Method for purifying branched chain amino acids

Abstract: The present invention provides a method for purifying an amino acid selected from the group consisting of valine, leucine and isoleucine easily in high yield and with high purity by use of an inexpensive precipitant. The method comprises allowing p-ethylbenzene sulfonic acid or its water-soluble salt to act on an aqueous solution containing an amino acid selected from the group consisting of valine, leucine and isoleucine to form crystals of the amino acid p-ethylbenzene sulfonate and then separating and decomposing said salt to purify and obtain said amino acid. The p-ethylbenzene sulfonic acid may be free acid or an alkali metal salt, ammonium salt etc.

Description: The present invention relates to a method for purifying branched chain amino acids through the use of a novel crystalline salt. In particular, the invention relates to novel valine ethylbenzene sulfonate crystals suitable for purification of valine and a method for purifying valine by use of said salt, to novel leucine ethylbenzene sulfonate crystals suitable for purification of leucine and a method for purifying leucine by use of said salt, and to novel isoleucine ethylbenzene sulfonate crystals suitable for purification of isoleucine and a method for purifying isoleucine by use of said salt. Valine is useful as a starting material for pharmaceutical amino acid preparations, as a synthetic intermediate for various pharmaceutical preparations and as an intermediate for chemicals such as agricultural chemicals. Leucine is very useful as a starting material for pharmaceutical amino acid preparations, nutrients etc. and as a synthetic intermediate for various pharmaceutical preparations. Isoleucine is very useful as a starting material for pharmaceutical amino acid preparations, nutrients etc. and as a synthetic intermediate for various pharmaceutical preparations. In general, valine, leucine and isoleucine are generically called branched chain amino acids. Valine is produced by a method of hydrolyzing proteins such as soybean protein etc. or by a method of culturing a microorganism having the ability to produce valine. The conventional methods of isolating and purifying valine from aqueous, valine-containing solutions such as protein hydrolysates, fermentation broth etc. obtained in these methods include e.g.: (1) a method of removing neutral amino acids other than valine by repeatedly re-crystallizing a neutral amino acid fraction collected by separating and removing from acidic and basic amino acids by treatment with ion exchange resin (Biochem. J., 48 , 313 (1951)); and (2) a method of adding hydrochloric acid to an aqueous solution containing valine and then repeatedly forming and precipitating crystals of valine hydrochloride (Japanese Patent Application Laid-Open Publication No. 16450/81). However, there were the problems that the former was very cumbersome in operation and difficult to separate valine from leucine and isoleucine, while the latter results in lower yield because of the high solubility of crystals of valine hydrochloride in water. Other purification methods include methods for purifying valine in which a precipitant forming an adduct (slightly soluble salt) selectively with valine, e.g. tetrachloro-orthophthalic acid, sulfoisophthalic acid, flavianic acid (Japanese Patent Publication No. 25059/67), or p-isopropylbenzene sulfonic acid (Japanese Patent Application Laid-Open Publication No. 333,312/96), is allowed to act on valine to form an adduct with valine in order to purify valine. However, there are the problems that the precipitants used for valine tetrafluorophthalate, sulfoisophthalate, and flavianate are expensive and hard to obtain industrially, and the solubility of the resulting adduct is so high that recovery of valine in high yield is difficult, the method of isolating valine from the adduct is cumbersome. On the other hand, p-isopropylbenzene sulfonic acid is very effective as a precipitant for valine, but there are the problems that p-isopropylbenzene sulfonic acid itself is not capable of decomposition with active sludge, so disposal of its waste fluid is difficult, and because of its hard formation of a sparingly soluble salt with isoleucine, it cannot be applied to other branched chain amino acid and its use is limited. Leucine is produced by a method of hydrolyzing proteins such as soybean protein etc. or by a method of culturing a microorganism having the ability to produce leucine. The conventional methods of isolating and purifying leucine