Free radical facilitated damage of ungual keratin

Thioglycolic acid (TA) and urea hydrogen peroxide (urea H2O2) are thought to disrupt α-keratin disulfide links in the nail. However, optimal clinical use of these agents to improve the treatment of nail disorders is currently hindered by a lack of fundamental data to support their mechanism of action. The aim of this study was to investigate how the redox environment of ungual keratin, when manipulated by TA and urea H2O2, influenced the properties of the nail barrier. Potentiometric and voltammetric measurements demonstrated that urea H2O2 obeyed the Nernst equation for a proton coupled one-electron transfer redox process while TA underwent a series of redox reactions that was complicated by electrode adsorption and dimer formation. The functional studies demonstrated that nail permeability, measured through TBF penetration (38.51 ± 10.94 μg/cm2/h) and nail swelling (244.10 ± 14.99% weight increase), was greatest when relatively low concentrations of the thiolate ion were present in the applied solution. Limiting the thiolate ion to low levels in the solution retards thiolate dimerisation and generates thiyl free radicals. It appeared that this free radical generation was fundamental in facilitating the redox-mediated keratin disruption of the ungual membrane.