Negatively charged surface sites on hair and skin proteins

Negatively charged surface sites on hair and skin proteins, primarily keratin, serve as the primary binding targets for cationic conditioning agents and topical treatments (Robbins, 2012). These sites arise from the ionization of acidic amino acid residues, such as glutamic and aspartic acid, as well as the oxidation of cystine into cysteic acid, which occurs during environmental or chemical damage (Bhushan, 2010). At physiological pH (approximately 4.5 to 5.5), the surface of hair and skin maintains a net negative charge because the environment is above the isoelectric point of keratin, which is roughly 3.7 (Ali & Yosipovitch, 2013). Cationic surfactants and polymers, often referred to as "quats," interact with these anionic sites through strong electrostatic attraction (Goddard & Gruber, 1999). This interaction neutralizes the surface charge, reduces static electricity, and deposits a lubricating layer that enhances barrier function and improves the physical properties of the substrate. In clinical and cosmetic applications, targeting these sites is essential for treating hair damage and managing skin conditions characterized by a compromised stratum corneum.

Other names

Anionic sites on keratinNegatively charged keratin surfacesCarboxylate and sulfonate groups on hair and skinSurface anionic sites

The mechanism involves the electrostatic attraction between positively charged quaternary ammonium groups of the drug/agent and the negatively charged carboxylate (COO-) or sulfonate (SO3-) groups on the keratin surface, leading to the formation of a neutralizing film (Goddard & Gruber, 1999).