from aqueous, leucine-containing solutions such as protein hydrolysates, fermentation broth etc. obtained in these methods include e.g.: (1) a method of removing neutral amino acids other than leucine by repeatedly re-crystallizing a neutral amino acid fraction collected by separating and removing from acidic and basic amino acids by treatment with ion exchange resin (Biochem. J., 48 , 313 (1951)); and (2) a method of adding hydrochloric acid to an aqueous solution containing leucine and then repeatedly forming and precipitating crystals of leucine hydrochloride (Experimental Chemistry Lecture, Vol. 23, Biochemistry I, 75, compiled by the Chemical Society of Japan and published by Maruzen (1957)). However, there are the problems that the former is very cumbersome in operation and difficult to separate leucine from valine and isoleucine, while the latter results in lower yield because of the high solubility of crystals of leucine hydrochloride in water. Other purification methods include methods for purifying leucine in which a precipitant forming an adduct (slightly soluble salt) selectively with leucine, e.g. naphthalene β-sulfonic acid, 2-bromotoluene-5-sulfonic acid (Experimental Chemistry Lecture, Vol. 23, Biochemistry I, 75, compiled by the Chemical Society of Japan and published by Maruzen (1957)), 1,2-dimethylbenzene-4-sulfonic acid (Japanese Patent Application Laid-Open Publication No. 11373/65), benzenesulfonic acid (Japanese Patent Application Laid-Open Publication No. 149,222/76), or p-toluenesulfonic acid (Japanese Patent Application Laid-Open Publication No. 3016/77), is allowed to act on leucine to form an adduct with leucine in order to purify leucine. However, there are the problems that the precipitants used for leucine naphthalene β-sulfonate and 2-bromotoluene-5-sulfonate are expensive and hard to obtain industrially, and the solubility of the resulting adduct is so high that recovery of leucine in high yield is difficult, and the method of isolating leucine from the adduct is cumbersome. On the other hand, 1,2-dimethylbenzene-4-sulfonic acid, benzene sulfonic acid and p-toluenesulfonic acid are very effective as precipitants for leucine, but 1,2-dimethylbene-4-sulfonic acid hardly forms a slightly soluble salt with valine and isoleucine; benzene sulfonic acid hardly forms a slightly soluble salt with isoleucine; and p-toluenesulfonic acid hardly forms a sparingly soluble salt with valine, so their limited use is disadvantageous. Isoleucine is produced by a method of hydrolyzing proteins such as soybean protein etc. or by a method of culturing a microorganism having the ability to produce isoleucine. The conventional methods of isolating and purifying isoleucine from aqueous, isoleucine-containing solutions such as protein hydrolysates, fermentation broth etc. obtained in these methods include e.g.: (1) a method of removing neutral amino acids other than isoleucine by repeatedly re-crystallizing a neutral amino acid fraction collected by separating and removing from acidic and basic amino acids by treatment with ion exchange resin (Biochem. J., 48 , 313 (1951)); and (2) a method of adding hydrochloric acid to an aqueous solution containing isoleucine and then repeatedly forming and precipitating crystals of isoleucine hydrochloride (J. Biologc. Chem., 118 , 78 (1973)). However, there are the problems that the former is very cumbersome in operation and difficult to separate isoleucine from valine and leucine, while the latter results in lower yield because of the high solubility of crystals of isoleucine hydrochloride in water. Other purification methods include methods for purifying isoleucine in which a precipitant forming an adduct (sparingly soluble salt) selectively with isoleucine, e.g. 4-nitro-4'-methyldiphenylamine-2-sulfonic acid (J. Biologc. Chem., 143 , 121 (1942)), 2-naphthol-6-sulfonic acid (Japanese Patent Application Laid-Open Publication No. 13515/73), or 1,5-napthalenedisulfonic acid (Japanese Patent Application Laid-Open Publication No. 109,953/79), is allowed to act on isoleucine to form an adduct in order to purify isoleucine. However, there are the problems that the used precipitants are expensive and hard to obtain industrially, the method of isolating isoleucine from the adducts is cumbersome, their difficult formation of salts with amino acids other than isoleucine makes their usage limited, and the toxicity of the precipitants themselves is high. In view of all these difficulties, it is desirable that embodiments of the present invention provide a method