Preparation method of polydihydroxyindole and application of polydihydroxyindole in hair dyeing

Abstract

The invention discloses a preparation method of polydihydroxyindole and application of the polydihydroxyindole in hair dyeing, and belongs to the field of functional materials and the field of daily cosmetics. The polydihydroxyindole is obtained in the mode that Boc-true melanin is formed after true melanin is protected through phenolic hydroxyl groups, the Boc-true melanin is subjected to oxidative degradation, separation and purification and then deprotection, the true melanin is naturally extracted melanin or chemically synthesized melanin or melanin synthesized through a microbiological method, and the oxidative degradation method is an enzymatic oxidative degradation method. The obtained polydihydroxyindole is moderate in molecular weight and has excellent permeability on hair, a strong alkaline condition is not needed in the oligomer dyeing process, and hair quality damage can be reduced; the polydihydroxyindole is oxidized again in the hair dyeing process to generate melanin, and under the condition that the pH is 5-7, and in the presence of ferric ions, the polydihydroxyindole can be more tightly combined with keratin on the hair to coat the outer layer of the hair, so that the hair color is deepened, and the effects of dyeing and protecting the hair are achieved.

Description

Technical Field

[0001] The invention relates to the fields of functional materials and daily cosmetics, and in particular to a method for preparing polydihydroxyindole and application technology thereof as a core component of a hair dye. Background Art

[0002] With the development of society and the progress of science, people's demand for covering age and graying is increasing. However, existing hair dyes mostly rely on aniline dyes (such as p-phenylenediamine and resorcinol compounds). Although they have a good dyeing effect on gray hair, they have problems such as carcinogenicity, sensitization, and environmental toxicity. Long-term and frequent use poses great health risks. Therefore, people's demand for safe hair dye products is increasing.

[0003] In response to the health risks brought by traditional hair dyes, the current market usually uses natural plant extracts to replace aniline dyes, but their cost is high and the dyeing effect is limited; the use of melanin as a hair dye raw material has received widespread attention in recent years. Melanin is a natural pigment and its structure is highly similar to hair pigment, but direct application has limitations such as excessive molecular weight and poor permeability.

[0004] Patent CN 113559017 A discloses a hair dye based on the copolymerization of 5,6-dihydroxyindole and 6-hydroxyindole, but 6-hydroxyindole does not have a catechol structure, does not undergo phenolquinone isomerization, and is difficult to form dark deposits. Although 5,6-dihydroxyindole can be effectively deposited on the surface of the hair, it is not a raw material within the scope of the "Catalogue of Used Cosmetic Raw Materials (2021 Edition)" and its use is restricted. In addition, 5,6-dihydroxyindole has active chemical properties and is easily oxidized. The raw material needs to be stored at -20°C under nitrogen conditions, which is not suitable for large-scale production and storage, and the product shelf life is too short.

[0005] Patent CN108670879B mentions a melanin hair dye and its preparation method. Its main raw material is dopamine hydrochloride. It also has problems such as not being a raw material within the scope of the "Catalogue of Used Cosmetic Raw Materials (2021 Edition)" and having restricted use.

[0006] In view of the above problems, it is urgent to develop a safe and efficient hair dye that meets Chinese regulations. Polydihydroxyindole is an intermediate in the oxidation of 5,6-dihydroxyindole to melanin. It has excellent permeability of small molecules and is more stable than small molecules in chemical properties. Patent CN 116889524 A proposes to use polydihydroxyindole after protection with liposomes, but the preparation of liposomes is complicated and costly, and there is no mention of how to obtain suitable polydihydroxyindole raw materials.

[0007] Therefore, there is no report on the preparation of polydihydroxyindole by controllable oxidative degradation of melanin to optimize the hair dyeing effect so far. SUMMARY OF THE INVENTION

[0008] The object of the present invention is to provide a preparation method of polydihydroxyindole and its application in hair dyeing, breaking the traditional chemical synthesis thinking. First, the phenolic hydroxyl groups of melanin are protected, and after oxidative degradation of acetylated melanin, deprotection is carried out, and the degradation process is adjusted to obtain polydihydroxyindole with a moderate molecular weight. Polydihydroxyindole meets the usage requirements of the "List of Cosmetic Raw Materials in Use (2021 Edition)", has a moderate molecular weight, has excellent permeability on hair, and the oligomer dyeing process does not require strong alkaline conditions, which can reduce hair quality damage; polydihydroxyindole regenerates melanin by re-oxidation during the hair dyeing process. Under the condition of pH 5-7, in the presence of ferric ions, it can bind more tightly to keratin on hair and coat the outer layer of hair, deepen the hair color, and achieve the effects of hair dyeing and hair care.

