Chitosan is a partially or fully deacetylated form of chitin, a naturally occurring polysaccharide. Structurally, chitin is a polysaccharide consisting of beta-(1.fwdarw.4) 2-acetamido-2-deoxy-D-glucose units, some of which are deacetylated: ##STR1## wherein x=0.85-0.95 and y=0.15-0.05. The degree of deacetylation usually varies between 8 and 15 percent, but depends on the species from which the chitin is obtained, and the method used for isolation and purification.
Chitin is not one polymer with a fixed stoichiometry, but a class of polymers of N-acetylglucosamine with different crystal structures and degrees of deacetylation, and with fairly large variability from species to species. The polysaccharide obtained by more extensive deacetylation of chitin is chitosan: ##STR2## wherein x=0.50-0.10, and y=0.50-0.90.
Like chitin, chitosan is a generic term for a group of polymers of acetylglucosamine, but with a degree of deacetylation of between 50 and 90 percent. Chitosan is the beta-(1-4)- polysaccharide of D-glucosamine, and is structurally similar to cellulose, except that the C-2 hydroxyl group in cellulose is substituted with a primary amine group in chitosan. The large number of free amine groups (pKa=6.3) makes chitosan a polymeric weak base. Both chitin and chitosan are insoluble in water, dilute aqueous bases, and most organic solvents. However, unlike chitin, chitosan is soluble in dilute aqueous acids, usually carboxylic acids, as the chitosonium salt. Solubility in dilute aqueous acid is therefore a simple way to distinguish chitin from chitosan.
Chitosan is unique in that it is a polysaccharide containing primary amine groups. Chitosan and its derivatives are therefore useful materials in precious metal recovery, ion-exchange resins, surgical dressings and sutures, ocular bandages and lenses, and other applications in the biomedical field. Chitosan forms water-soluble salts with many organic and inorganic acids, and these chitosonium derivatives are useful in biomedical applications for several reasons.
First, as hereinafter indicated, these salts are biologically compatible with skin, hair, and most living tissues, and second, these chitosan salts are known to accelerate the healing process in damaged tissue. The tissue compatibility and healing acceleration of chitosan salts are also shared by covalent chitosan derivatives, covalent chitin derivatives, chitosan, and chitin.
However, prior to the present invention widespread use of chitosan derivatives had been limited because of the difficulty in their preparation. Chitosan is a highly crystalline polymer, and without rendering it amorphous and swollen with diluent, it cannot be easily derivatized. The methods known today to render chitosan reactive involve unusual, expensive, and toxic solvents such as dimethyl formamide or dimethyl sulfoxide, or expensive mechanical processes such as repetitive freezing of a chitosan slurry. In addition, while chitosan salt solutions may be prepared by dissolution of chitosan in dilute aqueous acids, such solutions are cumbersome because they must be very dilute, about five percent or less, to be pourable. Isolating the solid chitosan salt from these solutions requires expensive processes such as freeze-drying or spray drying, and these isolation techniques do not remove any unreacted acid which may not be desired in the final dried chitosan salt.
A variety of other methods have been reported in the literature for decrystallizing chitosan and most have not been entirely successful in decrystallizing the product without the need for expensive and time consuming procedures as outlined above.
For example, in 1957, U.S. Pat. No. 2,795,579 issued to Warner-Lambert Pharmaceutical Company and disclosed a process for the purification of chitosan which utilized salicylic acid. The method comprised forming an aqueous solution of an acid salt of crude chitosan, adding a soluble salicylate, separating the resulting precipitate of chitosan salicylate and regenerating the chitosan by decomposing the salicylate with a base. In the two examples in the patent, solutions of the crude chitosan were prepared by treatment with acid and dilution of the mixture with water. In example I of the patent the solution which contained less than 3 weight percent chitosan was indicated to be viscous.
Chitosan derivatives as sequestering agents for heavy metals are disclosed in U.S. Pat. No. 4,031,025 which issued June 21, 1977 and is assigned to L'Oreal of Paris, France. These derivatives are prepared by forming an aqueous solution of chitosan by reaction with hydrochloric acid and further reacting with a base and an anhydride. In Table I at column 6 of the patent the concentration in solution of chitosan in the eleven examples ranged from 2 to 5 weight percent.
U.S. Pat. No. 3,879,376 issued Apr. 22, 1975 and is also assigned to L'Oreal of Paris, France and disclosed and claimed certain chitosan derivatives. The derivatives disclosed were formed by the acylation of chitosan with a saturated or unsaturated organic diacid anhydride. It is stated in the patent that the chitosan acylation reaction is performed by adding a base diluted solution to an aqueous solution of a chitosan salt, alternately and in fractional amounts, to release the amine function and the acid anhydride. A divisional application issued as U.S. Pat. No. 3,953,608 on Apr. 27, 1976 and claims cosmetic compositions for the skin containing the chitosan derivatives.
