SynopsisConformational transitions induced by pH changes in random copolymers of leucine and glutamic acid have been studied. Significant differences were observed in the potentiometric titration curves of copolymers with small (up to 4%) and large leucine contents. The helical stability of copolymers with small leucine content, determined from titration curves by the Zimm and Rice method, decreases slightly with an increase in the leucine content, whereas the helical stability of copolymers with large leucine content increases sharply with an increase of the leucine content. It is shown that copolymers with large leucine content aggregate in the region of transition into the helical state, but the increase of their helical state stability is not connected with intermolecular aggregation, as it was also observed for a nonaggregating fraction isolated from one of the copolymers by gel chromatography. A conclusion is made that the helix-coil equilibrium constants for leucine does not itself exceed the s constant for uncharged polyglutamic acid. The stabilization of the helical state in copolymers with large leucine content is due to intramolecular aggregation of helices in these copolymers. The analysis of the leucine residue distribution between helical and nonhelical regions in globular proteins also gives no real arguments to ascribe special helix-forming properties to leucine.
The synthesis of peptides in the presence of papain at pH 8–9.5 is described. Starting substances are acylamino acid alkyl esters (the carboxyl component) and amides or tert.‐butylesters of amino acids, as well as peptide (the amino component). Under such conditions secondary hydrolysis is not essential, making the synthesis of peptides soluble in aqueous medium. The yield of peptides is 50–94%. The effect of different factors (temperature, solvents, reagent concentrations) on the result of the reaction has been studied. It has been found that an excess of the carboxyl component is expedient to increase the yield of peptides.
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