Melting points, proteins

Ethylammoniinn niirate, l H jf HjNH tNOj, was the first limit liquid to he discovered. Its melting point of 12°C was reported in 1914 and it has since been used as a nonpolluting solvent for organic reactions and for facilitating the folding of proteins. 

The recoil-free fraction /a of transition metal complexes or proteins in frozen solution can be as small as 0.1-0.3, when measured just below the melting point, but the /-factor increases strongly when the temperature is lowered to fiquid nitrogen temperatures (77 K), and at fiquid helium temperatures (4.2 K) it may reach values of 0.7-0.9 [35]. This makes a substantial difference to the acquisition time of the spectra because of the square dependency on the signal (3.1). 

The A-> W mutation in all likelyhood took place in the core of the protein. Since W is much larger than A the hydrophobic core of the protein cannot pack as efficient as before and the melting point is as a consequence much lower. 

This chapter deals with the very important a-amino acids, the building blocks of the proteins that are necessary for the function and structure of living ceils. Enzymes, the highly specific biochemical catalysts are proteins. or-Amino acids are dipolar ions (zwitterions), RCH(N" H,)COO , as is indicated by their crystallinity, high melting point, and solubility in water rather than in nonpolar solvents. The standard (naturally occurring) amino acids are listed in Table 21-1 those marked with an asterisk are essential amino acids that cannot be synthesized in the body and so must be in the diet. They have 1° NHj s except for proline and hydroxyproline (2°). They have different R groups. 

Maltose. This sugar is found in suybean. and is produced by the action of ihe enzyme diastase of germinated barley (mall) on starch at 50°C. and is thus an intermediate product in the transformation of starch into alcohol. Maltose CnHiaOn- ILO. melting point IOtf C. when rapidly heated, may be crystallized from the concentrated malt syrup after removal of proteins and insoluble material. Maltose reduces ammonio-cupric salt solulion. and forms osazone. 

These compounds have remarkably high melting points (see Thble 21.2), considering their water content, e.g., [(i-C5H11)4N] + [F 38H20] has a melting point of 31 °C and contains 68 wtM of water [795], which is comparable to the water content in protein crystals. 

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