for purifying valine, leucine and isoleucine easily in high yield and with high purity using an inexpensive precipitant. It is further desirable that in one embodiment the invention should provide a precipitant being applicable in common to any of valine, leucine and isoleucine which due to similar chemical properties, are hard to separate and purify from each other, the precipitant being capable of assimilation with active sludge etc. and being used readily in industry. As a result of their eager research under these circumstances, the present inventors have found that crystals of amino acid p-ethylbenzene sulfonate are selectively precipitated by adding p-ethylbenzene sulfonic acid to an aqueous solution containing a branched chain amino acid such as valine, leucine, br isoleucine, then reacting and cooling them. Thereby the present inventors arrived at the present invention. That is, the present invention comprises: (1) A method for purifying a branched chain amino acid which comprises allowing p-ethylbenzene sulfonic acid or its water-soluble salt to act on an aqueous solution containing the amino acid to form and precipitate amino acid p-ethylbenzene sulfonate and then separating and decomposing said salt to obtain the amino acid. For example, p-ethylbenzene sulfonic acid or its salt may be added to an aqueous solution containing the amino acid. The branched chain amino acid is preferably selected from valine, leucine and isoleucine. (2) Novel valine p-ethylbenzene sulfonate crystal comprising 1 mole of valine and 1 mole of p-ethylbenzene sulfonic acid. (3) Novel leucine p-ethylbenzene sulfonate crystal comprising 1 mole of leucine and 1 mole of p-ethylbenzene sulfonic acid, and (4) Novel isoleucine p-ethylbenzene sulfonate crystal comprising 1 mole of isoleucine and 1 mole of p-ethylbenzene sulfonic acid. Valine, leucine or isoleucine to which the method of the present invention can be applied may be in the form of their optically active isomer (L or D body), racemate, or a mixture thereof. The aqueous solution containing valine, leucine or isoleucine may be any of a wide variety of aqueous solutions such as a mixture of amino acids obtained by separating and removing basic amino acids from a hydrolysate where proteins such as soybean protein etc. have been hydrolyzed, a fermentation broth obtained by culturing a microorganism having the ability to produce and accumulate valine, leucine or isoleucine, or a liquid obtained by removing the microorganism from the fermentation liquid, or a liquid obtained by treating it with ion exchange resin or adsorption resin, or an aqueous solution of crude D, L-valine, D, L-leucine or D, L-isoleucine obtained e.g. via a hydantoin derivative etc. by a chemical synthesis method. p-Ethylbenzene sulfonic acid used in the present invention can be easily produced by placing ethyl benzene and 1.5 molar excess of conc. sulfuric acid in a glass vessel and heating the mixture at 130 °C for 1 to 2 hours, so it can be obtained inexpensively in industry. p-Ethylbenzene sulfonic acid can be used as free acid or in the form of its water-soluble salts, for example alkali metal salts such as sodium salt, potassium salt etc., alkaline earth metal salts such as calcium salt etc., or ammonium salts etc. The amount of p-ethylbenzene sulfonic acid or its water-soluble salt used is preferably equimolar or more, preferably 1.0 to 1.1-fold molar excess relative to valine, leucine or isoleucine contained in the aqueous solution, and its use in particularly large excess is not required. The desired compound, valine p-ethylbenzene sulfonate, can be easily formed and precipitated for instance by adding p-ethylbenzene sulfonic acid or its water-soluble salt to an aqueous solution containing 60 g/L or more valine and adjusting it to about pH 1.5. The pH of the solution suitable for forming and precipitating the crystals of valine p-ethylbenzene sulfonate is generally in the range of 0.1 to 2.3, preferably 1.0 to 1.7. The acid used for pH adjustment is preferably an inorganic acid such as hydrochloric acid, sulfuric acid etc. If necessary, seed crystals of valine p-ethylbenzene sulfonate can be added to a mixture solution of valine and p-ethylbenzene sulfonic acid to efficiently precipitate crystals. If a dilute solution is used as the aqueous solution of valine, it may be concentrated to precipitate crystals of said salt. In this case, p-ethylbenzene sulfonic acid may be added at any stage before or after concentration. Although crystals of free valine are precipitated if the aqueous