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Potentiometric titrations and the helix–coil transition of poly( L ‐glutamic acid) and poly‐ L ‐lysine in aqueous salt solutions
Author(s) -
Ciferri A.,
Puett D.,
Rajagh L.,
Mermans Jan
Publication year - 1968
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.1968.360060803
Subject(s) - chemistry , potentiometric titration , aqueous solution , salt (chemistry) , molecule , lysine , titration , isoelectric point , crystallography , helix (gastropod) , ionization , inorganic chemistry , ion , amino acid , organic chemistry , biochemistry , ecology , snail , biology , enzyme
The objective have been to establish if those ions which are known to change the stability of the structure of proteins, have any influence on the properties of ionizable polypeptides. Potentiometric titrations and complementary optical rotation data are presented for aqueous solutions of poly‐ L ‐lysine (PLL) in the presence of KSCN, KCl, and KF, and for poly( L ‐glutamic acid) (PLGA) in the presence of KSCN, KCl, and LiCl. The following measured quantities which are affected by salt concentration were obtained: intrinsic p K (p K 0 ), slope of p K app versus degree of ionization (α) curves, the degree of ionization at which the helix to coil transition occurs, and the free energy of this transition for the uncharged molecule (δ G ° hel ). The effects of nonspecific salts (KCl and LiCl for PLL and KSCN and KCl for PLGA) are small, and about, as expected from general electrostatic considerations. In line with the observations made with isoelectric and cat ionic collagen, specific, effects were noted with KSCN–PLL and with LiCl–PLGA. In the presence of KSCN, the poly‐ L ‐lysine helix becomes stabilized at much lower degree of ionization than in the presence of KCl, and the slope of the p K app versus α plots is greatly reduced. However, Δ G ° hel (for the uncharged molecule) is not affected, and p K 0 is only slightly higher. We interpret these data in terms of binding of SCN − primarily to the side‐chain amino groups (both to RNH 3 + and to RNH 2 ) solutions. ( L ‐glutamic acid) in LiCl solution has its transition at the same α value as in KCl solution. However, both the slopes of the p K app versus α plots and the absolute values of Δ G ° hel are lower than in KCl solution. We interpret these results in terms of binding of Li + to side chains as well as to the peptide bond.