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Thermodynamic parameters of the helix–coil transition in polypeptide chains. III. Random copolymers of L ‐leucine with L ‐glutamic acid
Author(s) -
Bychkova V. E.,
Gudkov A. T.,
Miller W. G.,
Mitin Yu. V.,
Ptitsyn O. B.,
Shpungin I. L.
Publication year - 1975
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.1975.360140814
Subject(s) - chemistry , leucine , copolymer , random coil , helix (gastropod) , titration curve , titration , monomer , crystallography , polymer chemistry , glutamic acid , intramolecular force , amino acid , polymer , circular dichroism , stereochemistry , organic chemistry , biochemistry , ecology , snail , biology
Conformational 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 constant s 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.