solution of valine is concentrated at neutrality, crystals of valine p-ethylbenzene sulfonate can be easily formed by adding a suitable amount of p-ethylbenzene sulfonic acid and adjusting it to about pH 1.5. Alternatively, a dilute solution of valine in the coexistence of a suitable amount of p-ethylbenzene sulfonic acid may be concentrated to precipitate crystals of valine p-ethylbenzene sulfonate after adjusting it to about pH 2. To separate and obtain the precipitated crystals of valine p-ethylbenzene sulfonate, conventional methods for solid-liquid separation, for example, filtration, centrifugation etc. may be used. Although the separated crystals are of high purity, they can further be purified by conventional purification methods such as rinsing re-crystallizing etc. The resulting valine p-ethylbenzene sulfonate crystal consisting of 1 mole of valine and 1 mole of p-ethylbenzene sulfonic acid is a novel substance with the following physical properties: White plate crystal: soluble in water and ethanol. Solubility in water: 14.5 wt % (pH 1.4, 5 °C) Crystal structure: rhombic system Crystal density: 1.31 g/cm3 Elemental analysis: C, 51.6%; H, 6.9%; N, 4.5%; S, 10.5% (calc.: C, 51.5 %; H, 6.9 %; N, 4.6 %; S, 10.6 %) To isolate free valine from the crystals of valine p-ethylbenzene sulfonate, the crystals are dissolved in a large amount of hot water, and then the solution may be contacted with weakly basic ion exchange resin (OH form), or an alkali such as sodium hydroxide may be added to the solution. If ion exchange resin is used, p-ethylbenzene sulfonic acid is adsorbed onto it to give free valine as an eluting solution. It is subjected to conventional methods e.g. crystallization by concentration to give valine crystals. The precipitant (p-ethylbenzene sulfonic acid) adsorbed onto the resin is eluted as alkali salt upon regeneration of the resin with an alkali solution such as sodium hydroxide solution etc. In the method of adding an alkali, an alkali such as sodium hydroxide etc. is added as a solid or as an aqueous solution to an aqueous suspension of crystals of valine p-ethylbenzene sulfonate, then the pH is adjusted in the range of 5 to 8 or thereabout, preferably 6 to 7, whereby p-ethylbenzene sulfonic acid is dissolved in the solution as an alkali salt while free valine is precipitated, and the precipitated valine is separated from the solution. The precipitant (p-ethylbenzene sulfonic acid) itself separated and recovered as the alkali salt can be used again as the precipitant in the next operation. The desired compound, leucine p-ethylbenzene sulfonate, can be easily formed and precipitated for instance by adding p-ethylbenzene sulfonic acid or its water-soluble salt to an aqueous solution containing 30 g/L or more leucine and adjusting it to about pH 1.5. The pH of the solution suitable for forming and precipitating the crystals of leucine p-ethylbenzene sulfonate is generally in the range of 0.1 to 2.3, preferably 1.0 to 1.7. The acid used for pH adjustment is preferably an inorganic acid such as hydrochloric acid, sulfuric acid etc. If necessary, seed crystals of leucine p-ethylbenzene sulfonate can be added to a mixture solution of p-ethylbenzene sulfonic acid to efficiently precipitate crystals. If a dilute solution is used as the aqueous solution of leucine, it may be concentrated to precipitate crystals of said salt. In this case, p-ethylbenzene sulfonic acid may be added at any stage before or after concentration. Although crystals of free leucine are precipitated if the aqueous solution of leucine is concentrated at neutrality, crystals of leucine p-ethylbenzene sulfonate can be easily formed by adding a suitable amount of p-ethylbenzene sulfonic acid and adjusting it to about pH 1.5. Alternatively, a dilute solution of leucine in the coexistence of a suitable amount of p-ethylbenzene sulfonic acid may be concentrated to precipitate crystals of leucine p-ethylbenzene sulfonate after adjusting it to about pH 2. To separate and obtain the precipitated crystals of leucine p-ethylbenzene sulfonate, conventional methods for solid-liquid separation, for example, filtration, centrifugation etc. may be used. Although the separated crystals are of high purity, they can further be purified by conventionally used purification methods such as rinsing re-crystallizing etc. The resulting leucine p-ethylbenzene sulfonate crystal consisting of 1 mole of leucine and 1 mole of p-ethylbenzene sulfonic acid is a novel substance with the following physical properties: White plate crystal: soluble