A kind of melanin and its preparation method

Abstract

A kind of melanin and its preparation method, which relates to the field of functional materials. The melanin mainly consists of three types of components, namely A, B, and C. Among them, component A is dihydroxyacetone; component B is one or more mixtures selected from 3-hydroxy-L-tyrosine, lysine, glycine, histidine, methionine, arginine, cystine, cysteine, tyrosine, acetyltyrosine, proline, amidinoprolin, hydroxyproline, phenylalanine; component C is one or more mixtures selected from dopamine hydrochloride, caffeic acid, chlorogenic acid, ellagic acid, gallic acid, protocatechuic acid, honeysuckle extract, dihydroxyindole, octanoyl epicatechin gallate, epigallocatechin galloyl glucoside, epigallocatechin gallate, catechol gum, catecholamine, catechu leaves / wood powder, uncaria rhynchophylla extract. The present invention can obtain natural and safe synthetic melanin, and the reaction time is short, and the synthesis process is safe and simple.

Description

Technical Field

[0001] The present invention relates to the field of functional materials, and more particularly, to a melanin and a preparation method thereof. Background Art

[0002] Melanin is mainly formed by the oxidation of polyhydroxyphenols and is the most common type of known biological pigment.

[0003] Melanin is a macromolecule with phenolic hydroxyl groups, amino groups and imino groups in its structural units. It is widely present in animals and plants and has special chemical structures and many excellent biological functions, such as photoprotective properties, free radical capture, photothermal conversion functions, etc. Natural melanin is an amorphous pigment that is insoluble in water and conventional organic solvents, making it difficult to conduct physical and chemical analyses on it. Therefore, although the basic units of melanin are known, its precise chemical structure remains inconclusive to date.

[0004] There are many types of melanin in nature. The most widely studied ones include eumelanin and pheomelanin. Melanin can be extracted from plant sources such as black sesame, black glutinous rice, and natural tea seeds, animal sources such as cuttlefish and animal hair, and microbial sources. The conventional separation and purification of melanin include water washing method, chemical method and enzymatic method. However, due to its different sources and purification methods, melanin varies greatly, so its application is also limited.

[0005] In recent years, scientists have turned their focus to artificially synthesized melanin-like substances. Deziderio synthesized melanin-like substances by oxidizing L-dopa with benzoyl peroxide, but this reaction takes 28 days, which is time-consuming [J.Non-Cryst.Solids,2004,338(12):634-638]. Dopamine is a derivative of dopa and has the same properties as dopa. It will oxidize and polymerize in an alkaline solution to form melanin-like polydopamine nanoparticles. Its formation mechanism is very similar to that of natural eumelanin and has the same physical and chemical properties. Lee oxidized and self-polymerized dopamine in the presence of oxygen in a sodium hydroxide (NaOH) solution to form melanin-like nanoparticles [Biomacromolecules,2011,12(3):625-632], but the raw material dopamine is expensive, which greatly limits its application.

[0006] Therefore, how to reduce the cost of artificially synthesized melanin, simplify the synthesis process, and shorten the reaction time has become a current research hotspot. Summary of the Invention

[0007] To solve the problems in the prior art, the present invention provides a melanin and a preparation method thereof.

[0008] Dihydroxyacetone is mostly produced in large quantities by microbial fermentation in industry, and its price cost is far lower than that of monomers such as dopamine. The Maillard reaction browning of dihydroxyacetone with components such as amino acids is synergistic with the oxidation of the ortho-dihydroxyphenol structure. The ortho-dihydroxyphenol structure is oxidized to a quinone group, which easily combines with amino acids in the system to form Schiff bases or Michael adducts, and the conjugated electron transfer can further promote the oxidation of Maillard products into melanin. The present invention can obtain natural and safe synthetic melanin, and the reaction time is short, and the synthesis process is safe and simple.

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

[0010] A kind of melanin, the melanin is mainly composed of three types of components A, B, and C, wherein the mass ratio of the three types of components A, B, and C is 100:(20-50):(1-20).

[0011] Furthermore: (1) Component A is dihydroxyacetone; (2) Component B is one or a mixture of two or more of 3-hydroxy-L-tyrosine, lysine, glycine, histidine, methionine, arginine, cystine, cysteine, tyrosine, acetyltyrosine, proline, amidinoprolin, hydroxyproline, phenylalanine; (3) Component C is one or a mixture of two or more of dopamine hydrochloride, caffeic acid, chlorogenic acid, ellagic acid, gallic acid, protocatechuic acid, honeysuckle extract, dihydroxyindole, octanoyl epicatechin gallate, epigallocatechin galloyl glucoside, epigallocatechin gallate, catechol gum, catecholamine, catechu leaves / wood powder, Uncaria gambir extract.

