Cyclodextrin absorbing material and its use

Abstract: A cyclodextrin adsorbing material composed of a water-insoluble resin substrate to which a ligand having such a size as to be included by cyclodextrin is chemical­ ly bonded via a spacer radical; a method of separating and purifying cyclodextrin from an aqueous solution containing it, which comprises bringing said aqueous solution into contact with the above adsorbing material to adsorb cyclodextrin on the adsorbing material, and subjecting the adsorbing material to a desorption treat­ ment to recover cyclodextrin; and a method of increasing the amount of cyclodextrin yielded, which comprises carrying out a cyclodextrin-forming reaction by an enzy­ matic method in the presence of the above adsorbent material.

Description: This invention relates to an adsorbent material capable of adsorbing α-cyclodextrin thereon. More specifically, this invention relates to an adsorbent material capable of adsorbing α-cyclodextrin thereon with a high efficiency and a high selectivity from a solution containing cyclodextrin, for example, a reaction product mixture containing cyclodextrin as a main product, the unreacted material and various by-products, and to its use. Cyclodextrin is usually produced by treating starch with cyclodextrin glycosyl transferase (CGTase). The resulting reaction mixture contains not only cyclodextrin as the final product but also the unreacted starch, various decomposition products such as oligodextrin and glucose, and the enzyme used. One method proposed for separating cyclodextrin from the reaction product mixture of cyclodextrin containing the unreacted material and by-products comprises adding an organic solvent capable of being included by cyclodextrin to the reaction mixture, and precipitating and separating cyclodextrin as a solid. The use of such an organic solvent, however, is undesirable for medicines and foods which are the main uses of cyclodextrin, and it has been desired to develop a separating method which does not involve the use of organic solvents. In an attempt to separate or purify cyclodextrin without using an organic solvent, there have previously been proposed (1) a method which comprises adsorbing cyclodextrin on a porous styrene/divinylbenzene copolymer resin (Japanese Laid-Open Patent Publication No. 805/1981), (2) a method which comprises treating a mixture of cyclodextrin and reduced sugar with an anion exchange resin to adsorb the reduced sugar alone on it, and separating cyclodextrin (Japanese Laid-Open Patent Publication No. 136,889/1976), (3) and a method which comprises treating a reaction product mixture containing cyclodextrin on a column packed with an alkali or alkaline earth metal salt of a strong acid-type cation exchange resin to fractionate it into a cyclodextrin fraction and a glucose fraction (Japanese Laid-Open Patent Publication No. 146,600/1982). However, the porous resin and ion exchange resins used in these prior methods have low selectivity, and in separating and recovering cyclodextrin adsorbed thereon, a complex elution treatment is required. Such methods are not entirely satisfactory for industrial operations. The present inventors have made extensive investigations on an adsorbent material which has high selectivity and can permit separation of cyclodextrin by a simple eluting operation. These investigations have led to the present invention. According to this invention, there is provided an α-cyclodextrin adsorbing material composed of a water-insoluble resin substrate to which a ligand having such a size as to be included by α-cyclodextrin is chemically bonded via a spacer radical whereby the resin substrate is a polysaccharide, and the ligand is a myristic acid residue. The cyclodextrin adsorbent material will be described below in detail. Ligand The characteristic feature of this invention is that a resin substrate to which a ligand having such a size as to be included by cyclodextrin is chemically bonded via a spacer radical is used as an adsorbent material for cyclodextrin in place of conventional porous resins or ion exchange resins. The ligand used in the adsorbent material of this invention is a myristic acid residue which enters a ring-like cavity in α-cyclodextrin and is adsorbed onto its inner surface by hydrophobic bonding; namely it has such a size as to be included by α-cyclodextrin. The size of the ligand included by α-cyclodextrin is restricted by the diameter of the ring-like cavity of α-cyclodextrin. If the shape of the ligand is taken as an ellipse just surrounding the entire atomic grouping of the ligand, the suitable size of the ligand is such that the short diameter of the ellipse is within the range of about 0.4 to 1.0 nm (about 4 to about 10 Å) preferably about 0.5 to about 0.75 nm (about 5 to about 7.5 Å). Water-insoluble resin substrate The term "resin substrate", as used in the present specification and the appended claims, denotes a skeletal structure portion constituting the main portion of a resin, and when the resin has functional group-containing side chains for introduction of ligands, it denotes a portion of the resin resulting from removal of the side chains. In the following, not