[0009] The technical solution of the present invention is as follows:

[0010] A preparation method of polydihydroxyindole, wherein the polydihydroxyindole is obtained by protecting the phenolic hydroxyl groups of eumelanin to form Boc-eumelanin, separating and purifying Boc-eumelanin after oxidative degradation, and then deprotecting. Among them, the eumelanin is natural extracted melanin, chemically synthesized melanin or melanin synthesized by the microbial method, and the oxidative degradation method is an enzymatic oxidative degradation method.

[0011] Furthermore, the Boc-eumelanin refers to eumelanin with phenolic hydroxyl groups protected by Boc groups.

[0012] Furthermore, the preparation of the Boc-eumelanin includes the following steps:

[0013] (1) Disperse eumelanin in a solvent, and add a catalyst, and stir to form a uniform dispersion;

[0014] (2) Slowly and gradually add di-tert-butyl dicarbonate to the solution in step (1), and react at room temperature;

[0015] (3) Filter or centrifuge to separate the reaction product, wash and then dry in vacuum to obtain Boc-eumelanin.

[0016] Even further, the mass concentration of the eumelanin in step (1) is 1-10 wt%, the catalyst is 4-dimethylaminopyridine (DMAP) or triethylamine (Et2N), and the dosage of the catalyst is 0.2-1 wt% of the eumelanin; the solvent is one of dichloromethane (DCM), tetrahydrofuran (THF), and N,N-dimethylformamide (DMF).

[0017] Further, the dosage of di-tert-butyl dicarbonate in step (2) is 5-10 wt% of eumelanin, and the reaction time is 4-10 hours.

[0018] Further, the preparation method of the polydihydroxyindole includes the following steps:

[0019] (1) Disperse Boc-eumelanin in an acetate buffer solution and adjust the pH of the system to 5.0-6.5;

[0020] (2) Add laccase to the solution in step (1) and react in the dark on a shaker;

[0021] (3) Add ascorbic acid as a reducing agent to inhibit the activity of laccase, remove solid substances through a filter membrane, concentrate and then dry at low temperature to obtain a solid;

[0022] (4) Dissolve the solid in a trifluoroacetic acid / dichloromethane solution, stir in an ice bath, continue the reaction after returning to room temperature, evaporate the solvent under reduced pressure, precipitate with ether and dry to obtain the final product.

[0023] Further, in step (1), the mass concentration of Boc-eumelanin is 1-10 wt%; in step (2), the reaction temperature is room temperature, and the reaction time is 1-6 hours; the concentration of laccase in the system of step (2) is 10-50 U / mL; in step (3), the dosage of ascorbic acid is 5-20 wt% of eumelanin, and the filter membrane range is 0.45-1.2 um; in step (4), the concentration of the solid in the trifluoroacetic acid / dichloromethane solution is 60-160 g / L; the volume ratio of trifluoroacetic acid / dichloromethane is 1:1, the ice bath stirring time is 20-50 minutes, and the reaction time after returning to room temperature is 0.5-1 hour.

[0024] Further, during the product separation and purification process, that is, after evaporating the solvent under reduced pressure in step (4), it can also be separated by a chromatographic column and detected, and the subsequent preparation method process is determined after the target product appears.

[0025] The molecular weight of the polydihydroxyindole is 300-800 Da and it can dissolve in water and re-oxidize and polymerize to form melanin.

[0026] An application of polydihydroxyindole in hair dyeing, and the usage method of the polydihydroxyindole in the hair dye is as follows:

[0027] (1) Prepare agent A by formulating an aqueous solution containing 0.05-0.5 wt% of polydihydroxyindole with a thickener;

[0028] (2) Prepare agent B by formulating an aqueous solution of a compound containing 0.05-1.5 wt% of ferric ions with a thickener;

[0029] (3) Mix agent A and agent B, adjust the pH to 5 - 7, then evenly apply the mixture to the hair, fix the color at room temperature, and wash it with clear water.