[0012] The preparation method of the above-mentioned melanin includes the following steps:

[0013] (1) Weigh the three types of components A, B, and C according to the ratio and add them to the reaction vessel;

[0014] (2) Add a solvent and uniformly stir and disperse the substances added in step (1);

[0015] (3) Heat up to the reaction temperature and stir mechanically;

[0016] (4) Add an alkali to the system obtained in step (3), adjust the pH or add an oxidant to carry out the reaction;

[0017] (5) After the reaction is completed, adjust the pH to 7 with an acidic solution, and then carry out pressure filtration drying, freeze drying or centrifugal heat drying.

[0018] Further, in step (2), the solvent is deionized water or an alcohol-water mixture; wherein, the alcohol-water mixture is a mixture of deionized water and alcohols such as ethanol, propylene glycol, butylene glycol, etc., and the mass ratio of deionized water to the alcohol is (50 - 90):(10 - 50).

[0019] Further, in step (2), the concentration of the solution is 10 - 180 g / L.

[0020] Further, in step (3), the reaction temperature is 40 - 90 °C, and the mechanical stirring speed is 200 - 1000 rpm. Preferably, the reaction temperature is 40 - 70 °C, and the mechanical stirring rate is 250 - 500 rpm.

[0021] Further, in step (4), the base is one or a mixture of two or more of alkali metal hydroxides, alkaline earth metal hydroxides, alkali metal carbonates, alkaline earth metal carbonates, alkali metal bicarbonates, alkaline earth metal bicarbonates, alkali metal acetates, alkali metal phosphates, alkali metal alkoxides, and ammonia water, and the pH is adjusted to 8 - 13.

[0022] Further, in step (4), the base is one or a mixture of two or more of ammonium hydroxide, potassium hydroxide, sodium hydroxide, lithium hydroxide, and strontium hydroxide.

[0023] Further, in step (4), the oxidant is one or a mixture of two or more of ammonium persulfate, potassium persulfate, hydrogen peroxide, and oxidase; the concentration of the oxidant in the system is 0.1 - 2 g / L, and the reaction time is 1 - 5 h.

[0024] Advantages of the present invention:

[0025] (1) The raw materials of the present invention are natural and safe. Among them, dihydroxyacetone and various amino acids, o-dihydroxy phenol-containing compounds are all produced in large quantities through production processes such as biological fermentation and plant extraction. The raw material cost is very low, and it is safe and non-toxic.

[0026] (2) Utilize the Maillard reaction between dihydroxyacetone and components such as amino groups and carboxyl groups to produce browning and blackening to generate polymers with ultraviolet absorption functions. This reaction is mild and is commonly used in various food productions, and the reaction process is safe.

[0027] (3) Coordinate with the oxidation of the o-dihydroxy phenol structure. The o-dihydroxy phenol structure is oxidized to a quinone group, which is easily combined with amino acids in the system to form Schiff bases or Michael adducts. The conjugated electron transfer can further promote the oxidation of Maillard products into melanin. It can effectively combine components A, B, and C through chemical promotion reactions to obtain natural, safe, and low-cost artificially synthesized melanin, and the reaction time is short, and the synthesis process is safe and simple. Description of the Drawings

[0028] Figure 1 UV-visible absorption spectra (0.02 mg / mL) for Examples 4 and 7 and Comparative Examples 6 and 7.

[0029] Figure 2 Visible light (420 nm) absorbance analysis diagrams for browning analysis of Examples 3 and 7 and Comparative Examples 1-6. Specific implementation methods

[0030] Except as otherwise specifically specified, the reagents used in the embodiments of the present invention can be obtained by purchasing through commercial channels.

[0031] It should be understood that the specific implementation manners described herein are only for explaining and illustrating the present invention, and are not used to limit the present invention. Except for the raw materials used in the embodiments of the present invention, raw material components with the same functional groups as those in the raw materials used in the embodiments or containing the same structural units involved in the present invention and using equivalent replacement should be included in the protection scope of the present invention. The present invention will be further described below with specific embodiments.

