Abstract
The cortex of human hair is essentially a fiber-reinforced composite of partly crystalline keratin intermediate filaments (IFs), which are embedded in an amorphous matrix of keratin-associated proteins (IFAPs). The amorphous matrix proteins are highly cross-linked by disulphide bonds. The composition of these fibre/matrix- components, which strongly interact in the composite, is the basis for the high stress and flexural resistance of human hair.
Oxidative hair treatments affect the chemical structure of the amorphous keratin matrix. The oxidative cleavage of disulphide bonds leads to the formation of cysteic acid up to concentrations of 6 - 8 mole%. Since cysteic acid (pK=1.3) is the strongest acid within the protein structure, the keratin-associated proteins become highly negatively charged and as a consequence their native structure energetically less favourable.
The oxidized matrix becomes hydrophilic and more accessible for water. Increased rates and degrees of swelling are direct consequences. Increased swelling is associated with the decrease of denaturation temperatures and cysteine content.
The determination of the denaturation temperatures TD by means of Differential Scanning Calorimetry in water (DSCw) shows that a pH-readjustment after bleaching to pH≈4.5, close to the isoionic point, stabilizes the hair matrix. An increase of TD by more than 20 oC is observed. This stabilization effect is attributed to the protonation of the IFAPs, which leads to a reduction of the repellent forces due to the high concentration of cysteic acid.
This stabilisation effect is further strongly boosted through multivalent ions, such as, Ca2+ and Al3+. Treatments with Al3+-salts are shown to bring the level of swelling back to the level of virgin hair even after intensive oxidative treatments. Further tests show that treatments with multivalent ions increase tensile strength and the denaturation temperature of oxidized hair significantly.
Oxidative hair treatments affect the chemical structure of the amorphous keratin matrix. The oxidative cleavage of disulphide bonds leads to the formation of cysteic acid up to concentrations of 6 - 8 mole%. Since cysteic acid (pK=1.3) is the strongest acid within the protein structure, the keratin-associated proteins become highly negatively charged and as a consequence their native structure energetically less favourable.
The oxidized matrix becomes hydrophilic and more accessible for water. Increased rates and degrees of swelling are direct consequences. Increased swelling is associated with the decrease of denaturation temperatures and cysteine content.
The determination of the denaturation temperatures TD by means of Differential Scanning Calorimetry in water (DSCw) shows that a pH-readjustment after bleaching to pH≈4.5, close to the isoionic point, stabilizes the hair matrix. An increase of TD by more than 20 oC is observed. This stabilization effect is attributed to the protonation of the IFAPs, which leads to a reduction of the repellent forces due to the high concentration of cysteic acid.
This stabilisation effect is further strongly boosted through multivalent ions, such as, Ca2+ and Al3+. Treatments with Al3+-salts are shown to bring the level of swelling back to the level of virgin hair even after intensive oxidative treatments. Further tests show that treatments with multivalent ions increase tensile strength and the denaturation temperature of oxidized hair significantly.
| Original language | English |
|---|---|
| Title of host publication | 20th International Hair-Science Symposium, Book of Abstracts |
| Place of Publication | Aachen |
| Publisher | DWI - Leibniz-Institute for Interactive Materials |
| Publication status | Published - Sept 2017 |
Keywords
- human hair
- bleaching
- isoelectric point
- ions
- DSC
- morphology
- matrix stabilization