Proteins inverse temperature transitions

Urry, D.W., Entropic elastic processes in protein mechanisms. 1. Elastic structures due to an inverse temperature transition and elasticity due to internal chain dynamics, J. Prot. Chem., 7, 1-34, 1988. 

The azo-modified, elastin-like polypeptide XIV illustrated in Scheme 9 exhibits a so-called inverse temperature transition" that is, the compound gives cross-linked gels that remain swollen in water at temperature below 25 °C but deswell and contract upon a rise of temperature. The trans-cis photoisomerization of the azo units, obtained through alternating irradiation at 350 and 450 nm, permits photomodulation of the inverse temperature transition.[S9] The result indicates that attachment of a small proportion of azobenzene chromophores is sufficient to render inverse temperature transition of elastin-like polypeptides photoresponsive, and provides a route to protein-based polymeric materials capable of photomechanical transduction. 

Some polymers have been purified on the basis of their physicochemical properties. For example, silk-like polymers have been purified by taking advantage of their low solubility in aqueous medium (13). Elastin-like polymers (ELPs) have been purified by temperature cycling above and below their inverse temperature transition (Tt) (14). This technique has been extended to produce an ELP-tag that can be used to purify a number of recombinant proteins by temperature cycling, which may be faster and less expensive than affinity chromatography (15). 

Table 1. Hydrophobicity scale for protein-based polymers and proteins based on the properties of the inverse temperature transition of elastic protein-based polymers, poly[/v(GVGVP), (GXGVP)]. ...

Heating Reversibly Increases Protein Order, an Inverse Temperature Transition... 

Quite the inverse occurs for water-dissolved protein of interest here that is, by the consilient mechanism, heating from below to above the folding transition increases the order of the model protein. Because heating increases protein order, the transition is called an inverse temperature transition. 

Figure 2.7. These crystals of cyclo(GVGVAPGVG-VAP) form when the temperature of aqueous solutions is raised and dissolve when the temperature is lowered. This finding represents an unambiguous demonstration that the model protein component of the aqueous solution becomes more ordered on higher temperature and is one of the reasons that the transition is called an inverse temperature transition. (Adapted with permission from Urry et al. )...

Figure 2.8. Ordered water molecules surround oillike groups, as first shown by Stackelberg and Muller. As oil-like groups associate, this structured water becomes less ordered liquid water this is the large positive change in entropy responsible for the inverse temperature transition, the phase transition that is fundamental to protein function. As shown in the upper left, hydration of a proximal carboxylate...

Inverse Temperature Transitions Provide Negative Entropy to Protein... 

The inverse temperature transition is a specific mechanism whereby thermal energy (heat) provides an increase in order of the protein part of the system. A decrease in entropy of this sort has been termed negative entropy by Schrodinger. ° While the total entropy (disorder) for the complete system of protein and water increases as the temperature is raised, the structural protein component, critical to the conversion of thermal energy to mechanical work, increases in negative entropy. The protein component increases in order by the folding that shortens length and by the assembly of oillike domains that builds structures. 

The above equations for photosynthesis and respiration, exactly balanced with respect to CO2, H2O, [C(H20)]6, and O2, mask an extraordinarily intricate set of reactions where balance tends to be masked by blurring detail. The objective here, however, is to dissect out sufficient detail to expose the primary energyconverting steps common to both processes and to demonstrate that model proteins, utilizing inverse temperature transitions, emulate key elements of those energy-converting steps. ... 

Consequences of Protein Machines Based on the Inverse Temperature Transitions... 

The essential aspect of the capacity of the inverse temperature transition to achieve diverse energy conversions resides within large chain molecules, which were just becoming known when the first edition of Schrodinger s book appeared. As we have sketched above, the functional properties of the model protein-... 

Figure 2.18. Energies are shown that can be inter-converted by means of elastic-contractile model proteins capable of exhibiting inverse temperature transitions functioning by means of the competition for hydration between oil-like and charged groups called an apolar-polar repulsive free energy of hydration. See Chapter 5 for a more complete development of the phenomenology and physical basis and Chapter 8 for details of the molecular process.

Due to the struggle to survive under circumstances of limited food supply, organisms evolve to use the most efficient mechanism available to their composition. The most efficient mechanism available to the proteins that sustain Life would seem to be the apolar-polar repulsive free energy of hydration as observed for the inverse temperature transitions for hydrophobic association. The efficiency of designed elastic-contractile protein-based machines and a number of additional properties make designed protein-based materials of substantial promise for the marketplace of the future. 

A smart plastic would harmlessly disintegrate once its useful Life were completed. Plastics made of plastic-contractile model proteins with controllable inverse temperature transitions can be designed as smart plastics. A smart protein-based plastic, having fulfilled its role, would swell and become a fragile, edible gelatin-like substance. Rather than foretell death for the fishes, a smart protein-based plastic could provide food for the fishes, once its useful Life as a plastic were complete. 

D-amino acid residue on the right (an optical isomer that does occur in biology, but in those peptides not encoded for by the genetic code). C The effect of insertion of a D-amino acid residue in an otherwise L-amino acid residue protein in the P-spiral structure of the elastic-contractile model protein of our focus would be to disrupt the regular structure. This is difficult to avoid completely in chemical synthesis, and it increases the temperature at which occurs the inverse temperature transition and decreases the heat of the transition due to less optimal association of oil-like groupings. 

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