[0032] The present invention provides the attached drawings of the detection results of some embodiments. Other embodiments and comparative examples adopt the same detection method. Those skilled in the art can directly and unambiguously determine the content of the embodiments of the present invention through the detection method provided by the present invention.

[0033] Example 1

[0034] Add 10 g of dihydroxyacetone, 1 g of lysine, 1 g of 3-hydroxy-L-tyrosine, and 0.1 g of caffeic acid to 1 L of deionized water, stir to disperse and dissolve; raise the temperature to 50 °C, and under the condition of mechanical stirring at 300 rpm, add sodium hydroxide solution to adjust the pH of the system to 8. After reacting for 1 h, adjust the pH to 7, filter under pressure and dry, and then dry in an oven to obtain synthetic melanin 1.

[0035] Example 2

[0036] Add 10 g of dihydroxyacetone, 1 g of lysine, 1 g of 3-hydroxy-L-tyrosine, and 0.1 g of caffeic acid to 1 L of deionized water, stir to disperse and dissolve; raise the temperature to 90 °C, and under the condition of mechanical stirring at 300 rpm, add sodium hydroxide solution to adjust the pH of the system to 8. After reacting for 1 h, adjust the pH to 7, filter under pressure and dry, and then dry in an oven to obtain synthetic melanin 2.

[0037] Example 3

[0038] Add 50 g of dihydroxyacetone, 10 g of lysine, 10 g of 3-hydroxy-L-tyrosine, and 1 g of caffeic acid to 1 L of deionized water, stir to disperse and dissolve; raise the temperature to 50 °C, under the condition of mechanical stirring at 300 rpm, add sodium hydroxide solution to adjust the pH of the system to 8, after reacting for 1 h, adjust the pH to 7, filter press and dry, and then dry in an oven to obtain synthetic melanin 3.

[0039] Example 4

[0040] Add 50 g of dihydroxyacetone, 10 g of lysine, 10 g of 3-hydroxy-L-tyrosine, and 1 g of caffeic acid to 1 L of deionized water, stir to disperse and dissolve; raise the temperature to 50 °C, under the condition of mechanical stirring at 300 rpm, add sodium hydroxide solution to adjust the pH of the system to 8, after reacting for 5 h, adjust the pH to 7, filter press and dry, and then dry in an oven to obtain synthetic melanin 4.

[0041] Example 5

[0042] Add 100 g of dihydroxyacetone, 25 g of lysine, 25 g of 3-hydroxy-L-tyrosine, and 20 g of caffeic acid to 1 L of deionized water, stir to disperse and dissolve; raise the temperature to 50 °C, under the condition of mechanical stirring at 300 rpm, add sodium hydroxide solution to adjust the pH of the system to 8, after reacting for 1 h, adjust the pH to 7, filter press and dry, and then dry in an oven to obtain synthetic melanin 5.

[0043] Example 6

[0044] Add 100 g of dihydroxyacetone, 25 g of lysine, 25 g of 3-hydroxy-L-tyrosine, and 20 g of caffeic acid to 1 L of deionized water, stir to disperse and dissolve; raise the temperature to 50 °C, under the condition of mechanical stirring at 300 rpm, add sodium hydroxide solution to adjust the pH of the system to 13, after reacting for 1 h, adjust the pH to 7, filter press and dry, and then dry in an oven to obtain synthetic melanin 6.

[0045] Example 7

[0046] Add 50 g of dihydroxyacetone, 10 g of phenylalanine, 15 g of 3-hydroxy-L-tyrosine, 1 g of dopamine hydrochloride, and 0.5 g of uncaria rhynchophylla extract to 1 L of an aqueous alcohol solution (500 ml of deionized water, 300 ml of propylene glycol, 200 ml of butylene glycol), stir to disperse and dissolve; raise the temperature to 40 °C, under the condition of mechanical stirring at 500 rpm, add ammonia water to adjust the pH of the system to 9, add 2 g of oxidase laccase, after reacting for 3 h, adjust the pH to 7, filter press and dry, and then dry in an oven to obtain synthetic melanin 7.

