With hair analysis a crucial part of forensic toxicology and biomonitoring, it is important to understand the effect of cosmetic treatment. Using several techniques, including inelastic and quasi-elastic neutron scattering, a research group has studied the chemical and structural changes to hair caused by bleaching.
Chemical bleaching of hair is known to change its chemical, structural and mechanical properties. This study, published in The European Physical Journal Special Topics, investigated how the oxidative damage that occurs is caused.
Although neutron spectroscopy is rarely used in this area of science, it is an ideal tool to study the structure and dynamics of confined water and proteins in the hair, and the strength of the H-bond interactions present.
A single hair is made up of a central cortex surrounded by the cuticle. Inside the cortex is a crystalline helical phase embedded in a matrix of proteins. The group studied the changes to both the central helix and the matrix around it to see what affect the bleaching had.
Their results showed that lighter hair was actually lighter, as the process of bleaching reduced the mass of the hair. Loss of the crystalline material, proteins and lipids caused the hair to weaken.
The researchers then delved deeper into the structural changes of to look at the hydrogen atoms on the surface of the hair. By using inelastic neutron scattering measurements on Tosca and quasi-elastic neutron scattering on Iris, they found that, for bleached hair, the hydrogen atoms are able to move more freely, indicating that the structure is more flexible.
This freedom is caused by the chemical bleaching attacking the thioester groups that bind lipids to the cuticle. This promotes the cleavage of cystine amino acids of the cuticle and cortex, predominantly via S-S fission, forming residues of cysteine and cysteic acid. This chemical modification makes the structure less rigid and, in turn, the proton mobility increases.
This study shows that neutrons are an ideal method for studying these structural changes in human hair. The ability of the technique to measure traces of cysteine is an additional benefit, and indicates that this approach could be extended to other biological systems.
Dr Cibele de Castro Lima, Postdoctoral researcher at the Universidade de Sao Paulo and expert in characterization of hair fibers, explains; “Neutron scattering allowed us to identify the alterations in the dynamics of the SH-residual groups, an extremely important piece of information for the evaluation of the damage caused by classical harsh hair bleaching conditions."
“Having access to this data in very difficult, if not impossible, using other experimental techniques," she adds; “The formation of this residue reflects negatively in the mechanical proprieties of the hair, such as elasticity and resistance."
The full paper can be found online at: https://doi.org/10.1140/epjst/e2020-900217-4