secondary peak is related to the increase of intermolecular constraints. In our case this increase of constraints is due to the low mobility component of the mixture (tristyrene)

The Johari-Goldstein beta-relaxation of glass-forming binary mixtures RID A-8503-2012

The present paper shows, by means of broadband dielectric measurements, that the primary alpha- and the secondary Johari-Goldstein (JG) beta-processes in binary mixtures are strongly correlated. This occurs for different polar rigid probes dissolved in apolar glass-forming solvents, over a wide temperature and pressure range. reserved. We found that the coupling parameter n = 1 - beta(KWW) and the ratio between alpha- and beta-relaxation time reduce on increasing the size of the solute solved within the same apolar matrix. Moreover, such a ratio is invariant when calculated at different combinations of P and T maintaining either the primary or the JG relaxation times constant. Dielectric spectra measured at different T-P combinations but with an invariant alpha-relaxation time are well superposed in both the alpha- and beta-frequency ranges. Experimental results can be rationalized by Coupling Model equation. (C) 2010 Elsevier B.V. All rights reserved

Genuine Johari-Goldstein beta-relaxations in glass-forming binary mixtures

Broadband dielectric spectroscopy measurements were performed on glass-forming binary mixtures, composed of rigid polar molecules dissolved at low concentration in apolar viscous solvent (tristyrene). Dielectric spectra were dominated by the polar molecule contribution, so enabling the study of its dynamic behavior. A well resolved secondary relaxation, not attributable to internal degrees of freedom, was visible in both the liquid and glassy states: for its intermolecular nature, it can be called a 'genuine' Johari-Goldstein (JG) relaxation, in the sense that it is a local and non-cooperative process but entailing the motion of the molecule as a whole. Among our results, the following ones are noteworthy: (a) polar systems in the neat state showed an excess wing that became a well resolved JG-peak on mixing with the apolar solvent; (b) the time-scale distance between structural and JG loss peak increased with the apolar solvent fraction; (c) broader the structural loss peak was, larger was the separation in the frequency scale between structural and JG peak; (d) the JG relaxation time showed a non-Arrhenius temperature behavior above T, paralleling that of the structural relaxation time. All the results can be rationalized in the framework of Coupling Model. (c) 2006 Elsevier B.V. All rights reserved

Correlation of structural and Johari-Goldstein relaxations in systems vitrifying along isobaric and isothermal paths

The effect of isobaric cooling ( over the range 190 - 350 K) and isothermal compression (up to 700 MPa) on structural alpha- and secondary beta-relaxations has been studied for low molecular weight glass-forming systems. The shape of the a- loss peak was found to change with temperature T and pressure P but to be constant for a combination of T and P giving the same tau(alpha)(T, P). The invariance of shape at constant tau(alpha)(T, P) involved also the excess wing, i.e. the process showing up at the high-frequency tail of the alpha-loss peak in systems with no well-resolved beta-process. Likewise, systems where the excess wing evolved to a well-resolved beta-peak showed that the timescale of the beta-process was strongly related to that of the alpha-peak. Also in this case, once a given value tau(alpha)(T, P) was fixed, a corresponding value tau beta(T, P) was found for different T and P. Same results were found also for a binary mixture of a polar rigid molecule dissolved in an apolar solvent, i.e. a model system for Johari-Goldstein intermolecular relaxation. These evidences imply that a strong correlation exists between structural alpha- and Johari-Goldstein relaxation over a wide interval of temperature and density

Effect of pressure on relaxation dynamics at different time scales in supercooled systems

For over 10 decades, the dynamics of two glass-forming systems have been investigated by dielectric spectroscopy under cooling (from melting point to well below the glass transition temperature) and under compression (from atmospheric pressure up to 700 MPa). The alpha-relaxation time tau(alpha) was significantly affected by both thermodynamic variables, showing equal roles in slowing down the dynamics. Some similarities have been found; for instance, the dispersion of the alpha-process was shown to increase with decreasing temperature T and increasing pressure P. Furthermore, the same shape for relaxation dynamics over a broad time-scale was found by comparing two dielectric loss spectra obtained at different T and P but characterized by the same tau(alpha)(T, P). Additionally, it is noteworthy that the effect of T and P on slowing down the time scale of fast relaxation processes (beta-relaxation and excess wing), although less strong than in the case of alpha-process, was again comparable. The evidence demonstrates that in the investigated systems: (a) slow and fast relaxations are strongly related; (b) the shape of alpha-relaxation and (c) the separation between alpha- and alpha-relaxation time scale are controlled by tau(alpha)(T, P) and not by separate thermodynamic variables

The Johari-Goldstein beta-relaxation of glass-forming binary mixtures

The present paper shows, by means of broadband dielectric measurements, that the primary alpha- and the secondary Johari-Goldstein (JG) beta-processes in binary mixtures are strongly correlated. This occurs for different polar rigid probes dissolved in apolar glass-forming solvents, over a wide temperature and pressure range. reserved. We found that the coupling parameter n = 1 - beta(KWW) and the ratio between alpha- and beta-relaxation time reduce on increasing the size of the solute solved within the same apolar matrix. Moreover, such a ratio is invariant when calculated at different combinations of P and T maintaining either the primary or the JG relaxation times constant. Dielectric spectra measured at different T-P combinations but with an invariant alpha-relaxation time are well superposed in both the alpha- and beta-frequency ranges. Experimental results can be rationalized by Coupling Model equation. (C) 2010 Elsevier B.V. All rights reserved

Relationship between structural and secondary relaxation in glass formers: Ratio between glass transition temperature and activation energy

The relationship between structural and secondary dynamics is one of the most recently considered and possibly important issues of the dynamics in glass formers. One relation of interest is the ratio between the activation energy of the secondary relaxation and RT(g), where T(g) is the glass transition temperature, and R is the gas constant. This relationship has been recently rationalized by the Coupling Model in terms of many-body dynamics and was applied to the intermolecular Johari-Goldstein secondary relaxation. This article investigates the pressure dependence of this ratio and attempts to verify the validity of the Coupling Model predictions under such conditions

Secondary dynamics in glass formers: Relation with the structural dynamics and the glass transition

In this paper, we study secondary relaxations in a neat glass former close and below the glass transition. The pressure and temperature dependences of the characteristic relaxation frequencies were investigated to find a connection between structural and secondary relaxations and the microscopic mechanism at the basis of the latter. We found that the ratio between the relaxation time of the structural and secondary processes is almost constant when considered at different values of temperature and pressure corresponding to the glass transition (same value of structural relaxation time). This result is also related to the finding of a constant broadness of the structural peak. We propose to use such dynamic relation between the two processes to distinguish between intramolecular secondary relaxation, reflecting the local motion of molecular subgroups, and intermolecular secondary relaxation, called also Johari-Goldstein process, originating by a local motion of the whole molecule. (c) 2007 Elsevier B.V. All rights reserved