By Roger L. McMullen
May 15, 2017
Peptides are commonly used in hair care products designed to repair the structural proteins of the hair fiber. Until recently, very little was known about the mechanism of interaction between peptides and hair proteins. In a recent study, researchers at the University of Minho in Portugal examined the interactions between low molecular weight peptides (about 10 amino acids in length) and isolated structural proteins from hair. In total, they studied over 1,000 peptides that were designed from the proteome of human keratin and keratin associated proteins.1
Peptides target structural proteins that are situated in the internal part of fiber. Hair is a complex structure and is comprised of thin, overlapping cuticle cells that protect its inner core, known as the cortex. The cuticle cells are mechanically tough and very resistant to environmental insults, and to the penetration of foreign molecules, due to the high degree of isodipeptide and disulfide bonds present in its various lamellar layers. The cortex, which constitutes most of the fiber mass, contains spindle-shaped cortical cells filled with macrofibrils (higher ordered structures of alpha-keratin proteins) embedded in an amorphous protein matrix (primarily comprised of keratin associated proteins). A cell membrane complex joins cuticle cells with other cuticle cells, cortical cells with other cortical cells, and cuticle and cortical cells.
Treating hair with peptides presents two challenges. The first is the successul penetration of the peptide into the hair cortex. This is not an easy endeavor, especially in virgin, undamaged hair. It should be noted that molecular size and polarity play a large role in determining the ability of a molecule to gain access to the interior of the fiber structure. Also, damaged hair is more permeable than undamaged hair. More than likely this is due to the increased porosity of damaged hair, especially hair that has undergone chemical treatments.2 Once penetration has been achieved, the second challenge is understanding how the peptides influence the physicochemical properties of the fiber. In fact, several studies have demonstrated the ability of low molecular weight peptides to gain access to the cortex of damaged hair and increase its mechanical properties.3-6 Improving mechanical strength is highly desirable in hair that has undergone chemical or thermo-mechanical treatments in which case its structural integrity becomes compromised.
The recent study by reasearchers at the University of Minho brings to light a mechanistic understanding of how peptides could interact and fortify endogeonous hair proteins. In their investigations, they examined the influence of isolectric point, charge, and amino acid side chain functionality on the binding interactions of the screened peptides with hair proteins. Their findings suggest that hydrophobic bonding and disulfide bonds are the most impactful mechanisms of mediating interactions between cosmetic peptides and the structural proteins of hair. Such a finding offers much insight into the design of new peptides capable of modulating the physicochemical properties of hair.
1. C.F. Cruz, N.G. Azoia, T. Matamá, and A. Cavaco-Paulo, Peptide—protein interactions within human hair keratins, Int. J. Biol. Macromol., 101, 805-814 (2017).
2. Y.Z. Hessefort, B.T. Holland, and R.W. Cloud, True porosity measurement of hair: A new way to study hair damage mechanisms, J. Cosmet. Sci., 59, 303-315 (2008).
3. E. Oshimura, H. Abe, and R. Oota, Hair and amino acids: The interactions and the effects, J. Cosmet. Sci., 58, 347-357 (2007).
4. M. Fernandes and A. Cavaco-Paulo, Protein disulphide isomerase-mediated grafting of cysteine- containing peptides onto over-bleached hair, Biocatal. Biotransform., 30, 10-19 (2012).
5. M.M. Fernandes, C.F. Lima, A. Loureiro, A. C. Gomes, and A. Cavaco-Paulo, Keratin-based peptide: biological evaluation and strengthening properties on relaxed hair, Int. J. Cosmet. Sci., 34, 338–346 (2012).
6. A. Ribeiro, T. Matama, C.F. Cruz, A.C. Gomes, and A.M. Cavaco-Paulo, Potential of human γD-crystallin for hair damage repair: insights into the mechanical properties and biocompatibility, Int. J. Cosmet. Sci., 35, 458-466 (2013).