in water and ethanol. Solubility in water: 7.1 wt % (pH 1.6, 5°C ) Crystal structure: monoclinic system Crystal density: 1.32 g/cm3 Elemental analysis: C, 53.1%; H, 7.3%; N, 4.3%; S, 9.9%. (calc.: C, 53.0%; H, 7.3%; N, 4.4%; S, 10.1 %) To isolate free leucine from the crystals of leucine p-ethylbenzene sulfonate, the method of isolating free valine from crystals of valine p-ethylbenzene sulfonate as described above may be followed. The precipitant (p-ethylbenzene sulfonic acid) itself separated and recovered as an alkali salt can be used again as the precipitant in the next operation. The desired compound, isoleucine p-ethylbenzene sulfonate, can be easily formed and precipitated for instance by adding p-ethylbenzene sulfonic acid or its water-soluble salt to an aqueous solution containing 50 g/L or more isoleucine and adjusting it to about pH 1.5. The pH of the solution suitable for forming and precipitating the crystals of isoleucine p-ethylbenzene sulfonate is generally in the range of 0.1 to 2.3, preferably 1.0 to 1.7. The acid used for pH adjustment is preferably an inorganic acid such as hydrochloric acid, sulfuric acid etc. If necessary, seed crystals of isoleucine p-ethylbenzene sulfonate can be added to a mixture solution of isoleucine and p-ethylbenzene sulfonic acid to efficiently precipitate crystals. If a dilute solution is used as the aqueous solution of isoleucine, it may be concentrated to precipitate crystals of said salt. In this case, p-ethylbenzene sulfonic acid may be added at any stage before or after concentration. Although crystals of free isoleucine are precipitated if the aqueous solution of isoleucine is concentrated at neutrality, crystals of isoleucine p-ethylbenzene sulfonate can be easily formed by adding a suitable amount of p-ethylbenzene sulfonic acid and adjusting it to about pH 1.5. Alternatively, a dilute solution of isoleucine in the coexistence of a suitable amount of p-ethylbenzene sulfonic acid may be concentrated to precipitate crystals of isoleucine p-ethylbenzene sulfonate after adjusting it to about pH 2. To separate and obtain the precipitated crystals of isoleucine p-ethylbenzene sulfonate, conventional methods for solid-liquid separation, for example, filtration, centrifugation etc. may be used. Although the separated crystals are of high purity, they can further be purified by conventionally used purification methods such as rinsing, re-crystallizing etc. The resulting isoleucine p-ethylbenzene sulfonate crystal consisting of 1 mole of isoleucine and 1 mole of p-ethylbenzene sulfonic acid is a novel substance with the following physical properties: White plate crystal: soluble in water and ethanol. Solubility in water: 11.6 wt % (pH 1.5, 5°C) Crystal structure: monoclinic system Crystal density: 1.27 g/cm 3 Elemental analysis: C, 53.1%; H, 7.3%; N, 4.3%; S, 9.8% (calc.: C, 53.0%; H, 7.3%; N, 4.4%; S, 10.1%) To isolate free isoleucine from the crystals of isoleucine p-ethylbenzene sulfonate, the method of isolating free valine from crystals of valine p-ethylbenzene sulfonate as described above may be followed. The precipitant (p-ethylbenzene sulfonic acid) itself separated and recovered as an alkali salt can be used again as the precipitant in the next operation. Examples Hereinafter, embodiments of the present invention are described in more detail by reference to Examples. The quantification of leucine, isoleucine, valine and other amino acids was carried out with Hitachi L-8500 type amino acid analyzer. In the illustrative figures: FIG. 1 is a 1 H-NMR spectrum (D 2 O) of sodium p-ethylbenzene sulfonate obtained in Reference Example 1. FIG. 2 is an X-ray diffraction pattern of the crystal powder of L-valine p-ethylbenzene sulfonate obtained in Example 1. FIG. 3 is an X-ray diffraction pattern of the crystal powder of L-leucine p-ethylbenzene sulfonate obtained in Example 5. FIG. 4 is an X-ray diffraction pattern of the crystal powder of L-isoleucine p-ethylbenzene sulfonate obtained in Example 9. Reference Example 1 (Production and Assimilability of p-Ethylbenzene Sulfonic Acid) 33 ml (0.6 mol) of conc. sulfuric acid was added to 62 ml (0.5 mol) of ethyl benzene, and the mixture was stirred for 1.5 hours under heating at 120 to 130°C. Layer separation will occur if unreacted ethyl benzene still remains, so the reaction was terminated when no layer separation was confirmed, and a solution containing p-ethylbenzene sulfonic acid as the major component was thus obtained. This solution was poured into 150 ml water and neutralized partially with sodium