<P> X-ray crystallography and calorimetry indicates that there is no general mechanism that describes the effect of temperature change on the functions and structure of proteins . This is due to the fact that proteins do not represent a uniform class of chemical entities from an energetic point of view . The structure and stability of an individual protein depends on the ratio of its polar and non-polar residues . They contribute to the conformational and the net enthalpies of local and non-local interactions . </P> <P> Taking the weak intermolecular interactions responsible for structural integrity into consideration, it is hard to predict the effects of temperature because there are too many unknown factors contributing to the hypothetical free energy balance and its temperature dependence . Internal salt linkages produce thermal stability, and whether cold temperature results in the destabilization of these linkages is unknown . </P> <P> In principle, the free energy of stabilization of soluble globular proteins does not exceed 50 - 100 kJ / mol . The stabilization is based on the equivalent of few hydrogen bonds, ion pairs, or hydrophobic interactions, even though numerous intramolecular interactions results in stabilization . Taking into consideration the large number of hydrogen bonds that take place for the stabilization of secondary structures, and the stabilization of the inner core through hydrophobic interactions, the free energy of stabilization emerges as small difference between large numbers . Therefore, the structure of a native protein is not optimized for the maximum stability . </P> <P> Proteins are frequently described as consisting of several structural units . These units include domains, motifs, and folds . Despite the fact that there are about 100,000 different proteins expressed in eukaryotic systems, there are many fewer different domains, structural motifs and folds . </P>

Techniques used to determine the primary structure of protein