Hair Damage 372 Ker–S–S–R + Ker–S- à R–S–S- + Ker–S–Ker à R–S- + S + Ker–S–Ker lanthionine lanthionine The Effect of Stretching Astbury15 and Speakman16 suggested that the disulphide bonds of stretched wool are more reactive than when in an unstretched state. However, experiments involving oxidative and reduction agents on stretched and unstretched hair were inconclusive when comparing the reaction rates for bundles of hair.17,18 However, despite the findings of the aforementioned studies, direct experimental proof was acquired recently. A single protein molecule was stretched by means of an atomic force microscopy (AFM) and reduced with dithiothreitol (DTT) and it was observed that the reactivity of the protein disulphide bond increased exponentially with the force applied, by a factor of 10 over a range of 300 pN.19 It may be concluded that stretched hair fibers are more susceptible to damage when under attack from external factors. The Effect of Water Water is known to swell hair fibers both in length and in diameter. The effect is anisotropic, as fibers subjected to 100% relative humidity increase about 16% in diameter, compared with only 2‑3% increase in length.20 The anisotropic swelling of the fibers increases the stretching force on radially oriented S-S bonds, and accordingly, increases the reactivity of these bonds. For example, it was found19 that during the transition state of a thiol/disulphide exchange reaction, the S-S bond lengthens by 34 pm, which is about 16% of the unstretched disulphide length (~206 pm). Based on the aforementioned swelling data, this corresponds to the effect of 100% relative humidity on the fiber, which suggests that at high humidity the radially oriented S-S bonds are in an activated state. As a consequence, swelling in water reduces the energy barrier to the activated state and eases the occurrence of reactions which damage the sulphur-sulphur bridges.21