Entropy protein stability

Water-soluble globular proteins usually have an interior composed almost entirely of non polar, hydrophobic amino acids such as phenylalanine, tryptophan, valine and leucine witl polar and charged amino acids such as lysine and arginine located on the surface of thi molecule. This packing of hydrophobic residues is a consequence of the hydrophobic effeci which is the most important factor that contributes to protein stability. The molecula basis for the hydrophobic effect continues to be the subject of some debate but is general considered to be entropic in origin. Moreover, it is the entropy change of the solvent that i... 

Disulfides. The introduction of disulfide bonds can have various effects on protein stability. In T4 lyso2yme, for example, the incorporation of some disulfides increases thermal stability others reduce stability (47—49). Stabili2ation is thought to result from reduction of the conformational entropy of the unfolded state, whereas in most cases the cause of destabili2ation is the introduction of dihedral angle stress. In natural proteins, placement of a disulfide bond at most positions within the polypeptide chain would result in unacceptable constraint of the a-carbon chain. 

M. J. Stone, NMR relaxation studies of the role of conformational entropy in protein stability and ligand binding,... 

Additives to enhance protein stability are generally compatible with the process. Contrary to its effect on other modes of chromatography, reducing the temperature decreases the retention in HIC due to its being an entropy-driven technique. 

A. A. Rashin, Biopolymers, 23, 1605 (1984). Buried Surface Area, Conformational Entropy, and Protein Stability. 

B. Tidor and M. Karplus, Proteins, 15, 71 (1993). The Contribution of Cross-Links to Protein Stability A Normal Mode Analysis of the Configurational Entropy of the Native State. 

In HIC, molecules are bound with a high concentration of salt, usually ammonium or sodium sulfate (1-2 A/) in a buffer (0.02-0.05 M). Elution is attained by a gradient to a lower concentration of salt in the buffer. The pH is controlled and is usually in the range of 6-8, but it is not a critical factor in selectivity. Additives to enhance protein stability are generally compatible with the process. Contrary to its effect on other modes of chromatography, reducing the temperature decreases the retention in HIC due to its being an entropy-driven technique. 

In brief, a DSC instrument comprises two cells fixed in an adiabatic chamber. One cell contains the sample to be tested, the second cell contains a reference solution or an empty DSC pan. The adiabatic chamber is maintained under pressure to avoid the evaporation of the sample (Plum, 2009). A DSC-thermogram represents the plot of heat capacity difference ACp (between the sample and the reference) as a function of temperature. Thermodynamic parameters, such as T, AH and AS, could be determined by the DSC curve analysis. T is the temperature at which the concentration of denatured and native forms of the protein are equal. This specific temperature is also called the midpoint of the thermal transition. AH represents the enthalpy of thermal transition determined from the integration of the DSC curve. The entropy (AS) of the thermodynamic transition of the protein may be calculated from the integrated area under the curve of AC /T vs. T. The free energy (AG), which gives an indication of the protein stability, can also be determined at any temperature from the values of AH and AS (O Brien and Haq, 2004 Plum, 2009). Thermal and thermodynamic properties of proteins analyzed by DSC are greatly affected by the experimental conditions used, such as pH, salts, alcohols, and the presence of other food components (e.g., lipids, polysaccharides) (Grinberg et al, 2009). 

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