SYNOPSISApplying differential scanning calorimetry (DSC) for temperatures up to 18O"C, the denaturation transition of the a-helical material of various keratins in excess water was studied at conditions of equilibrium water vapor pressure (high-pressure DSC) . The results show a generally cystine content and material invariate denaturation range of 20-30°C with peak temperatures around 140°C. Though analysis of variance and multiple comparison tests show a pronounced inhomogeneity of the enthalpy data, the results generally support the material invariance of the denaturation enthalpy and hence of the helix content of the keratins. The denaturation enthalpy for the helical coiled-coil-structures in the intermediate filaments is determined as AH = 6.5-7.8 kJ/mol. A significant positive correlation was found between the denaturation temperatures and the cystine content. It is concluded, that the helix denaturation temperatures are kinetically controlled by the amount and/or the chemical composition of the surrounding nonhelical matrix, and that the double-peak endotherms observed for wool and other keratins originate from two cell types that are sufficiently different in sulfur content to allow endotherm separation. In the case of wool the cell types can be identified as "ortho-" and "paracortical" cells.

The nature of the bimodal denaturation endotherm, which has been observed for some α‐keratin materials, was investigated in order to evaluate the validity of the two conflicting theories to interpret the phenomenon. Ortho‐ and para‐cortical cells were isolated from Merino wool and subjected to measurements by differential scanning calorimetry in water at elevated temperatures (80–170°C). The results show for the isolated cell fractions denaturation peaks at 138°C (ortho) and 144°C (para) that are with respect to their location and temperature difference in good agreement with the results obtained for the whole fiber material. The denaturation enthalpy of the para‐ (21 J/g) was found to be higher than of the ortho‐cortical cells (17 J/g) in contrast to expectations from electron diffraction studies. The fact that both of these values are higher than for the whole fiber (15 J/g) is attributed to the removal of the cuticle prior and to the enzymatic digestion of some further material during cell separation. The results give unequivocal evidence for the validity of the ortho/para‐hypothesis for the interpretation of the endothermic denaturation doublet of keratins. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1991–1995, 1998