United Kingdom patent application 2,107,340A, published Apr. 27, 1983, and assigned to Wella Aktiengesellschaft discloses certain surface active chitosan salts and their use in cosmetic applications. These salts are prepared by reacting chitosan and a surfactant in an aqueous or aqueous-alcohol solution. It is indicated in the patent that although solutions can be prepared in any desired quantity, for solubility reasons, a content of 0.05 to 10 weight percent is preferred. In example 1 deacetylated chitosan is dissolved in lauryl alcohol polyglycolethercarboxylic acid mixed with isopropanol and water to give a 4 weight percent solution which was indicated to be viscous. Other examples disclosed the preparation of solutions containing 0.5 to about 4 weight per cent chitosan salt.
In laid open Japanese patent publication Sho 59/1984 106409 of June 20, 1984 by Ichimaru Falcos K.K. Co. Ltd., cosmetic compositions were disclosed containing various kinds of chitin compounds including glycol chitin, carboxymethyl chitin and chitin sulfate. For example, it was indicated in this reference that the normally water-insoluble chitin is made soluble by glycolating chitin in the 6 position by a caustic soda treatment over a period of several days, followed by a freezing procedure, agitation with ethylene oxide and finally dialysis using a cellophane membrane.
In January, 1985, there was disclosed in Japanese patent 58/75561 by Ajinimoto K.K., wetting agents containing water soluble deacetylated non-crystalline chitin which contained 40-60 percent D-glucosamine units. Several compounds were disclosed for neutralization of the amino groups in the D-glucosamine units and included inorganic acids such as hydrochloric, sulfuric, phosphoric and organic acids such as acetic, citric, butyric, lactic, malic, succinic, gluconic, pyrrolidone carboxylic acid or acidic amino acids such as aspartic, glutamic, cysteine or homocysteine.
In U.S. Pat. No. 4,528,283 which issued July 9, 1985 to Gunter Lang et al. and is assigned to Wella Aktiengesellschaft of The Federal Republic of Germany, there are disclosed cosmetic compositions useful for the treatment of hair or skin and which contain certain glyceryl-chitosan macromolecular compounds. These novel compounds are prepared by reacting chitosan with glycidol at a temperature between 10.degree. C. and 100.degree. C. It is indicated in the patent that the chitosan is structurally modified before the reaction by precipitation and deep freezing. It is also stated in the patent that the few known water soluble derivatives of chitosan require expensive equipment and involved techniques in their production, and some are even known to be physiologically harmful.
A hair treating composition was patented by the Gillette Company as described in U.S. Pat. No. 4,542,014 and utilized in its formulations adducts of low molecular weight aminopolysaccharides derived from chitosan with hydrolyzed keratin protein. The patentee indicates in Example 1 that a 5 weight percent solution of low molecular weight chitosan became viscous when prepared.
Dry, free-flowing, water-soluble carboxylic acid complexes of chitosan are prepared as described in U.S. Pat. No. 4,574,150 to Austin, assigned to the University of Delaware, using a heterogeneous reaction system and select dispersant liquids and carboxylic acids. The patentee states that from zero to about 30 weight percent water based on the weight of the acid is used and demonstrates in Example 5 that large amounts of water are undesirable.
It will therefore be evident that while the prior art has succeeded in preparing water soluble derivatives and aqueous solutions of chitosan, it was only accomplished with great difficulty or the types of derivative were limited in number. For example, it is clearly demonstrated by the aforementioned art that attempts to prepare aqueous solutions of chitosan have been limited to those of very low concentration, since solubility and viscosity considerations do not permit the formation of solutions of solids content much above 5 weight percent chitosan. This, of course, is a decided disadvantage in the preparation of chitosan derivatives since, the low concentration of chitosan in solution will of necessity dictate a low concentration of the derivatized product. Moreover, notwithstanding the low solids content in aqueous solutions, the high viscosity renders such solutions largely unsuitable for chemical processing techniques utilizing an aqueous medium.
It has now been discovered that one way to obviate the problems of high solution viscosity and polymer recovery is to conduct the synthesis of aminopolysaccharide derivatives, particularly chitosan derivatives, by a heterogeneous process. Hence, it has now been found that chitosan can be conveniently decrystallized and derivatives prepared therefrom by a novel and inexpensive process hereinafter referred to as "acid decrystallization".
In the process of this invention chitosan is slurried in an appropriate diluent system containing water and an organic diluent, such as acetone. Addition of an acid, usually a carboxylic acid, provides a swollen, decrystallized slurry of the chitosan salt, which may be directly isolated by merely filtering and desiccating the solid. Derivatives prepared from the decrystallized chitosan can be either covalent or ionic (salts) compositions.
Accordingly, one or more of the following objects can be achieved by the practice of this invention. It is an object of this invention to provide a novel process for the acid decrystallization of chitosan. Another object of the invention is to provide a process for the decrystallization of chitosan which is simple and efficient. A further object of this invention is to provide a convenient and economically attractive process for the preparation of a variety of derivatives of chitosan. Another object is to provide such derivatives which have utility in the pharmaceutical, biomedical and personal care fields. These and other objects will readily become apparent to those skilled in the art in the light of the teachings herein set forth.