hydrogen carbonate, and sodium chloride was added to precipitate crystals of sodium p-ethylbenzene sulfonate. The resulting crystals were separated by filtration and dried in vacuo. This sodium p-ethylbenzene sulfonate is easily soluble in water and sparingly soluble in ethanol. A 1 H-NMR spectrum of the resulting crystals is shown in FIG 1. With respect to the ability of p-ethylbenzene sulfonic acid to be assimilated by active sludge, it has been confirmed that about 80 % of 100 mg/L p-ethylbenzene sulfonic acid is assimilated for 2 weeks at pH 7.0 at a temperature of 25°C ("Yukagaku" (Oil Chemistry), 28(5) , 354 (1979)). Example 1 400 ml water was added to 100 g of L-valine and 159 g of p-ethylbenzene sulfonic acid, and the temperature of the solution was made 40°C to dissolve the solid. Then, the solution was cooled at 5°C to precipitate L-valine p-ethylbenzene sulfonate crystals, and the precipitated crystals were separated by filtration and then dried in vacuo. The resulting L-valine p-ethylbenzene sulfonate was fine white crystal in the rhombic system with 1.31 g/cm 3 of a crystal density. An X-ray diffraction pattern of the crystal powder is shown in FIG. 2. The X-ray diffraction was determined using Cu-Ka ray as a radiation source. The result of elemental analysis was C, 51.6 %; H, 7.0 %; N, 4.5 %; S, 10.5 %. Comparative Example 1 400 ml water was added to 100 g of L-valine and 147 g of p-toluene sulfonic acid, and the temperature of the solution was made 40°C to dissolve the solid. Then, the solution was cooled at 5°C, but no crystal was obtained. Example 2 35 g of L-valine and 3.5 g each of L-leucine and L-isoleucine were dispersed in 125 ml water, and 55.6 g of p-ethylbenzene sulfonic acid (equimolar to valine) was added thereto, dissolved by heating, and adjusted to pH 1.1. Then, the solution was cooled to precipitate L-valine p-ethylbenzene sulfonate crystals. The precipitated crystals were recovered by centrifugation, then dissolved in a large amount of hot water, and passed through weakly basic ion exchange resin (OH form) to remove the p-ethylbenzene sulfonic acid. The eluate was crystallized by concentration to give 21 g of free L-valine crystals. Analysis of the mother liquor indicated that the ratio of precipitation of valine was 63 %, the purity of free L-valine was 92 %, and the content of the other amino acids was 8 % or less, and 76 % of the impurities before crystallization were removed. Example 3 35 g of L-valine and 1.1 g each of L-leucine and L-isoleucine were dispersed in 100 ml water, and 55.6 g of p-ethylbenzene sulfonic acid (equimolar to valine) was added thereto, dissolved by heating, and adjusted to pH 1.1. Then, the solution was cooled to precipitate L-valine p-ethylbenzene sulfonate crystals. The precipitated crystals were recovered by centrifugation, then dissolved in a large amount of hot water, and passed through weakly basic ion exchange resin (OH form) to remove the p-ethylbenzene sulfonic acid. The eluate was crystallized by concentration to give 27 g of free L-valine crystals. Analysis of the mother liquor indicated that the ratio of precipitation of valine was 80 %, the purity of free L-valine was 97 %, and the content of the other amino acids was 3 % or less, and 67 % % of the impurities before crystallization were removed. Example 4 10 g of D-valine and 0.3 g each of D-leucine and D-isoleucine were added to 30 ml water, and 15.8 g of p-ethylbenzene sulfonic acid (equimolar to valine) was added thereto, adjusted to pH 1.1, and dissolved by heating. Then, the solution was cooled to precipitate D-valine p-ethylbenzene sulfonate crystals. The precipitated crystals were recovered by centrifugation, then dissolved in a large amount of hot water, and passed through weakly basic ion exchange resin (OH form) to remove the p-ethylbenzene sulfonic acid. The eluate was crystallized by concentration to give 7.5 g of free D-valine crystals. Analysis of the mother liquor indicated that the ratio of precipitation of valine was 80 %, the purity of free D-valine was 97 %, and the content of the other amino acids was 3 % or less, and 67 % of the impurities before crystallization were removed. Example 5 400 ml water was added to 100 g of L-leucine and 142 g of p-ethylbenzene sulfonic acid, and the temperature of the solution was made 40°C to dissolve the solid. Then, the solution was cooled at 5°C to precipitate L-leucine p-ethylbenzene sulfonate crystals, and the precipitated crystals were separated by filtration and then dried in vacuo. The resulting L-leucine p-ethylbenzene