Letter to the Editor To the Editor: Hair geometry is the result of processes of keratinization and follicular extrusion that transform a viscous mixture of proteins into a strong and resilient fiber. Waving can be viewed as a restage of these processes entailing softening of keratin, molding it to a desired shape, and stabiliz- ing the newly imparted configuration. The cleavage of disulfide bonds is essen- tial to the softening process, and their reformation is required for imparting a durable wave. While both the sulfite and thioglycolate waving systems rely upon the sequence of such reactions, the differ- --= Sulfite Thioglycolate 0.8 0.4 - 0.2 I I • I i 6 8 lO pH Figure 1. Fraction of -S-S- Bond Cleavage (e•) as a Function of 363

364 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ences in the chemical nature of the reac- tants and in the pH profile of the cleavage step [(1), Figure 1] imply system-specific mechanisms for the rebuilding of disul- fide bonds. Longitudinal setting of hair serves as an informative guide to the physicochemical phenomena occurring in the fiber in the course of its conformational change. In particular, owing to the inherently high stresses favoring the length recovery of the fiber, this technique is a sensitive measure of the formation of stabilizing cross-links. Hair fibers, mounted in setting frames (2), were extended by 40 percent of their original length, treated in a setting solu- tion for 30 minutes at 35øC, rinsed, then released in water (30 minutes at 35øC). Imparted length change, calculated as a fraction of original extension, is expressed as percent set. The results (Figure 2) indicate a profound difference between sulfite and thioglycolate in the mecha- nism of stabilization which is a direct consequence of the chemistry involved. Thus, reduction of disulfide bonds (KSSK) in hair by thioglycolate (RSH) produces cysteine residues (KSH) incapa- ble on their own of reforming disulfide bonds: KSSK + 2RSH • 2KSH + RSSR The latter can, of course, be readily gener- ated by oxidation: % Set 6O 4O 2O Thioglycolate ,,, I I I i I 6 5 6 7 8 9 pH Figure 2. Setting of Hair in Sulfite and Thioglycolate.