Figure 5. Dye formed in the HDAP–AHT couple. Figure 6. Correlation of UV absorbance of dye extracted versus copper level in hair. and this formed color is subsequently washed out during rinsing off the colorant. This leads to a lighter fi nal hair color. CHELANT EXPERIMENTS Experiments were performed to demons trate that chelants added to a shampoo can re- move copper from hair and improve color uptake and color intensity over multiple washes. The chelant chosen was L-histidine, an amino acid known to have high binding JOURNAL OF COSMETIC SCIENCE 276

effi ciency to copper because of its imidazole group and relatively poor binding to calcium and magnesium (7). In addition, it has been shown to reduce protein damage in hair ex- posed to UV by reducing copper levels in hair (5). Hair was fi rst washed in water contain- ing copper (0 .06–0.09 μg/g) for 12 cycles with a sham poo containing 0.1% histidine and control shampoo containing no histidine. To note, both shampoos contained ethylene diamine tetraacetic acid chelant at 0.13%. Copper levels measured showed lower accu- mulation in hair washed with 0.1% histidine, a 44% decrease (Table II). A small decrease in calcium (6%) and magnesium (5%) was also measured. After coloring with shades 77/44 and 4/6, color levels initially were darker (lower L) for hair washed with the histidine-containing shampoo, and this color remained more in- tense over 12 washes post-coloring (Figure 7). The change in color, delta color change, is also lower than initial color to 12 wash cycles (Table III), indicating improved color fade over time. Color differences were noticeable at all time points between hair pretreated with a 0.1% histidine shampoo versus 0% histidine. CONCLUSION The rate of color formatio n in hair w as shown to be accelerated by the addition of low levels of copper, and this chemistry is proposed to be due to one-electron chemistry dur- ing the rate-determining initial oxidation step of the primary intermediate dye precursor. This color acceleration was demonstrated across three dye couples (PPD–AHT, HDAP– AHT, and PPD–res) in solution dosed with three different levels of copper. On h air tresses pretreated with different levels of copper, this color acceleration led to a lower level of dye formed inside the hair and consequently a lighter fi nal color. It is proposed Table II Copper Levels in Hair before Coloring (Standard Error in Parenthesis) Shampoo details Metal levels after 12 wash cycles (μg/g) Copper Calcium Magnesium 0% histidine 79.2 (1.7) 5,315 (30) 582 (3) 0.1% histidine 44.2 (1.4) 5,012 (72) 553 (7) Figure 7. (A) shade 77/44 and (B) shade 4/6—L value versus wash cycle number. REDOX METALS IN COLOR FORMATION IN A HAIR COLORANT 277