[0030] Furthermore, in step (1), the thickener is one of hydroxypropyl methylcellulose, hydroxyethyl cellulose, hydroxypropyl starch phosphate, xanthan gum, chitosan, carrageenan; the mass ratio of the thickener to polydihydroxyindole is 1:(0.1 - 1); in step (2), the compound of trivalent iron ions is one of ammonium ferrocyanide, ammonium ferric citrate, ferric chloride, ferric citrate; the mass ratio of the thickener to the compound of trivalent iron ions is 1:(0.1 - 3); in step (3), the mixing volume ratio of agent A to agent B is (1 - 1.2):1; the color fixation time is 30 - 90 min; surfactants, emulsifiers, stabilizers, preservatives and / or hair conditioners are selectively added to agents A and B.

[0031] Advantages of the present invention:

[0032] (1) The present invention breaks through the traditional chemical synthesis method, degrades eumelanin by oxidative degradation, is applicable to the degradation of various types of eumelanin, and solves the problem of difficult penetration of traditional melanin.

[0033] (2) The enzyme degradation reaction conditions are mild and highly specific. During the oxidative degradation process, other biomolecules or substance structures will not be damaged, reducing the occurrence of unnecessary side reactions.

[0034] (3) The phenolic hydroxyl groups of eumelanin are protected in the present invention, which can effectively prevent it from being over-oxidized during the oxidative degradation process and then degraded to the carboxylic acid structure and unable to be re-oxidized; acid deprotection can effectively prevent it from undergoing oxidative polymerization under alkaline conditions.

[0035] (4) The polydihydroxyindole prepared by the process method of the present invention has a moderate molecular weight, excellent permeability, and meets the usage requirements of the "List of Cosmetic Raw Materials in Use (2021 Edition)".

[0036] (5) Polydihydroxyindole can be re-oxidized to form melanin without an alkaline environment during the hair dyeing process. At pH 5 - 7, in the presence of trivalent iron ions, it can bind more tightly to the keratin on the hair and coat the outer layer of the hair, deepen the hair color, achieve the effects of hair dyeing and hair care, and reduce hair quality damage. Description of the Drawings

[0037] Figure 1 It is the NMR spectrum of Example 10.

[0038] Figure 2 It is the microscopic pictures of the hair cross-sections of Example 14, Comparative Example 2, Comparative Example 3, and Comparative Example 4.

[0039] Figure 3Pictures of hair after dyeing in Example 14, Comparative Example 2, Comparative Example 3, and Comparative Example 4.

[0040] Figure 4 The breaking strength of the hair in Example 14, Comparative Example 2, Comparative Example 3, and Comparative Example 4 after 96 h of ultraviolet damage test. Detailed implementation mode

[0041] The present invention will be further described below in conjunction with embodiments. The following embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. Those skilled in the art can make some non-essential improvements and adjustments based on the above invention content. All raw materials used in the embodiments are commercially available.

[0042] Melanin extracted from cuttlefish is obtained by crushing the cuttlefish ink sac, removing proteins, washing, dispersing the ink juice in deionized water, soaking for a period of time, and then centrifuging (18000 rpm, 15 min) to obtain a black solid, which is vacuum dried at 60 °C for 24 hours.

[0043] Chemically synthesized melanin is prepared according to "Bioinspired Polymerization of Dopamine to Generate Melanin-Like Nanoparticles Having an Excellent Free-Radical-Scavenging Property": Dissolve dopamine in deionized water, control the temperature at 50 °C, and in the presence of oxygen in an alkaline environment, the dopamine solution will quickly turn dark brown. After 5 h, stop the reaction, centrifuge (15000 rpm, 30 min) to obtain a black solid, and dry it in a vacuum oven at 45 °C for 24 h;

[0044] Fermentation-synthesized melanin is prepared according to "Fermentation Production, Biological Activity and Synthetic Pathway of Melanin by Streptomyces sp. ZL--24": The ZL-24 strain is cultured in 50 mL of LB medium on a shaker at 140 rpm and 30 °C for 18 - 24 h. When the strain grows into a spherical shape, transfer it to a 250 mL Erlenmeyer flask with an inoculation amount of 1% and culture it on a shaker at 200 rpm and 30 °C for 5 days; Centrifuge the fermentation broth at 4 °C for 20 min to remove cells and debris, adjust the pH of the supernatant of the fermentation broth to 2 with 6 M HCl aqueous solution, let it stand for 7 days until no more precipitation occurs, centrifuge for 10 min, and freeze-dry to obtain the product.