[0047] Example 8

[0048] Add 50 g of dihydroxyacetone, 10 g of phenylalanine, 15 g of 3-hydroxy-L-tyrosine, 1 g of dopamine hydrochloride, and 0.5 g of Uncaria rhynchophylla extract to 1 L of an aqueous alcohol solution (500 ml of deionized water, 300 ml of propylene glycol, and 200 ml of butanediol), and stir to disperse and dissolve; raise the temperature to 70 °C, and under the condition of mechanical stirring at 500 rpm, add ammonia water to adjust the pH of the system to 9, add 0.1 g of ammonium persulfate, after reacting for 3 h, adjust the pH to 7, filter press and dry, and then dry in an oven to obtain synthetic melanin 8.

[0049] Example 9

[0050] Add 100 g of dihydroxyacetone, 10 g of glycine, 10 g of tyrosine, 5 g of proline, 5 g of 3-hydroxy-L-tyrosine, 1 g of octanoyl epicatechin gallate, 1 g of epigallocatechin galloyl glucoside, 0.5 g of catecholamine, and 0.5 g of chlorogenic acid to 1 L of an aqueous alcohol solution (900 ml of deionized water and 100 ml of propylene glycol), and stir to disperse and dissolve; raise the temperature to 60 °C, and under the condition of mechanical stirring at 500 rpm, add ammonia water to adjust the pH of the system to 10, after reacting for 5 h, adjust the pH to 7, centrifuge and dry, and then dry in an oven to obtain synthetic melanin 9.

[0051] Example 10

[0052] Add 100 g of dihydroxyacetone, 10 g of glycine, 10 g of tyrosine, 5 g of proline, 5 g of 3-hydroxy-L-tyrosine, 1 g of octanoyl epicatechin gallate, 1 g of epigallocatechin galloyl glucoside, 0.5 g of catecholamine, and 0.5 g of chlorogenic acid to 1 L of an aqueous alcohol solution (900 ml of deionized water and 100 ml of propylene glycol), and stir to disperse and dissolve; raise the temperature to 80 °C, and under the condition of mechanical stirring at 800 rpm, add ammonia water to adjust the pH of the system to 10, after reacting for 5 h, adjust the pH to 7, centrifuge and dry, and then dry in an oven to obtain synthetic melanin 10.

[0053] Example 11

[0054] Add 100 g of dihydroxyacetone, 10 g of glycine, 10 g of tyrosine, 5 g of proline, 5 g of 3-hydroxy-L-tyrosine, 1 g of octanoyl epicatechin gallate, 1 g of epigallocatechin galloyl glucoside, 0.5 g of catecholamine, and 0.5 g of chlorogenic acid to 1 L of an aqueous alcohol solution (900 ml of deionized water and 100 ml of propylene glycol), and stir to disperse and dissolve; raise the temperature to 90 °C, and under the condition of mechanical stirring at 1000 rpm, add ammonia water to adjust the pH of the system to 10, after reacting for 5 h, adjust the pH to 7, centrifuge and dry, and then dry in an oven to obtain synthetic melanin 11.

[0055] Comparative Example 1 (AB)

[0056] Dissolve 50 g of dihydroxyacetone, 10 g of lysine, and 10 g of 3-hydroxy-L-tyrosine in 1 L of deionized water by stirring and dispersing; raise the temperature to 50 °C, and under the condition of mechanical stirring at 300 rpm, add sodium hydroxide solution to adjust the pH of the system to 8. After reacting for 1 h, adjust the pH to 7, filter press and dry, and then dry in an oven to obtain Comparative Sample 1.

[0057] Comparative Example 2 (C)

[0058] Dissolve 1 g of caffeic acid in 1 L of deionized water by stirring and dispersing; raise the temperature to 50 °C, and under the condition of mechanical stirring at 300 rpm, add sodium hydroxide solution to adjust the pH of the system to 8. After reacting for 1 h, adjust the pH to 7, filter press and dry, and then dry in an oven to obtain Comparative Sample 2.

[0059] Comparative Example 3 (AC)

[0060] Dissolve 50 g of dihydroxyacetone, 1 g of dopamine hydrochloride, and 0.5 g of Uncaria rhynchophylla extract in 1 L of an aqueous alcohol solution (500 ml of deionized water, 300 ml of propylene glycol, and 200 ml of butanediol) by stirring and dispersing; raise the temperature to 40 °C, and under the condition of mechanical stirring at 500 rpm, add ammonia water to adjust the pH of the system to 9, add 2 g of oxidase laccase, after reacting for 3 h, adjust the pH to 7, filter press and dry, and then dry in an oven to obtain Comparative Sample 3.

