Polypeptide melting temperature

Lysozyme from bacteriophage T4 is a 164 amino acid polypeptide chain that folds into two domains (Figure 17.3) There are no disulfide bridges the two cysteine residues in the amino acid sequence, Cys 54 and Cys 97, are far apart in the folded structure. The stability of both the wild-type and mutant proteins is expressed as the melting temperature, Tm, which is the temperature at which 50% of the enzyme is inactivated during reversible beat denat-uration. For the wild-type T4 lysozyme the Tm is 41.9 °C. 

When a native protein in solution is heated to a high enough temperature, its polypeptide chain will unfold to become the denatured protein. The temperature at which a large portion of the protein unfolds is called the melting temperature. The melting temperature of a certain protein is found to be 46°C, and the enthalpy of denaturation is 382 kJ/mol. Estimate the entropy... 

On melting the crystals, a rapid decarboxylation takes place with the formation of a low molecular weight polypeptide. These spontaneous polymerisations were first noticed by Leuchs (7, 2), and were subsequently thoroughly studied by Katchalski and his co-workers (6) who carried out these reactions in vacuum at elevated temperatures. Under these conditions the conversion of the anhydride into polypeptide appears to be quantitative. 

At this temperature, the entropy change for dissolution of liquid hydrocarbons in water is zero. However, the entropy of protein denaturation is far from zero at this temperature but amounts to 17.6 J - K l per mole of amino acid residues (Privalov, 1979), a value that corresponds to an 8-fold increase of the number of possible configurations and is close to the value expected for the helix-coil transition of polypeptides (Schellman, 1955). This difference shows that an oil drop is an inadequate model for a globular protein. A more suitable model resembles that of a small crystal with a quite definite positive melting entropy (see also Bellow, 1977, 1978). 

Influence of Water. Even in desiccated corneum there is a weak, broad endotherm centering at about 120°-130°C as well as the higher temperature doublet melts at 194 °C and 210 °C. Both melts appear to be somewhat lower in temperature for corneum than for wool, which is consistent with the accepted higher degree of orientation and helical content of wool. The 130°C transition has been reported in other polypeptide systems by DSC, IR, and x-ray diffraction where it was found to be highly moisture sensitive (67). It has been suggested that this endothermic process which occurs between 110° and 150 °C in a number of polypeptide systems is a partial conversion from the alpha-... 

Lavialle et al. [74] have also obtained information on the lipid-protein boundary. These authors studied the interaction of melittin, a polypeptide consisting of 26 amino acid residues, with dimyristoyl phosphatidylcholine. The results illustrated in Fig. 20 show that for a lipid-melittin molar ratio of 14 1 two order-disorder transitions are observed, one above (at 29 °C) an one below (at 17 °C) the transition for the pure lipid (at 22.5 °C). The low temperature transition is associated with a depression of the main lipid phase transition while the 29°C transition is associated with the melting behavior of approximately seven inunobilized boundary lipids which surround the hydrophobic portion of the melittin. 

In the pioneer paper published by Thierry and Skoulios [20], it was advanced that polypeptides with sufficient long polymethylene side chains could display liquid-crystalline phases upon melting of the side chain. They in fact reported a high temperature nematic phase that reversed upon cooling into a lower temperature phase, whose structure was not elucidated. Since then, a good amount of research has been made on comblike polyglutamates, with the focus mainly toward the development of thin films with thermochromic properties. The liquid-crystal structure anticipated for these systems should be cholesteric according to the chiral nature of the main chain a-helix. However other mesomorphs have been described under certain conditions and for certain side chain compositions. 

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