[0045] I. Preparation of Boc-eumelanin

[0046] Example 1

[0047] Disperse 1 g (1 wt%) of eumelanin (extracted from cuttlefish) in 100 g of dichloromethane, add 0.002 g of 4-dimethylaminopyridine (0.2 wt% of eumelanin), and stir until evenly dispersed; slowly add 0.05 g of di-tert-butyl dicarbonate (5 wt% of eumelanin) to the above solution, react at room temperature for 4 hours, filter the solution through a 0.45 μm filter membrane, wash the solid with deionized water, and then dry it under vacuum to obtain Boc-eumelanin 1.

[0048] Example 2

[0049] Disperse 1 g (1 wt%) of eumelanin (chemically synthesized) in 100 g of dichloromethane, add 0.002 g of 4-dimethylaminopyridine (0.2 wt% of eumelanin), and stir until evenly dispersed; slowly add 0.05 g of di-tert-butyl dicarbonate (5 wt% of eumelanin) to the above solution, react at room temperature for 4 hours, filter the solution through a 0.45 μm filter membrane, wash the solid with deionized water, and then dry it under vacuum to obtain Boc-eumelanin 2.

[0050] Example 3

[0051] Disperse 1 g (1 wt%) of eumelanin (fermentation synthesized) in 100 g of dichloromethane, add 0.002 g of 4-dimethylaminopyridine (0.2 wt% of eumelanin), and stir until evenly dispersed; slowly add 0.05 g of di-tert-butyl dicarbonate (5 wt% of eumelanin) to the above solution, react at room temperature for 4 hours, filter the solution through a 0.45 μm filter membrane, wash the solid with deionized water, and then dry it under vacuum to obtain Boc-eumelanin 3.

[0052] Example 4

[0053] Disperse 5 g (5 wt%) of eumelanin (chemically synthesized) in 100 g of dichloromethane, add 0.006 g of 4-dimethylaminopyridine (0.6 wt% of eumelanin), and stir until evenly dispersed; slowly add 0.08 g of di-tert-butyl dicarbonate (8 wt% of eumelanin) to the above solution, react at room temperature for 6 hours, filter the solution through a 0.45 μm filter membrane, wash the solid with deionized water, and then dry it under vacuum to obtain Boc-eumelanin 4.

[0054] Example 5

[0055] Disperse 10 g (10 wt%) of eumelanin (chemically synthesized) in 100 g of dichloromethane, add 0.01 g of 4-dimethylaminopyridine (1 wt% of eumelanin), and stir until evenly dispersed; slowly add 0.1 g of di-tert-butyl dicarbonate (10 wt% of eumelanin) to the above solution, react at room temperature for 10 hours, filter the solution through a 0.45 μm filter membrane, wash the solid with deionized water, and then dry it under vacuum to obtain Boc-eumelanin 5.

[0056] II. Preparation of Polydihydroxyindole

[0057] Example 6

[0058] Disperse 1 g (1 wt%) of Boc-eumelanin 1 in 100 g of acetate buffer solution, and adjust the pH of the system to 6.5; at room temperature, add laccase with a concentration of 10 U / mL to the above solution, react in the shaker in the dark for 1 hour, add 0.05 g (5 wt% of Boc-eumelanin) of ascorbic acid, mix evenly, remove solid substances through a 0.45 μm filter membrane, concentrate and then dry at low temperature to obtain a solid; dissolve the above 60 g of solid in 1 L of TFA / DCM (1:1) solution, stir in an ice bath for 20 minutes, restore to room temperature and continue to react for 0.5 hour, distill off the solvent under reduced pressure, precipitate with ether and dry to obtain the product polydihydroxyindole 1. The molecular weight of polydihydroxyindole 1 is about 750 Da.

[0059] Example 7

[0060] Disperse 1 g (1 wt%) of Boc-eumelanin 2 in 100 g of acetate buffer solution, and adjust the pH of the system to 5.5; at room temperature, add laccase with a concentration of 50 U / mL to the above solution, react in the shaker in the dark for 1 hour, add 0.05 g (5 wt% of Boc-eumelanin) of ascorbic acid, mix evenly, remove solid substances through a 0.45 μm filter membrane, concentrate and then dry at low temperature to obtain a solid; dissolve the above 100 g of solid in 1 L of TFA / DCM (1:1) solution, stir in an ice bath for 30 minutes, restore to room temperature and continue to react for 1 hour, distill off the solvent under reduced pressure, precipitate with ether and dry to obtain the product polydihydroxyindole 2. The molecular weight of polydihydroxyindole 2 is about 350 Da.

