Electrostatic interactions in ionic homopolypeptides in solutions of moderate ionic strength

Enthalpy Changes upon Dilution and Ionization of Poly(l-glutamic acid) in Aqueous Solutions

The enthalpy changes accompanying the dilution and ionization of poly(L-glutamic acid) in water have been measured at 25 degrees C for two degrees of polymerization (DP = 115 and DP = 480) at various degrees of ionization, alpha, for a concentration range from about 0.2 to 0.002 monomol/L. The heat of dilution displays an unusual dependence on the degree of ionization, which is in sharp contrast to the behavior of other weak carboxylic polyelectrolytes, such as poly(acrylic acid). The exothermic heat effects observed at low values of alpha become endothermic for the region where the helix-coil transition is most pronounced, and for high degrees of ionization, they are exothermic again. Evidently, an endothermic heat effect, produced by an additional conformational transition in the dilution process, is superimposed on the exothermic enthalpy of dilution, and it overweighs the latter in the region of alpha where the conformational transition is prevailing. The calorimetric titration curve, which gives the dependence of the heat of ionization, deltaH(i), on alpha, has a maximum and is typical for poly(carboxylic acids) which undergo pH-induced conformational transition, such as poly(methacrylic acid). The values of deltaH(i) obtained at two polymer concentrations indicate that the enthalpy of ionization depends on the polypeptide concentration.

Effects of temperature, pH and detergents on the molecular conformation of the enterotoxin of Clostridium perfringens

The effects of temperature, pH and sodium dodecyl sulfate on the conformation of the enterotoxin from Clostridium perfringens type A were followed by circular dichroism in both the peptide and aromatic regions. At near-physiological conditions (35 degrees C, pH 6.7) the enterotoxin exhibited a conformation consisting of approximately 60% pleated sheet, 40% non-periodic, and essentially no helix. The peptide region was relatively stable at temperatures up to 55 degrees C and at pH values ranging from 4-10. The aromatic region demonstrated profound, time-dependent changes at 55 degrees C. At temperatures greater than 55 degrees C, extremes of pH, and in the presence of SDS, the spectra in both regions showed major structural reorganization; in most cases a gain in helical content at the expense of sheet structure was observed. The conformational properties of the protein are very similar to those observed for the lectins, a group of carbohydrate-binding proteins.