NATURAL-BASED HAIR COLORING PROCESS 135 pigment particles, and therefore the observed color is dependent on the size and shape of the particles, their distribution in the keratin matrix, and their concentration. In addition, there is probably some contribution from the weak chromophores thought to be present in the pigments. In line with this, the two pigment types have been found to have significantly different physical properties. In a recent publication (2), we described an efficient way to convert dopa into 5,6- dihydroxyindole on hair using selected oxidants, thus allowing us to form eumelanins within the hair by a process that closely follows the natural pathway. We have now made additional significant developments in this area by establishing conditions that also allow us to exploit the pheomelanin pathway, thereby markedly broadening the color palette and the consumer appeal of this novel, natural-based dyeing system. MATERIALS AND METHODS All dye materials were either purchased from commercial suppliers or synthesized by the methods referred to in the text. Where necessary, appropriate techniques were used to verify the structures. Hair dyeing followed standard industry practice and was generally carried out by soaking 2-g hair tresses (usually white hair) in 5 g of dye solution for the required time. Water was the solvent of choice, although frequently a small amount of methanol or diethyleneglycolmethylether was needed to dissolve the dye precursors. After a rapid rinse under running tap water, the tress was either shampooed or the second oxidative treatment was applied prior to the shampoo. Shampoo resistance was evaluated by washing the tress by hand with a commercial shampoo after the dyed tress had been air dried. Color hue and intensity were evaluated by eye and where necessary by Hunter- Lab Tristimulus instrumentation. RESULTS AND DISCUSSION EUMELANIN FORMATION Conditions for polymerizing 5,6-dihydroxindole (DHI) and 5,6-dihydroxyindole-2- carboxylic acid (DHICA) into eumelanins are well known. In general, any oxidant including atmospheric oxygen is effective, although copper ions or the combination of hydrogen peroxide and iodide ions seem to be the most effective reagents according to the patent literature (3,4). Our recent publication (2) describes a simple and convenient process for converting dopa into a mixture of DHI and DHICA by use of a controlled amount of potassium ferricyanide in buffered solution. By combining these two pro- cesses in sequence, gray hair is dyed a very intense black color by forming high yields of melanin from only low concentrations of dopa. In order to produce colors other than black and therefore broaden the consumer appeal, a number of modifications and improvements to the original process were developed. Some of the reactive sites on dopa have been blocked by less reactive substituents so that the subsequent chemistry is then modified. In this way, much lighter brown colors are produced on hair from materials such as dopa methyl ester (which cannot easily decar-

136 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS boxylate) and adrenaline (which is less likely to undergo extensive polymerization due to blocking groups at the l- and 3-positions and is known to form only adrenolutin and various isatin derivatives on oxidation). In addition, a variety of nucleophi!es are able to react with dopaquinone and radically alter the subsequent chemistry. Many currently used hair dye ingredients, including permanent developers such a p-phenylenediamine and p-aminophenol, couplers such as m-aminophenol and resorcinol, and direct dyes, are extremely efficient trapping reagents for dopaquinone and result in a wide variety of yellow to red shades being formed from the colored adducts through which the chro- mophore becomes chemically bound into the resulting melanins. Examples of these processes are given in Table I. PHEOMELANIN FORMATION Intermediates in the pheomelanin pathway are somewhat more difficult to synthesize than the relatively easily available dopa and indole derivatives. However, we have been able to prepare quantities of cysteinyldopa analogs and hydroxybenzothiazines to act as model compounds for pheomelanin formation. In general, these derivatives formed pigments on hair under oxidative conditions and resulted in stable colors in the yel- Table I Dyeing With Dopa and Its Derivatives Dye Oxidant Process Color 1% Dopa, 6% ferricyanide 1% Dopa, 1% Copper sulfate 6% ferricyanide pretreatment 1% s-Methyl dopa, 1% Copper sulfate 6% ferricyanide pretreatment 1% Adrenaline, 1% Copper sulfate 6% ferricyanide pretreatment 1% Dopa methyl ester, 1% Copper sulfate 6% ferricyanide pretreatment 1% Dopa, 1% Copper sulfate 6% ferricyanide, pretreatment 1% resorcinol 1% Dopa, 5% Sodium iodate 6% ferricyanide 0.3% 2-methyl-5- aminophenol 1% Dopa, 5 % Sodium iodate 6% ferricyanide, 0.3% 2-nitro-p- phenylenediamine 1% Dopa, Pretreatrnent with 1% 6% ferricyanide, copper sulfate 1% p-aminophenol At pH 8, for 30 min Gray L 29.6, a 0.3, b 5.2 Pretreat for 5 rnin at pH 8 Black Dye for 30 rnin at pH 8 L 14.1, a 0.4, b 0.6 Pretreat for 5 rnin at pH 8 Dark brown Dye for 30 rnin at pH 8 L 20.5, a 1.9, b 2.5 Pretreat for 5 rnin at pH 8 Yellow brown Dye for 30 min at pH 8 L 22.4, a 1.8, b 6.1 Pretreat for 5 min at pH 8 Medium brown Dye for 30 min at pH 8 L 27.8, a 2.5, b 5.5 Pretreat for 5 min at pH 8 Dark brown Dye for 30 min at pH 8 L 17.3, a 0.6, b 2.5 Dye for 30 min at pH 8 Red brown Post-treat with oxidant for L 23.5, a 2.0, b 4.8 2 min Dye for 20 min at pH 8 Post-treat with oxidant for 2 min Pretreat for 5 min at pH 8 Dye for 30 min at pH 8 Red brown L 19.6, a 4.3, b 4.0 rown 23.1, a 1.8, b 3.7