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

that faster dye formation chemistry causes more dye to form outside versus inside hair, and this color is subsequently washed off during the rinsing step. The same effect was seen when prewashing hair in tap water containing copper with a shampoo product containing either no histidine or 0.1% histidine. Addition of low levels of a copper- specifi c chelant such as histidine in a shampoo prevents copper accumulation in hair as shown in the ICP-OES data and leads to improved color uptake and color over multiple wash cycles. ACKNOWLEDGMENTS: The authors would like to thank the Procter & Gamble Company for support of this work. REFERENCES (1) J. M. Marsh, “Hair coloring,”in Practical Modern Hair Science: Allured Business Media, Carol Stream, IL, T. Evans and R. R. Wickett. Eds. (2012), pp. 117–156 . (2) J. F. Corbett, Benzoquinone imines. Part 1. p-Phenylenediamine-Ferricyanide and p-amino phenol- Ferricyanide redox systems, J Chem Soc (B)., 3, 207–212 (1969). (3) J. F. Corbett, Benzoquinone imines. Part II. Hydrolysis of p-benzoquinone monoimine and p-benzoquinone di-imine, J Chem Soc (B), 3, 213–216 (1969). (4) S. Godfrey, W. Staite, Bowtell P., and J. Marsh, Metals in female scalp hair globally and its impact on perceived hair health, Int. J. Cosmet. Sci., 35(3), 264–271 (2013). (5) J. M. Marsh, M. G. Davis, R. L. Lucas, R. Reilman, P. B. Styczynski, Li C., M. Mamak, D. W. McComb, R. E. A. Williams, S. Godfrey, K. R. Navqi, and V. Chechikand, Preserving fi bre health: reducing oxida- tive stress throughout the life of the hair fi bre, Int. J. Cosmet. Sci., 37(S2), 16–24 (2015). (6) J. M. Marsh, M. G. Davis, M. J. Flagler, Y. Sun, T. Chaudhary, M. Mamak, D. W. McComb, R. E. A. Williams, K. D. Greis, L. Rubio, and L. Coderchand, Advanced hair damage model from ultra-violet radiation in the presence of copper, Int. J. Cosmet. Sci., 37(5), 532–541 (2015). (7) B. Sarkar and Y. Wigfi eld, The structure of copper (II)-histidine chelate, J. Biol. Chem., 242(23), 5572– 5577 (1967). Table III dE Values for Initial Color versus Wash Cycles (Std Error in Parenthesis) Shampoo details Shade dE color change between initial color and Wash cycles 2 5 8 12 0% histidine 77/44 7.59 (0.2) 7.73 (0.3) 6.99 (0.2) 6.70 (0.2) 0.1% histidine 77/44 4.93 (0.3) 5.32 (0.3) 5.71 (0.3) 5.60 (0.3) 0% histidine 4/6 3.42 (0.2) 4.57 (0.2) 5.43 (0.2) 6.12 (0.2) 0.1% histidine 4/6 2.58 (0.3) 3.65 (0.3) 4.68 (0.2) 5.68 (0.2) JOURNAL OF COSMETIC SCIENCE 278