[0061] Comparative Example 4 (BC)

[0062] Dissolve 10 g of phenylalanine, 15 g of 3-hydroxy-L-tyrosine, 1 g of dopamine hydrochloride, and 0.5 g of Uncaria rhynchophylla extract in 1 L of an aqueous alcohol solution (500 ml of deionized water, 300 ml of propylene glycol, and 200 ml of butanediol) by stirring and dispersing; raise the temperature to 40 °C, and under the condition of mechanical stirring at 500 rpm, add ammonia water to adjust the pH of the system to 9, add 2 g of oxidase laccase, after reacting for 3 h, adjust the pH to 7, filter press and dry, and then dry in an oven to obtain Comparative Sample 4.

[0063] Comparative Example 5 (AB)

[0064] Dissolve 50 g of dihydroxyacetone, 10 g of phenylalanine, and 15 g of 3-hydroxy-L-tyrosine in 1 L of an aqueous alcohol solution (500 ml of deionized water, 300 ml of propylene glycol, and 200 ml of butanediol) by stirring and dispersing; raise the temperature to 40 °C, and under the condition of mechanical stirring at 500 rpm, add ammonia water to adjust the pH of the system to 9, add 2 g of oxidase laccase, after reacting for 3 h, adjust the pH to 7, filter press and dry, and then dry in an oven to obtain Comparative Sample 5.

[0065] Comparative Example 6 (C)

[0066] Add 1 g of dopamine hydrochloride and 0.5 g of Uncaria rhynchophylla extract to 1 L of an aqueous alcohol solution (500 ml of deionized water, 300 ml of propylene glycol, and 200 ml of butanediol), and stir to disperse and dissolve. Raise the temperature to 40 °C, and under the condition of mechanical stirring at 500 rpm, add ammonia water to adjust the pH of the system to 9. Add 2 g of oxidase laccase, and after reacting for 3 h, adjust the pH to 7. Filter under pressure and dry, and then bake to obtain Comparative Sample 6.

[0067] Comparative Example 7 (Convert the weight of all components to C)

[0068] Add 56.5 g of dopamine hydrochloride to 1 L of deionized water, and stir to disperse and dissolve. Raise the temperature to 40 °C, and under the condition of mechanical stirring at 500 rpm, add ammonia water to adjust the pH of the system to 9. Add 2 g of oxidase laccase, and after reacting for 3 h, adjust the pH to 7. Filter under pressure and dry, and then bake to obtain Comparative Sample 7.

[0069] The relevant data of Examples 1-11 and Comparative Examples 1-7 are listed in Table 1. Among them, the calculation of the ultraviolet (200-400 nm) absorption area percentage is the ratio of the area formed by the ultraviolet absorption curve of the same concentration of melanin and the X-axis to the total area in the 200-400 nm interval (the area is obtained by integrating with origin software).

[0070] Browning measurement: The Maillard reaction is usually accompanied by non-enzymatic browning, which can be confirmed by measuring the browning of the product. The final product of the Maillard reaction can be monitored by measuring the absorbance at 420 nm. Prepare a 0.2 mg / mL sample, and use a UV-visible spectrophotometer to measure the absorbance of the sample at 420 nm (reference method: Food Chemistry, 2019, 295: 120-128), and the data results are listed in Table 2.

[0071] Table 1

[0072]

[0073]

[0074] Table 2

[0075] sample browning value Example 3 1.21 Comparative Example 1 0.87 Comparative Example 2 0.05 Example 7 1.91 Comparative Example 3 0.71 Comparative Example 4 0.85 Comparative Example 5 1.04 Comparative Example 6 0.67

[0076] By comparing Example 1 and Example 2, it can be seen that the increase in reaction temperature is beneficial to the formation of melanin; by comparing Example 3 and Example 4, it can be seen that the extension of reaction time is beneficial to the formation of melanin; by comparing Example 1 with Example 4 and 5, it can be seen that as the concentration of the reaction solution increases, the formation of melanin is more complete; by comparing Example 5 and 6, it is found that an alkaline environment is more conducive to the formation of melanin in the present invention; considering the experimental effects and production costs of Examples 1-11, the synthesis process of the present invention is preferably carried out at a reaction temperature of 40-70 °C and a mechanical stirring rate of 250-500 rpm.