only the resin substrate alone, but also resins resulting from introduction of functional-group containing side chains for introduction of ligands will be illustrated. Polysaccharide compounds and derivative thereof The polysaccharide compounds may, for example, include cellulose, dexran, agarose, chitin and chitosan. Derivatives of such polysaccharides include compounds obtained by introducing side chains containing functional groups such as an epoxy group, an activated carboxyl group, a haloacetyl group or a tertiary or quaternary ammonium salt group into the polysaccharide compounds by utilizing crosslinked products of the polysaccharide compounds or the hydroxyl groups present in the polysaccharide compounds [for the method of introducing such groups, see, for example, P. Cautrecasas, J. Biol. Chem., 245 , 3059 (1970); P. Cautrecasas, I. Parikh, Biochemistry, 11 : and L. Sundberg, J. Porath, J. Chromatgr., 90 , 87 (1974)]. Such compounds are available, for example, from Pharmacia Fine Chemicals under the tradenames AH-Sepharose 4B®, CH-Sepharose 4B®, Epoxy-Activated Sepharose 6B®, Diethylaminoethyl-Sephadex®, Quaternary Aminoethyl-Sephadex® and Sulfopropyl-Sephadex®; and from Fuji Boseki Co., Ltd. under the tradenames Chitopearl® and Chitobeads®. Among the resins exemplified above, the polysaccharide compound chitosan and its crosslinked product are especially preferred. Production of the α-cyclodextrin adsorbent material in accordance with this invention The adsorbent material of this invention can be produced by introducing the aforesaid ligand into the water-insoluble resin substrate via a suitable spacer radical by methods to be described below. (1) Method utilizing a covalent bond reaction In the scheme:- P ○ is the water-insoluble resin substrate, L is the ligand, X and Y are functional groups capable of being reacted with each other to form a covalent bonded, Z₁ reperesents a spacer radical, Q₁ constitutes part of the spacer radical Z₁ and represents part of a functional group-containing side chain possessed by, or introduced into, the water-insoluble resin resulting from removal of the functional group X therefrom, Q₂ constitutes another part of the spacer radical Z₁ and represents a divalent residue linking the ligand L and the functional group Y, and p and q are each 0 or 1. Table 1 below shows examples of the functional groups X and Y and the type of reaction in covalent bonding. In Table 1, Examples of the reactive derivatives of the carboxyl group (-COOH) include carboxylic acid halids (e.g., -COCl, -COBr), a carboxylic acid anhydride or mixed anhydride, and active carboxylate esters (e.g., (the same examples are given with regard to Table 2 below). Examples of the starting materials of formulae (I) and (II) used in reaction scheme A and the adsorbent resin of formula (III) are given below. wherein R represents a hydrogen atom or an acetyl group, and n=100,000 to 500,000, In the formula, represents a water-insoluble resin substrate in which the hydroxyl groups in the basic skeleton of cellulose are partially omitted. and L are the same as and L in Nos. 1 to 18 and 33 to 41 in Table 2. The radical corresponds to the spacer radical. In the formula, represents a water-insoluble resin substrate in which the hydroxyl groups in the basic skeleton of crosslinked dextrin are partially omitted. and L are the same as and L in Nos. 1 to 18 and Nos. 33 to 41 in Table 1, and the radical corresponds to the spacer radical. (2) Method utilizing an ion bonding reaction In the formula:- P ○ and L are as defined above, A and B are functional groups which can be reacted with each other to form an ionic bond, Z₂ represents the spacer radical, Q₃ constitutes part of the spacer radical Z₂ and represents a part resulting from removal of the functional group A from a functional group-containing side chain possessed by, or introduced into, the water-insoluble resin, Q₄ forms another part of the spacer group Z₂ and represents a divalent residue linking the ligand L and the functional group B, and a and b are each 0 or 1. Specific examples of the reaction scheme B are shown below. In the schemes (a) and (b), specific examples of are given below. In the production of the adsorbent material of this invention utilizing the covalent bonding or ionic bonding reaction in (1) and (2) above, the reaction of the water-insoluble resin component of formula (I) or (IV) with the ligand introducing component of formula (II) or (V) can be carried out by methods known per se , for example by the methods described in K. Mosbach, ed., "Methods in Enzymology", vol. 44, Academic Press, New York (1976), and R. Axen, S. Ernback, Eur. J. Biochem., 18 , 351 (1971). These references are cited herein instead of giving a detailed description of these methods. The spacer radical interposed between the ligand and the resin substrate in the adsorbent resin of this invention serves to prevent the resin substrate from becoming a steric hindrance when cyclodextrin approaches the adsorbent resin and includes the ligand. For this purpose, the spacer radical desirably has a length sufficient to space the ligand from the resin substrate (support) by a distance above a