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Anion effects on equilibria and kinetics of the disorder–order transition of κ‐carrageenan
Author(s) -
Austen Kevan R. J.,
Goodall David M.,
Norton Ian T.
Publication year - 1988
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.360270111
Subject(s) - chemistry , tetramethylammonium , hofmeister series , kinetics , lyotropic , nucleation , reaction rate constant , ion , phase transition , crystallography , salt (chemistry) , lyotropic liquid crystal , phase (matter) , thermodynamics , organic chemistry , liquid crystalline , physics , quantum mechanics
The effect of a number of tetramethylammonium salts on the equilibria and kinetics of the disorder to order transition in the polysaccharide κ‐carrageenan have been investigated. Data from the temperature dependence of optical rotation show that anion stabilization of the ordered form follows the lyotropic series I − > Br − > NO − 3> Cl − > F − . Stopped‐flow polarimetry was used to study the kinetics of conformational ordering following a rapid increase in salt concentration. The transition to the new equilibrium position was shown to be biphasic for all of the tetramethylammonium salts studied. The rate equation for the fast phase and the temperature dependence of the observed forward rate constant accord with a cooperative dimerization process. Activation parameters for helix nucleation, Δ H * and Δ S * , vary with both salt concentration and (at constant ionic strength) the anion type, increasing through the lyotropic series from I − to F − . The slow phase shows second‐order kinetics, and is interpreted as further stabilization of the ordered form either through limited aggregation or annealing. The rate constant for the slow phase also follows the lyotropic series. Thus we have shown that both the growth and nucleation processes are anion dependent.