[0077] By comparing Example 3 with Comparative Example 1 and Comparative Example 2, it can be seen that when only components A and B are added without component C or only component C is present without components A and B, the ultraviolet absorption of the formed melanin is poor; moreover, the ultraviolet absorption of the melanin 3 formed in Example 3 is higher than the sum of the ultraviolet absorptions of the samples formed in Comparative Example 1 and Comparative Example 2. This shows that the presence of component C combines with the amino acid of component B in the system to form a Schiff base or a Michael adduct, and the conjugate electron transfer can further promote the oxidation of the Maillard product formed between components A and B into melanin. By comparing Comparative Examples 3-6 with Example 7, it can be known that the ultraviolet absorption ability of the melanin formed by separately not adding component B, not adding component A alone, using only component C, or using component C with the same concentration is weaker than that of the melanin formed in the present invention, and the ultraviolet absorption effect of Example 7 is higher than the sum of the ultraviolet absorption area percentages of the products of Comparative Example 5 and Comparative Example 6. This shows that the reaction composition composed of components A, B, and C of the present invention has a synergistic effect on each other, and the formed melanin has a more excellent ultraviolet protection performance.

[0078] Those of ordinary skill in the art should understand that the above are only specific embodiments of the present invention and are not used to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing melanin, characterized in that, It includes the following steps: (1) Weigh components A, B, and C according to the ratio and add them to the reaction vessel; (2) Add a solvent and uniformly stir and disperse the substances added in step (1); (3) Raise the temperature to the reaction temperature and stir mechanically; (4) Add an alkali to the system obtained in step (3), adjust the pH or add an oxidant, and carry out the reaction; (5) After the reaction ends, adjust the pH to 7 with an acidic solution, and then carry out pressure filtration drying, freeze drying, or centrifugal heat drying; (1) Component A is dihydroxyacetone; (2) Component B is one or a mixture of two or more of 3-hydroxy-L-tyrosine, lysine, glycine, histidine, methionine, arginine, cystine, cysteine, tyrosine, acetyltyrosine, proline, amidinoprolin, hydroxyproline, phenylalanine; (3) Component C is one or a mixture of two or more of dopamine hydrochloride, caffeic acid, chlorogenic acid, ellagic acid, gallic acid, protocatechuic acid, honeysuckle extract, dihydroxyindole, octanoyl epicatechin gallate, epigallocatechin galloyl glucoside, epigallocatechin gallate, catechu gum, catecholamine, catechu leaves / wood powder, Uncaria extract; The mass ratio of components A, B, and C is 100:(20 - 50):(1 - 20).

2. The method for preparing melanin according to claim 1, characterized in that, In step (2), the solvent is deionized water or an alcohol-water mixture; among them, the alcohol-water mixture is a mixture of deionized water and alcohol substances such as ethanol, propylene glycol, and butylene glycol, and the mass ratio of deionized water to alcohol substances is (50 - 90):(10 - 50).

3. The method for preparing melanin according to claim 1, characterized in that, In step (2), the concentration of the solution is 10 - 180 g / L.

4. The method for preparing melanin according to claim 1, characterized in that, In step (3), the reaction temperature is 40 - 90 °C, and the mechanical stirring speed is 200 - 1000 rpm.

5. The method for preparing melanin according to claim 1, characterized in that, In step (4), the alkali is one or a mixture of two or more of alkali metal hydroxides, alkaline earth metal hydroxides, alkali metal carbonates, alkaline earth metal carbonates, alkali metal bicarbonates, alkaline earth metal bicarbonates, alkali metal acetates, alkali metal phosphates, alkali metal alkoxides, and ammonia water, and the pH is adjusted to 8 - 13.

6. The preparation method of melanin according to any one of claims 1-5, characterized in that, In step (4), the alkali is one or a mixture of two or more of ammonium hydroxide, potassium hydroxide, sodium hydroxide, lithium hydroxide, and strontium hydroxide.

7. The method for preparing melanin according to claim 1, wherein In step (4), the oxidant is one or a mixture of two or more of ammonium persulfate, potassium persulfate, hydrogen peroxide, and oxidase; the concentration of the oxidant in the system is 0.1 - 2 g / L.

8. The method for preparing melanin according to claim 1, characterized in that, In step (4), the reaction time is 1 - 5 h.

9. The method for preparing melanin according to claim 4, characterized in that, In step (3), the reaction temperature is 40 - 70 °C, and the mechanical stirring rate is 250 - 500 rpm.

10. A melanin prepared by the preparation method according to any one of claims 1 - 9.

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