WATER HARDNESS METALS AND HUMAN HAIR 389 and amino groups participating in electrostatic interactions with carboxyl groups (4,22). Similar pH-dependent increases in the uptake of cationic moieties by keratin (14,23) and other charged macromolecules (24) have been reported. It is realized that the test pH range exceeds the observed pKa values of key ionizable protein groups that are capable of bind- ing metals, e.g., sulfonate groups of cysteic acid (pKa = 1.3), terminal carboxyl groups (pKa = 3.5-4.3), and carboxyl groups of aspartic and glutamic acids (pKa = 3.9 and 4.3, respectively) (25). However, since keratin is a polyelectrolyte, the effective pKa values of the amino acid residues are often different from the intrinsic pKa values of the isolated amino acid monomers due to electrostatic interactions, hydrogen bonding, solvation effects, conformational changes, and the presence of appreciable levels of counterions (26,27). All of these factors can infl uence the ionization of protein groups and their ability to bind cations. Creighton (25) reported that the difference in electrostatic environments can cause the pKa values of one type of amino acid residue to differ by 3– 4 pH units within a single protein. It is, therefore, plausible that the pH dependence of metal uptake was infl uenced by this. Based on these fi ndings, we can conclude that the hair of consumers who use alkaline water will contain higher levels of water hardness metals than hair from those who use neutral or less alkaline water. This effect will be compounded if the consumer has chemically treated hair. CONCLUSIONS Interesting insights about the interaction between water hardness metals and hair have resulted from this work. Our fi ndings suggest that the uptake of water hardness metals is driven primarily by the condition of the hair. Hair that contains more anionic moieties, Figure 3. Effect of water pH on the uptake of calcium and magnesium by hair. Uptake was calculated by subtract- ing the average calcium and magnesium content of hair that was soaked in pure buffer from that of hair treated with buffer solutions containing hardness ions. Water pH infl uenced calcium and magnesium uptake. Asterisks (*, **) represent p 0.05 and p 0.001, respectively, obtained by Tukey-Kramer HSD analysis.

JOURNAL OF COSMETIC SCIENCE 390 the result of chemical treatments such as bleaching and chemical relaxing, has a higher cationic binding capacity and is thus more susceptible to water hardness metal uptake than virgin hair. At a certain level, this binding capacity dominates the effect of water hardness levels, such that the hair will attract signifi cant levels of metal from water that has even a low degree of hardness. This suggests that the effects of water hardness are not just confi ned to consumers that reside in areas of very hard water, but to a much wider population that encompasses residents of lower water hardness areas. Additionally, the pH of the rinse water can also infl uence uptake. We have clearly established the signifi - cance of water hardness metal uptake by human hair and the conditions under which this occurs. This uptake could potentially impact the structural properties of human hair and the performance of hair care formulations. An investigation of the effect of water hardness on hair properties will be detailed in a future publication. REFERENCES (1) P. Kar and M. Misra, Use of keratin fi ber for separation of heavy metals from water, J. Chem. Technol. Biotechnol., 79, 1313–1319 (2004). (2) S. Kokot, J. Cheng, and N. Gill, Comparative study of metal ion interactions with wool keratin using chemometrics, Analyst, 119, 677–681 (1994). (3) A. Sheffi eld and M. Doyle, Uptake of copper(II) by wool, Textile Res. J., 75(3), 203–207 (2005). (4) P. Taddei, P. Monti, G. Freddi, T. Arai, and M. Tsukada, Binding of Co(II) and Cu(II) cations to chem- ically modifi ed wool fi bres: An IR investigation, J. Mol. Struct., 650, 105–113 (2003). (5) J. M. Marsh, J. Flood, D. Domaschko, and N. Ramji, Hair coloring systems delivering color with reduced fi ber damage, J. Cosmet. Sci., 58, 495–503 (2007). (6) I. M. Kempson, W. M. Skinner, and P. K. Kirkbride, Calcium distributions in human hair by ToF-SIMS, Biochim. Biophys. Acta, 1624, 1–5 (2003). (7) C. Mérigoux, F. Briki, F. Sarrot-Reynauld, M. Salomé, B. Fayard, J. Susina, and J. Doucet, Evidence for various calcium sites in human hair shaft revealed by sub-micrometer X-ray fl uorescence, Biochim. Bio- phys. Acta, 1619, 53–58 (2003). (8) K. M. Attar, M. A. Abdel-Aal, and P. Debayle, Distribution of trace elements in the lipid and nonlipid matter of hair, Clin. Chem., 36(3): 477–480 (1990). (9) L. Bertrand, J. Doucet, A. Simionovici, G. Tsoucaris, and P. Walter, Lead-revealed lipid organization in human hair, Biochim. Biophys. Acta, 1620, 218–224 (2003). (10) K. E. Smart, M. Kilburn, M. Schroeder, B. G. H. Martin, C. Hawes, J. M. Marsh, and C. R. M. Grovenor, Copper and calcium uptake in colored hair, J. Cosmet. Sci., 60, 337–345 (2009). (11) G. R. Bhat, E. R. Lukenbach, and R. R. Kennedy, The green hair problem: A preliminary investigation, J. Soc. Cosmet. Chem., 30, 1–8 (1979). (12) T. C. Tan, C. K. Chia, and C. K. Teo, Uptake of metal ions by chemically treated human hair, Water Res., 19(2), 157–162 (1985). (13) A. Uzu, T. Watanabe, H. Ogino, and H. Hirota, Study on the sorption of calcium ion to human hair, J. Soc. Cosmet. Chem. Japan, 22(2), 88–95 (1988). (14) R. E. Noble, Uptake of calcium and magnesium by human scalp hair from waters for different geo- graphic locations, Sci. Total Environ., 239, 189–193 (1999). (15) M. A. Stranick, Determination of negative binding sites on hair surfaces using XPS and Ba2+ labeling, Surf. Interface Anal., 24, 522–528 (1996). (16) T. D. Doering, C. Brockmann, A. Wadle, D. Hollenberg, and T. Förster, Super mild oxidation coloring: Preventing hair damage at the molecular level, IFSCC Mag., 10(4), 323–329 (2007). (17) W. W. Edman and M. E. Marti, Properties of peroxide-bleached hair, J. Soc. Cosmet. Chem., 12, 122–145 (1961). (18) U. P. Strauss and Y. P. Leung, Volume changes as a criterion for site binding of counterions by polyelec- trolytes, J. Am. Chem. Soc., 87, 1476–1480 (1965). (19) B. Tansel, J. Sager, T. Rector, J. Garland, R. Strayer, L. Levine, M. Roberts, M. Hummerick, and J. Bauer, Signifi cance of hydrated radius and hydration shells on ionic permeability during nanofi ltration in dead end and cross fl ow modes, Sep. Purif. Technol., 51, 40–47 (2006).