[0061] Example 8

[0062] Disperse 1 g (1 wt%) of Boc-eumelanin 3 in 100 g of acetate buffer solution, and adjust the pH of the system to 5.0; at room temperature, add laccase with a concentration of 10 U / mL to the above solution, react in the shaker in the dark for 1 hour, add 0.05 g (5 wt% of Boc-eumelanin) of ascorbic acid, mix evenly, remove solid substances through a 1.2 μm filter membrane, concentrate and then dry at low temperature to obtain a solid; dissolve the above 160 g of solid in 1 L of TFA / DCM (1:1) solution, stir in an ice bath for 50 minutes, restore to room temperature and continue to react for 1 hour, distill off the solvent under reduced pressure, precipitate with ether and dry to obtain the product polydihydroxyindole 3. The molecular weight of polydihydroxyindole 3 is about 800 Da.

[0063] Example 9

[0064] Disperse 5 g (5 wt%) of Boc-true melanin 4 in 100 g of acetate buffer, and adjust the pH of the system to 5.5; at room temperature, add laccase with a concentration of 30 U / mL to the above solution, react in the dark on a shaker for 3 hours, add 0.5 g (10 wt% of Boc-true melanin) of ascorbic acid, mix evenly, remove solid substances through a 0.45-μm filter membrane, concentrate and then dry at low temperature to obtain a solid; dissolve 100 g of the above solid in 1 L of TFA / DCM (1:1) solution, stir in an ice bath for 30 minutes, restore to room temperature and continue to react for 0.5 hour, evaporate the solvent under reduced pressure, precipitate with ether and dry to obtain the product polydihydroxyindole 4. The molecular weight of polydihydroxyindole 4 is about 720 Da.

[0065] Example 10

[0066] Disperse 10 g (10 wt%) of Boc-true melanin 5 in 100 g of acetate buffer, and adjust the pH of the system to 5.5; at room temperature, add laccase with a concentration of 50 U / mL to the above solution, react in the dark on a shaker for 6 hours, add 2 g (20 wt% of Boc-true melanin) of ascorbic acid, mix evenly, remove solid substances through a 0.45-μm filter membrane, concentrate and then dry at low temperature to obtain a solid; dissolve 100 g of the above solid in 1 L of TFA / DCM (1:1) solution, stir in an ice bath for 30 minutes, restore to room temperature and continue to react for 1 hour, evaporate the solvent under reduced pressure, precipitate with ether and dry to obtain the product polydihydroxyindole 5. The molecular weight of polydihydroxyindole 5 is about 650 Da.

[0067] Comparative Example 1

[0068] Disperse 10 g (10 wt%) of true melanin (chemically synthesized) in 100 g of acetate buffer, and adjust the pH of the system to 5.5; at room temperature, add laccase with a concentration of 50 U / mL to the above solution, react in the dark on a shaker for 6 hours, add 2 g (20 wt% of true melanin) of ascorbic acid, mix evenly, remove solid substances through a 0.45-μm filter membrane, concentrate and then dry at low temperature to obtain the comparative product 1.

[0069] Perform mass spectrometry analysis on the product, and list the main data of the mass-to-charge ratio in Table 1.

[0070] Table 1

[0071] Example 10 (m / z) 303 319 589 645 Comparative Example 1 (m / z) 155 178 201 222

[0072] From the data analysis, it can be seen that the polydihydroxyindole obtained by the preparation method of the present invention has a typical indole structure. The molecular weight analysis products are mostly dimers - tetramers, and the source of eumelanin does not affect the final polydihydroxyindole product. From the data of Comparative Example 1, it can be seen that the eumelanin without Boc protection cannot retain the indole structure after oxidative degradation and is basically degraded into pyrrole carboxylic acid structure.

