Premium
Helix–random chain transitions of polyamino acids in organic acid–salt solutions
Author(s) -
Franzen James S.,
Bobik Caroline,
Harry John B.
Publication year - 1966
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.1966.360040604
Subject(s) - chemistry , acetic acid , dichloroacetic acid , salt (chemistry) , carboxylate , dissociation (chemistry) , lithium (medication) , aqueous solution , inorganic chemistry , amide , polymer , side chain , hofmeister series , polymer chemistry , stereochemistry , medicinal chemistry , organic chemistry , medicine , endocrinology
Polyamino acids which are soluble and helical in acetic acid and dichloroacetic acid (DCA) have been observed to undergo a helix to random chain transition upon the addition of lithium salts of strong acids. The transition can be reversed by diluting the salt. Apparently only lithium cations are able to bring about the polycarbobenzoxy‐ L ‐lysine (PCBL) transition in acetic acid, whereas the anions display a varying degree of effectiveness; ClO 4 − > Br − > TSA − > Cl − > NO 3 − . The lithium salts of carboxylate anions such as OAc − and TFA − do not cause polymer unwinding in acetic acid. Neither do the acids, TSA, HCl, TFA, or DCA induce the transformation in acetic acid. Poly‐ L ‐alanine (PLA) in DCA unfolds as LiBr is added, but does not unfold in the presence of 0.5 M (CH 3 ) 4 NBr, 0.25 M CsBr, or 0.32 M HCl. These results are explained on the basis of a direct interaction of the lithium salt with the polymer amide groups to form an ion‐pair complex. The extent to which the union of the ion pair can dissociate from the complex in the low dielectric constant, environment determines the degree of unfolding of the polymer. The anion dissociation equilibrium presumably therefore would lie in the same order as given above. Acids such as HCl and TSA are considered to substantially protonate and ion‐pair with the polymer, but do not readily dissociate the anion partner from the complex, and therefore do not produce an unstable positively charged helical structure.