sulfonate was fine white crystal in the monoclinic system with 1.32 g/cm 3 of a crystal density. An X-ray diffraction pattern of the crystal powder is shown in FIG. 3. The X-ray diffraction was determined using Cu-Kα ray as a radiation source. The result of elemental analysis was C, 53.0 %; H, 7.4 %; N, 4.3 %; S, 9.8 %. Example 6 35 g of L-leucine and 3.5 g each of L-valine and L-isoleucine were dispersed in 300 ml water, and 49.7 g of p-ethylbenzene sulfonic acid (equimolar to leucine) was added thereto, dissolved by heating, and adjusted to pH 1.1. Then, the solution was cooled to precipitate L-leucine p-ethylbenzene sulfonate crystals. The precipitated crystals were recovered by centrifugation, then dissolved in a large amount of hot water, and passed through weakly basic ion exchange resin (OH form) to remove the p-ethylbenzene sulfonic acid. The eluate was crystallized by concentration to give 25 g of free L-leucine crystals. Analysis of the mother liquor indicated that the ratio of precipitation of leucine was 73 %, the purity of free L-leucine was 99 %, and the content of the other amino acids was 1 % or less, and 96 % of the impurities before crystallization were removed. Example 7 35 g of L-leucine and 1.1 g each of L-valine and L-isoleucine were dispersed in 220 ml water, and 49.7 g of p-ethylbenzene sulfonic acid (equimolar to leucine) was added thereto, dissolved by heating, and adjusted to pH 1.1. Then, the solution was cooled to precipitate L-leucine p-ethylbenzene sulfonate crystals. The precipitated crystals were recovered by centrifugation, then dissolved in a large amount of hot water, and passed through weakly basic ion exchange resin (OH form) to remove the p-ethylbenzene sulfonic acid. The eluate was crystallized by concentration to give 27 g of free L-leucine crystals. Analysis of the mother liquor indicated that the ratio of precipitation of leucine was 80 %, the purity of free L-leucine was 99 %, and the content of the other amino acids was 1 % or less, and 88 % of the impurities before crystallization were removed. Example 8 10 g of D-leucine and 0.3 g each of D-valine and D-isoleucine were added to 65 ml water, and 14.2 g of p-ethylbenzene sulfonic acid (equimolar to leucine) was added thereto, adjusted to pH 1.1 and dissolved by heating. Then, the solution was cooled to precipitate D-leucine p-ethylbenzene sulfonate crystals. The precipitated crystals were recovered by centrifugation, then dissolved in a large amount of hot water, and passed through weakly basic ion exchange resin (OH form) to remove the p-ethylbenzene sulfonic acid. The eluate was crystallized by concentration to give 7.5 g of free D-leucine crystals. Analysis of the mother liquor indicated that the ratio of precipitation of leucine was 80 %, the purity of free D-leucine was 99 %, and the content of the other amino acids was 1 % or less, and 88 % of the impurities before crystallization were removed. Example 9 400 ml water was added to 100 g of L-isoleucine and 142 g of p-ethylbenzene sulfonic acid, and the temperature of the solution was made 40°C to dissolve the solid. Then, the solution was cooled at 5°C to precipitate L-isoleucine p-ethylbenzene sulfonate crystals, and the precipitated crystals were separated by filtration and then dried in vacuo. The resulting L-isoleucine p-ethylbenzene sulfonate was fine white crystal in the monoclinic system with 1.27 g/cm 3 of crystal density. An X-ray diffraction pattern of the crystal powder is shown in FIG. 4. The X-ray diffraction was determined using Cu-Kα ray as a radiation source. The result of elemental analysis was C, 53.1 %; H, 7.3 %; N, 4.3 %; S, 9.9 %. Comparative Example 2 400 ml water was added to 100 g of L-isoleucine and 152 g of p-normalpropylbenzene sulfonic acid, and the temperature of the solution was made 40°C to dissolve the solid. Then, the solution was cooled at 5°C, but no crystal was obtained. Example 10 35 g of L-isoleucine and 3.5 g each of L-valine and L-leucine were dispersed in 125 ml water, and 49.7 g of p-ethylbenzene sulfonic acid (equimolar to isoleucine) was added thereto, dissolved by heating, and adjusted to pH 1.1. Then, the solution was cooled to precipitate L-isoleucine p-ethylbenzene sulfonate crystals. The precipitated crystals were recovered by centrifugation, then dissolved in a large amount of hot water, and passed through weakly basic ion exchange resin (OH form) to remove the p-ethylbenzene sulfonic acid. The eluate was crystallized by concentration to give 25 g of free L-isoleucine crystals. Analysis of the mother