certain value. Its length varies depending upon the type of the substrate resin and/or the ligand and cannot be definitely specified. Generally, the spacer radical may be selected such that the total length of the spacer radical and the ligand is at least 1 nm (10 angstroms), preferably 1 to 6 nm (10 to 60 angstroms), more preferably 1.5 to 3.0 nm (15 to 30 angstroms). The total length of the spacer radical and the ligand is calculated by a CPK (Corey-Pauling Koltun) molecule model (a product of Ealing Company, U. S. A.). The adsorbent resin provided by this invention may be molded into any desired shape suitable the intended uses to be described below. Preferably, it is, for example, in the form of beads, fibers, hollow fibers, cylinders, prisms, films, etc. Especially preferably, it is in the form of beads. The following Example illustrate the production of the adsorbent material of this invention more specifically. Example QAE A-25® resin (12.5 ml) which had been converted to the OH form by treatment with a 1% (W/V) aqueous solution of sodium hydroxide was packed into a column having a diameter of 15 mm, and washed with water fully and then with a 50% (V/V) aqueous solution of ethanol. 5.0 ml (34.5 millimoles) of myristic acid was dissolved in 125 ml of a 32% (V/V) aqueous solution of ethanol containing 1.0 ml of a 40% (V/V) aqueous solution of methylamine, and the solution was passed through the column of QAE A-25® (OH form) resin at a flow velocity (S.V.) of about 65 ml/hr. The column was then further washed with about 300 ml of a 50% (V/V) aqueous solution of ethanol, and then with water, and dried to give an adsorbent resin in which myristic acid was ionically bonded to the anion exchange part of QAE A-25® resin. Cyclodextrin adsorbing property of the adsorbent resin of this invention Tables 4 and 5 below show the cyclodextrin (to be abbreviated as "CD") adsorbing properties of the adsorbent resins of this invention. The adsorption test was carried out as follows:- 35 ml of each of the CD adsorbent resins indicated in these tables was packed into a column (1.5 cm x 25 cm) and fully washed with water, and then 45 ml of a CD-containing solution [α-CD, 1 (W/V) %; β-CD, 1 (W/V) %; γ-CD, 1 (W/V) % aqueous solutions] was passed through the column. The column was then washed with water by passing it in an amount of 350 ml and then 180 ml of a 1:1 mixture of ethanol and water was passed through the column. The CD content of each of he solutions that passed thorugh the column was measured by the HPLC method [Shodex®, RS-pak DC-613® column; CH₃CN/H₂O (65/35), 15 ml/min.]. The amount of CD measured was calculated for the amount (g) of CD adsorbed on each 1 liter of the resin, and the results are shown in Tables 5 to 9. Uses of the adsorbent resin of this invention Since the adsorbent resin of this invention has selective physical adsorbability with respect to its ligand and the hydrophobic cavity of cyclodextrin, it has the following specific uses. (1) Selective separation and purification of α-cyclodextrin from an aqueous solution containing cyclodextrin:- The adsorbent resin of this invention can be used to separate and purify α-CD selectively from a reaction product mixture containing α-CD. As stated hereinabove, CD is produced by causing various kinds of CGTase to act on an aqueous solution or suspension of a starch and separating and purifying α-CD from the resulting reaction mixture containing α-, β- and/or γ-CD, the unreacted starch and by-products such as oligodextrins and glucose. Since the adsorbent resin of this invention can selectively adsorb α-CD, α-CD can be separated and purified in good yields by absorbing α-CD on it by using it as a carrier in column chromatography or by contacting an aqueous solution containing α-CD such as the aforesaid reaction mixture with the adsorbent resin batchwise thereby to separate α-CD from the unreacted starch and the by-products, and then desorbing the adsorbed α-CD from the adsorbent resin using hot water or a mixed solvent of an alkanol and water. Contacting of the adsorbent material with an aqueous solution containing cyclodextrin can generally be carried out at ordinary temperature. If desired, it may be carried out at an elevated temperature of, for example, about 30 to 70°C. The contact time is not strictly limited, and usually a period of 1 to 4 hours suffices. The temperature of hot water used in desorbing cyclodextrin from the resin on which it is adsorbed may be within the range of about 70 to 90°C. The alkanol in the alkanol/water mixed solvent is a lower alkanol such as methanol, ethanol or propanol. The suitable mixing ratio of the alkanol to water is generally from 1:5 to 3:5 (V/V). Acetone, acetonitrile, tetrahydrofuran and dioxane may also be used as the desorption solvent. A reaction liquor in a chemical reaction catalyzed by CD, an enzymatic reaction liquor using CD as a protecting agent for the product and a microbial fermentation liquor may also be cited as examples of the aqueous solution containing cyclodextrion. (2) Increased production of a specific CD in the reaction of forming CDs The CD-forming reaction is an enzymatic reaction with CGTase and proceeds