[0073] III. Preparation of Polydihydroxyindole Hair Dye

[0074] Example 11

[0075] Dissolve 0.05 g of polydihydroxyindole 4 and 0.5 g of hydroxyethyl cellulose in 100 g of water to prepare Agent A; dissolve 0.05 g of ammonium ferrocyanide and 0.5 g of hydroxyethyl cellulose in 100 g of water to prepare Agent B; mix 100 g of Agent A and 100 g of Agent B, adjust the pH to 5, evenly apply the mixture to the hair, fix the color at room temperature for 30 minutes, and then wash it with clear water.

[0076] Example 12

[0077] Dissolve 0.5 g of polydihydroxyindole 4 and 0.5 g of hydroxyethyl cellulose in 100 g of water to prepare Agent A; dissolve 0.5 g of ammonium ferrocyanide and 0.5 g of hydroxyethyl cellulose in 100 g of water to prepare Agent B; mix 120 g of Agent A and 100 g of Agent B, adjust the pH to 5, evenly apply the mixture to the hair, fix the color at room temperature for 30 minutes, and then wash it with clear water.

[0078] Example 13

[0079] Dissolve 0.5 g of polydihydroxyindole 5 and 0.5 g of hydroxyethyl cellulose in 100 g of water to prepare Agent A; dissolve 0.5 g of ammonium ferrocyanide and 0.5 g of hydroxyethyl cellulose in 100 g of water to prepare Agent B; mix 100 g of Agent A and 100 g of Agent B, adjust the pH to 5, evenly apply the mixture to the hair, fix the color at room temperature for 30 minutes, and then wash it with clear water.

[0080] Example 14

[0081] Dissolve 0.5 g of polydihydroxyindole 5 and 0.5 g of hydroxyethyl cellulose in 100 g of water to prepare Agent A; dissolve 1.5 g of ammonium ferrocyanide and 0.5 g of hydroxyethyl cellulose in 100 g of water to prepare Agent B; mix 100 g of Agent A and 100 g of Agent B, adjust the pH to 5, evenly apply the mixture to the hair, fix the color at room temperature for 30 minutes, and then wash it with clear water.

[0082] Comparative Example 2

[0083] Evenly apply the commercially available hair dye on natural white hair, and after operating according to the instructions, rinse the hair with clear water.

[0084] Comparative Example 3

[0085] Wash the natural white hair and dry it.

[0086] Comparative Example 4

[0087] Add 0.5 g of Comparative Product 1 and 0.5 g of hydroxyethyl cellulose to 100 g of water to prepare Agent A; add 1.5 g of ammonium ferrocyanide and 0.5 g of hydroxyethyl cellulose to 100 g of water to prepare Agent B; mix 100 g of Agent A and 100 g of Agent B, adjust the pH to 5, evenly apply the mixture to the hair, fix the color at room temperature for 30 minutes, and wash it with clear water.

[0088] By analyzing its cross-sectional photos and surface morphology, it can be seen that for Comparative Example 2, the hair including the medulla shows obvious dark color inside when using the commercially available hair dye, indicating that the oxidative hair dye penetrates the dye into the hair interior and forms a color-developing substance through oxidative polymerization, resulting in obvious color inside the hair. While for the natural white hair in Comparative Example 3, the uncolored hair in Comparative Example 4, and the hair treated with the hair dye of the present invention, the inside shows the same color as the environment, indicating that the polydihydroxyindole hair dye of the present invention coats on the outer layer of the hair and does not enter the hair interior; from the dyeing photos, it can be seen that the dyeing effect of the present invention is comparable to that of the commercially available hair dye, but the melanin without phenolic hydroxyl protection loses the ability to be re-oxidized and polymerized into melanin after 6 hours of degradation and cannot dye the white hair; the tensile fracture data shows that after 96 h of ultraviolet aging test, the mechanical properties of the hair dyed with the hair dye provided by the present invention are more excellent than those of the hair dyed with the commercially available hair dye in Comparative Example 2, the natural white hair in Comparative Example 3, and the uncolored hair in Comparative Example 4, indicating that the hair dye of the present invention has an anti-ultraviolet protection effect on the hair.

Claims

1. A method for preparing polydihydroxyindole, characterized in that, The polydihydroxyindole is formed by protecting the phenolic hydroxyl groups of eumelanin to form Boc-eumelanin, which is separated and purified by oxidative degradation and then deprotected. Among them, the eumelanin is natural extracted melanin, chemically synthesized melanin or melanin synthesized by the microbial method, and the oxidative degradation method is the enzymatic oxidative degradation method.

