Heat induced denaturation of fibrous hard alpha-keratins and their reaction with various chemical reagents

This dissertation is concerned with the thermal behaviour of fibrous proteins encapsulated in rigid structures, among the most well-known representatives of this class being the alpha-keratins in human hair. In spite of a lot of work in the field, there is still no mechanism proposed for accounting on how thermal denaturation process occurs in hard alpha-keratins. This work aims at proposing a model for the alpha-keratin fibres and a mechanism for their thermal denaturation process. These are further used for understanding the effect of various cosmetic reagents on the thermal stability of the fibres. The use of differential scanning calorimetry, scanning electron microscopy and light microscopy revealed strong structural modifications induced by high temperature in case of heating keratin material in an opened atmosphere. The DSC in open environment was showed to supply misleading information, due to the interference of pyrolysis with the process of interest. Consequently the present work focuses mainly on using DSC of keratins in water excess. The study of the influence of pH, particularly acid values, on thermal behaviour of hard alpha-keratins, indicates limits of the two-phase model used so far to describe the fibrous proteins. We propose a three-phase model for explaining fibrous hard alpha-keratins high thermal stability and their reaction with various reagents. The approach is based on results from DSC study of keratins under various conditions, and is supported by amino-acid analysis, x-ray diffraction, Raman spectroscopy and tensile strength observations. According to the proposed model, the third phase, the interface between crystalline and matrix phases, made of nonhelical tail domains of keratin, scaffolds the intermediate filaments and controls their interaction with chemical reagents as well as their thermal properties. The differential scanning calorimetry measurements carried out in water excess and with different heating rates were used for the kinetic analysis of the endothermic process assigned to the denaturation of the helical material from human hair. We found that the kinetic mechanism is autocatalytic and that the value of the activation energy is rather close to disulphide bond scission than to protein denaturation. This allowed us proposing a multistep mechanism for the thermal denaturation of hard alpha-keratins in water excess that relies on the 3-phase model which describes their structure. The limiting step of the thermal denaturation process is then the scission of S-S bonds between the main morphological components, namely intermediate filaments (IF) and matrix (IFAP). The theoretical proposed model shows a good agreement with the experimental recorded data. The chemical damage induced by bleaching, permanent waving and oxidative dyeing on the structure of hard alpha-keratin fibres (human hair) as revealed by modifications in their thermal behaviour was investigated by using differential scanning calorimetry. Regression analysis of the data from hair samples treated differently shows a linear correlation between the enthalpy of the denaturation peak recorded by DSC and the cystine content of the fibre. The experimental results are evaluated within the framework of a three-phase model in which the nonhelical (globular) terminal domains of keratin promote filament interactions and control the thermal properties of keratin intermediate filaments. Amino-acid analysis, x-ray diffraction and tensile strength measurements provide evidence that the attack of chemical reagents occur preponderantly in the matrix and at the interface between filament and matrix. A possible intermediate state between native and denaturated crystalline helical material is suggested to account for the increased disorder in the IFs-IFAP package induced by harsh treatments. The DSC data suggests that hair keratin IFs can modulate their organisation and thermal properties through chemical induced interactions.

Grantor dc:publisher

Publikationsserver der RWTH Aachen University

Year dc:date

2011

Author dc:creator

• Istrate, Daniel

Contributors dc:contributor

• Möller, Martin

• info:eu-repo/classification/ddc/540
• Differential scanning calorimetry
• Keratine
• Chemie
• activation energy
• differential scanning calorimetry (DSC)
• interfaces
• proteins
• hair
• cosmetic treatments
• alpha-keratin
• kinetics
• bleaching
• permanent waving
• oxidative dyeing

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-124757%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:63322