ISSN:
0006-3525
Keywords:
Chemistry
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The enthalpy change associated with the isothermal pH-induced uncharged coil-to-helix transition ΔHh° in poly(L-ornithine) in 0.1 N KCl has been determnined calorimetrically to be -1530 ± 210 and -1270 ± 530 cal/mol at 10° and 25°C, respectively. Titration data provided information about the state of charge of the polymer in the calorimetric experiments, and optical rotatory dispersion data about its conformation. In order to compute ΔHh°, the observed calorimetric heat was corrected for the heat of breaking the sample cell, the heat of dilution of HCl, the heat of neutralization of the OH- ion, and the heat of ionization of the δ-amino group in the random coil. The latter was obtained from similar calorimetric measurements on poly(D,L-ornithine). Since it was discovered that poly(L-ornithine) undergoes chain cleavage at high pH, the calorimetric measurements were carried out under conditions where no degradation occurred. From the thermally induced uncharged helix-coil transition curve for poly(L-ornithine) at pH 11.68 in 0.1 N KCl in the 0°-40°C region, the transition temperature Ttr and the quantity (∂θh/∂T)Ttr have been obtained. From these values, together with the measured values of ΔHh°, the changes in the standard free energy ΔGh° and entropy ΔGh°, associated with the uncharged coil-to-helix transition at 10°C have been calculated to be -33 cal/mol and -5.3 cal/mol deg, respectively. The value of the Zimm-Bragg helix-coil stability constant σ has been calculated to be 1.4 × 10-2 and the value of s calculated to be 1.06 at 10°C, and between 0.60 and 0.92 at 25°C.
Additional Material:
8 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/bip.1976.360150913
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