2. The preparation method of a polydihydroxyindole according to claim 1, wherein The Boc-eumelanin refers to the eumelanin with the phenolic hydroxyl groups protected by the Boc group.

3. The preparation method of a polydihydroxyindole according to claim 1, characterized in that, The preparation of the Boc-eumelanin includes the following steps: (1) Disperse the eumelanin in a solvent and add a catalyst, and stir to form a uniform dispersion; (2) Slowly and gradually add di-tert-butyl dicarbonate to the solution in step (1) and react at room temperature; (3) Filter or centrifuge to separate the reaction product, wash and then dry in vacuum to obtain Boc-eumelanin.

4. The preparation method of a polydihydroxyindole according to claim 3, characterized in that, In step (1), the mass concentration of the eumelanin is 1-10 wt%, the catalyst is 4-dimethylaminopyridine or triethylamine, and the dosage of the catalyst is 0.2-1 wt% of the eumelanin; the solvent is one of dichloromethane, tetrahydrofuran, and N,N-dimethylformamide.

5. The preparation method of a polydihydroxyindole according to claim 3, characterized in that, In step (2), the dosage of di-tert-butyl dicarbonate is 5-10 wt% of the eumelanin, and the reaction time is 4-10 hours.

6. The preparation method of a polydihydroxyindole according to claim 1, characterized in that, The preparation method of the polydihydroxyindole includes the following steps: (1) Disperse the Boc-eumelanin in an acetate buffer solution and adjust the pH of the system to 5.0-6.5; (2) Add laccase to the solution in step (1) and react in the dark on a shaker; (3) Add ascorbic acid as a reducing agent to inhibit the activity of laccase, remove the solid substances through a filter membrane, concentrate and then dry at low temperature to obtain a solid; (4) Dissolve the solid in a trifluoroacetic acid / dichloromethane solution, stir in an ice bath, resume the reaction at room temperature, then distill off the solvent under reduced pressure, precipitate with ether and dry to obtain the final product.

7. The preparation method of a polydihydroxyindole according to claim 6, characterized in that, In step (1), the mass concentration of the Boc-eumelanin is 1-10 wt%; in step (2), the reaction temperature is room temperature, and the reaction time is 1-6 hours; the concentration of laccase in the system in step (2) is 10-50 U / mL; in step (3), the dosage of ascorbic acid is 5-20 wt% of the eumelanin, and the range of the filter membrane is 0.45-1.2 um; in step (4), the concentration of the solid in the trifluoroacetic acid / dichloromethane solution is 60-160 g / L; the volume ratio of trifluoroacetic acid / dichloromethane is 1:1, the ice bath stirring time is 20-50 minutes, and the reaction time for resuming to room temperature is 0.5-1 hour.

8. A polydihydroxyindole prepared by the preparation method according to any one of claims 1-7, characterized in that, The molecular weight of the polydihydroxyindole is 300-800 Da, and it can dissolve in water and re-oxidize and polymerize to form melanin.

9. Use of a polydihydroxyindole prepared by the preparation method according to any one of claims 1-7 in hair dyeing, characterized in that, The usage method of the polydihydroxyindole in hair dyes is as follows: (1) Prepare agent A by formulating an aqueous solution containing 0.05-0.5 wt% of the polydihydroxyindole with a thickener; (2) Prepare agent B by formulating an aqueous solution containing 0.05-1.5 wt% of a compound of trivalent iron ions with a thickener; (3) Mix agent A and agent B, adjust the pH to 5-7, then evenly apply the mixture to the hair, fix the color at room temperature, and wash it with clean water.

10. The use of a polydihydroxyindole in hair dyeing according to claim 9, characterized in that, In step (1), the thickener is one of hydroxypropyl methylcellulose, hydroxyethyl cellulose, hydroxypropyl starch phosphate, xanthan gum, chitosan, and carrageenan; the mass ratio of the thickener to polydihydroxyindole is 1:(0.1 - 1); in step (2), the trivalent iron ion compound is one of ammonium ferrocyanide, ammonium ferric citrate, ferric chloride, and ferric citrate; the mass ratio of the thickener to the trivalent iron ion compound is 1:(0.1 - 3); in step (3), the mixing volume ratio of agent A to agent B is (1 - 1.2):1; the fixing time is 30 - 90 min; surfactants, emulsifiers, stabilizers, preservatives, and / or hair conditioners are selectively added to agent A and agent B.

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