reversibly. It is known that by carrying out the reaction in the presence of an organic solvent capable of being included in CD, the resulting CD can be removed from the reaction system as a precipitate and the ratio of conversion of the starting starch to CD can be increased (see, for example, Japanese Laid-Open Patent Publication No. 156398/1985). Since the adsorbent resin of this invention has an organic residue as a ligand which can be included in CD, the aforesaid ratio of conversion can be increased by carrying out the reaction in the presence of the adsorbent resin in the reaction system, and at the same time, the ratio of formation of any desired one of α, β, and γ-CDs can be increased by selecting the ligand in the adsorbent resin. Since the resulting CD is selectively adsorbed on the adsorbent resin, CD of high purity can be easily obtained from the adsorbent resin by the CD desorption method described hereinabove. The CD-forming reaction using the adsorbent resin of this invention can be carried out by using starting materials known per se under reaction conditions known per se except that the reaction is carried out in the presence of the adsorbent resin [see, for example, British Patent No. 1,459,654, U. S. Patent No. 4,317,881, and Agricultural and Biological Chemistry, 49 , 1189-1191 (1985)]. For example, soluble starch is usually employed as a starting material, and by subjecting it to an enzymatic reaction with CGTase in an aqueous medium, CD is formed. CGTase may be any of those derived from Bacillus macerans , Bacillus circulans , Bacillus megaterium , Bacillus ohbensis , Klebsiella pneumoniae , Alkalophilic bacillus , Micrococcus variens , Micrococcus luteus , etc. The enzymatic reaction can be terminated in about 12 to 48 hours at a temperature of 30 to 65°C According to this invention, by performing the CD-forming reaction by this enzymatic method in the presence of the adsorbent resin, the amount of CD yielded can be increased by about 50 to 150% of that obtained in the prior art. This reaction can be carried out, for example, by charging the adsorbent resins in the form of beads, etc. into a reactor for CD formation, and performing the enzymatic reaction batchwise or continuously while suspending the reaction system with stirring; or by passing an aqueous solution containing the starting material and CGTase once or several times through a reaction column packed with the adsorbent resin molded into a suitable shape such as beads. One advantage of the CD-forming reaction in accordance with this invention is that it can be carried out by using the soluble starch in a higher concentration than in the prior art. In the general conventional CD-forming reaction, the concentration of the soluble starch is, for example, 5 to 7% (W/W). According to the method of this invention, the concentration of the starting starch can be increased to about 10 to about 20% (wt/wt) without substantially decreasing the efficiency of CD formation. The concentration of the starch used in this invention in the aqueous solution is not particularly limited. It may be selected within the range of 3 to 20% (wt/wt), preferably within the range of 5 to 15%. The optimum temperature for the reaction differs depending upon the type of the CGTase used, and is not critical. In the case of using CGTase obtained from Bacillus ohbensis , the reaction temperature may be 45 to 65°C, preferably 50 to 60°C, and the pH of the reaction system can be set at 5 to 11, preferably 6.5 to 7.5. The reaction time differs depending upon the mode of the reaction, and the type and amount of the CGTase used, and cannot be limited strictly. The reaction may be continued until the starch used as a substrate material is consumed almost completely. The amount of the adsorbent resin of this invention used in this reaction, irrespective of the mode of the reaction, may be above the amount which can sufficiently adsorb CD thereon under the assumption that 100% of starch used as the substrate material is completely converted to CD. The amount of CD adsorbed to a particular adsorbent resin in accordance with this invention can be empirically determined by a routine experiment conducted on the resin. According to the method of this invention described above, the resulting CD can be directly recovered from the reaction system in a form adsorbed on the adsorbent resin. Hence, the method of this invention does not require a complex separating and purifing operation for CD as in the prior art, and is very advantageous in industrial practice.

Claims: An α-cyclodextrin adsorbing material composed of a water-insoluble resin substrate to which a ligand having such a size as to be included by α-cyclodextrin is chemically bonded via a spacer radical whereby the resin substrate is a polysaccharide, and the ligand is a myristic acid residue.

Inventors: FUJII KOUKI, ISHIKURA TOMOYUKI, SATO MICHIKATSU, TSUCHIYAMA YUKIO, YAGI YOSHIAKI, YAMAMOTO KENICHI, FUJII, KOUKI, ISHIKURA, TOMOYUKI, SATO, MICHIKATSU, TSUCHIYAMA, YUKIO, YAGI, YOSHIAKI, YAMAMOTO, KENICHI
Applicants: SANRAKU INC, SANRAKU INCORPORATED
Application year: 1986
Main classification: C08B37
IPC codes: C08B37, B01D15, B01J20, G01N30