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Application of the nearest‐neighbor ising model to the helix–coil transition of polypeptides in mixed organic solvents
Author(s) -
Cabani S.,
Paci A.,
Rizzo V.
Publication year - 1976
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.1976.360150109
Subject(s) - chemistry , k nearest neighbors algorithm , ising model , thermodynamics , helix (gastropod) , dichloroacetic acid , electromagnetic coil , polymer , heat capacity , solvent , partition function (quantum field theory) , statistical physics , organic chemistry , physics , quantum mechanics , ecology , artificial intelligence , snail , computer science , biology
Using the formalism of nearest‐neighbor Ising model and assuming that the allowed states for a monomeric unity of a polypeptide chain in solutions containing strong acids are E (helix), C (coil), and CS (solvent‐bonded coil), the partition function of the system was deduced analytically. Equations were obtained which permitted the prediction of the characteristic thermodynamic behavior of the helix–coil transition under these conditions. These equations were used to examine critically the possible correlations between experimental data obtained using different techniques. Particular attention was devoted to quantities called “transition enthalpies,” obtained from the slope of the transition curves at the point where the helix fraction is one‐half (Δ H opt exp ), or for measurements of the heat of solution of the polymer over the total range of solvent composition (Δ H sol exp ), or from heat capacity measurements taken at various temperatures (Δ H cal exp ). Literature data of Δ H j exp (j = opt, sol, cal) for the system poly‐γ‐benzyl‐ L ‐glutamate in mixtures of dichloroacetic acid and 1,2‐dichloroethane were carefully analyzed.