Transitions model protein-water systems

In order to understand the effect of temperature on the water dynamics and how it leads to the glass transition of the protein, we have performed a study of a model protein-water system. The model is quite similar to the DEM, which deals with the collective dynamics within and outside the hydration layer. However, since we want to calculate the mean square displacement and diffusion coefficients, we are primarily interested in the single particle properties. The single particle dynamics is essentially the motion of a particle in an effective potential described by its neighbors and thus coupled to the collective dynamics. A schematic representation of the d)mamics of a water molecule within the hydration layer can be given by ... 

Four Phase Changes (Transitions) in Model Protein-Water Systems... 

Despite the absorption of heat for the transition and the overall increase in entropy of -(-4.0 EU for the water plus protein, the protein component actually increases in order on raising the temperature. As unambiguously demonstrated by crystallization of a cyclic analog (see Figure 2.7), in this case the protein component of the water plus protein system becomes more ordered as the temperature is raised. For this and additional reasons, noted below in section 5.1.3, we call this transition exhibited by our model protein, poly (GVGVP), an inverse temperature transition. 

A vital property of these model proteins is that they are more ordered above the transition temperature defined by the binodal or coexistence line in Figure 5.3. The polymer component of this water-polypeptide system becomes more ordered or structured on increased temperature from below to above the transition. This behavior is the inverse of that observed for most systems, as discussed above. In particular, we developed the term inverse temperature transition when the precursor protein and chemical fragmentation products of the mammalian elastic fiber changed from a dissolved state, and therefore when molecules were randomly dispersed in solution, to a state of parallel-aligned twisted filaments as the temperature was raised from below to above the phase transition. - ... 

Chapter 5 is the scientific core of the book. It begins with familiar phase transitions of ice-to-water and water-to-vapor, both of which demonstrate increased disorder with increased temperature. It then brings in the unique phase transitions of the two-component system, model proteins in water, in which the protein... 

Note that many of the molecules produced have few internal polar fimctional groups to which ions may bind. Instead, it is more likely that ion-water-channel interactions escort the ion through the pore. To that end. many of the models can then be viewed as methods to pull water into the lipidic core of a bilayer membrane and thereby stabilize ions in transport. Recent studies of molecular dynamics simulations of ion transportation in human aquaporin-1 and in the bacterial glycerol facilitator GlpF revealed the key role of water in the stabilization of ions in transit and in the molecular selectivity of channels. Synthetic compoxmds form less-defined stmctures than these complex proteins but apparently act as efficiently as more complex natural materials. It is likely that continued study of synthetic systems will continue to reveal the general details underlying all transport processes. 

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