liquor indicated that the ratio of precipitation of isoleucine was 68 %, the purity of free L-isoleucine was 96 %, and the content of the other amino acids was 4 % or less, and 86 % of the impurities before crystallization were removed. Example 11 35 g of L-isoleucine and 1.1 g each of L-valine and L-leucine were dispersed in 100 ml water, and 49.7 g of p-ethylbenzene sulfonic acid (equimolar to isoleucine) was added thereto, dissolved by heating, and adjusted to pH 1.1. Then, the solution was cooled to precipitate L-isoleucine p-ethylbenzene sulfonate crystals. The precipitated crystals were recovered by centrifugation, then dissolved in a large amount of hot water, and passed through weakly basic ion exchange resin (OH form) to remove the p-ethylbenzene sulfonic acid. The eluate was crystallized by concentration to give 28 g of free L-isoleucine crystals. Analysis of the mother liquor indicated that the ratio of precipitation of isoleucine was 85 %, the purity of free L-isoleucine was 98 %, and the content of the other amino acids was 2 % or less, and 75 % of the impurities before crystallization were removed. Example 12 10 g of D-isoleucine and 0.3 g each of D-valine and D-leucine were added to 30 ml water, and 14.2 g of p-ethylbenzene sulfonic acid (equimolar to isoleucine) was added thereto, adjusted to pH 1.1, and dissolved by heating. Then, the solution was cooled to precipitate D-isoleucine p-ethylbenzene sulfonate crystals. The precipitated crystals were recovered by centrifugation, then dissolved in a large amount of hot water, and passed through weakly basic ion exchange resin (OH form) to remove the p-ethylbenzene sulfonic acid. The eluate was crystallized by concentration to give 8 g of free D-isoleucine crystals. Analysis of the mother liquor indicated that the ratio of precipitation of isoleucine was 84 %, the purity of free D-isoleucine was 98 %, and the content of the other amino acids was 2 % or less, and 73 % of the impurities before crystallization were removed. Summary As described above, the amino acid p-ethylbenzene sulfonate crystals obtained by the invention consist of a branched chain amino acid which may be selected from the group consisting of valine, leucine and isoleucine, and of p-ethylbenzene sulfonic acid. By using such crystals, amino acid may be incorporated inexpensively and easily and may be regenerated with high purity. Thus they are very useful. Furthermore, even though the operation of producing said amino acid p-ethylbenzene sulfonate crystals and purifying said amino acid may be applicable to any amino acid selected from the group consisting of valine, leucine and isoleucine the method may nevertheless be used to separate the amino acids from each other because of the specificity of the salt. In addition, p-ethylbenzene sulfonic acid can be produced easily in industry by sulfonation of inexpensive ethylbenzene, is easily obtainable, and is further capable of assimilation with microorganisms, so its waste fluid can be treated by active sludge. Hence, the present invention is applicable to industry inexpensively and easily. Further, the method may facilitate separation and recovery of desired amino acid from the salt and the recovery and re-use of p-ethylbenzene sulfonic acid.

Claims: Crystalline amino acid p-ethylbenzene sulfonate comprising 1 mole of a branched chain amino acid selected from the group consisting of valine, leucine and isoleucine for each 1 mole of p-ethylbenzene sulfonic acid. A method for purifying an amino acid, which comprises allowing p-ethylbenzene sulfonic acid or its water-soluble salt to act on an aqueous solution containing a branched chain amino acid selected from the group consisting of valine, leucine and isoleucine to form crystals of amino acid p-ethylbenzene sulfonate and then separating and decomposing said salt to obtain said amino acid. A method according to claim 2, wherein the water-soluble salt of p-ethylbenzene sulfonic acid is an alkali metal salt.

Inventors: HASEGAWA KAZUHIRO, IITANI KINZO, KANEKO TETSUYA, NAKAMURA TORU, OKAMOTO YUTAKA, SANO CHIAKI, HASEGAWA, KAZUHIRO, IITANI, KINZO, KANEKO, TETSUYA, NAKAMURA, TORU, OKAMOTO, YUTAKA, SANO, CHIAKI, Hasegawa, Kazuhiro, c/o Ajinomoto Co., Inc., Iitani, Kinzo, c/o Ajinomoto Co., Inc., Kaneko, Tetsuya, c/o Ajinomoto Co., Inc., Nakamura, Toru, c/o Ajinomoto Co., Inc., Okamoto, Yutaka, c/o Ajinomoto Co., Inc., Sano, Chiaki, c/o Ajinomoto Co., Inc.
Applicants: AJINOMOTO KK, AJINOMOTO CO., INC.
Application year: 1998
Main classification: C07C227
IPC codes: C07C229, C07C227