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  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B3/00—Electrolytic production of organic compounds
  • C25B3/20—Processes
  • C25B3/25—Reduction
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B3/00—Electrolytic production of organic compounds
  • C25B3/01—Products
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B3/00—Electrolytic production of organic compounds
  • C25B3/01—Products
  • C25B3/07—Oxygen containing compounds
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B3/00—Electrolytic production of organic compounds
  • C25B3/01—Products
  • C25B3/09—Nitrogen containing compounds

Definitions

• the present invention relates to improved methods for the direct electrochemical synthesis of cysteine and its sulfhydryl analogues as salt-free amino acids, i.e. bases without production of intermediate acid salts.
• Cysteine is a sulfhydryl containing amino acid of increasing importance, used in hair wave formulations, nutritional supplements, and as an intermediate in the syntheses of certain pharmaceuticals.
• L-cysteine is derived from naturally occuring l-cystine, which is produced by hydrolysis of hair, feathers and other animal products; however, d-cysteine and the racemic optically inactive dl-mixture may also be derived by various methods. Cysteine is known to be unstable in neutral or alkaline media, and is easily oxidized by air to cystine.
• Cysteine may be prepared by reduction of cystine, a disulfide, according to the equation: (-S-CH2CH(NH2)CO2H)2+2H+ + 2e -> 2 HSCH2CH(NH2)CO2H
• This reduction has been conducted chemically with reagents such as Na/liquid NH3, Zn, Al or Sn in aqueous HCl, or solutions of NaBH4 have been employed.
• these methods lead to impure cysteine contaminated with inorganic by-products which are often difficult or costly to separate, and even minute traces of such impurities may be unacceptable for some uses, like nutritional supplements.
• the cathode is a sheet of Cu or a carbon rod
• SnCl2 is added to the catholyte
• the cathode is of Ag or Ni
• metallic Sn is added to the catholyte.
• Cysteine as the HCl salt is obtained after prolonged electrolysis. Additional steps are necessary to obtain pure cysteine as the free-base of the amino acid.
• Rambacher's method in order to prepare cysteine free-base electrochemically, it was necessary to first prepare the acid salt.
• the free amino acid cysteine was then prepared by dissolving the cysteine HCl in ethanol, carefully adding aqueous NH4OH solution to pH 6.2, and filtering off and drying the free cysteine.
• the electrochemical step gave a 92% yield of cysteine HCl product
• the neutralization step gave only an 80% yield of free cysteine.
• Cysteine is an expensive product, currently about $50/kg, hence losses of cysteine through precipitation steps or otherwise are costly.
• Mizuguchi et al Bull. Tokyo Inst. Technol . No. 64, 1-6 (1965) conducted electrolyses of cystine in aqueous acid media (HCl or H2SO4) and in aqueous alkaline media (NaOH, Na2CO3 and NH4OH), using a porous porcelain diaphragm in a first electrolysis cell to separate anode and cathode compartments.
• aqueous acid solutions were further electrolyzed in a second electrolysis cell containing an ion-exchange resin diaphragm, deacidification to free cysteine was demonstrated to occur in high yield.
• Mizuguchi et al concluded at page 6 that alkaline electrolysis provides lower yields of pure cysteine or its salts than acidic electrolysis. Based on actual results, Mizuguchi et al had a calculated yield of cysteine of about 58% and a current efficiency of about 12%, with about 25% of the valuable product and/or valuable starting material lost, presumably through the separator into the anode compartment. A low current efficiency of about 12% under these conditions signifies that most of the cathodic current was used wastefully for H2 evolution.
• Japanese patent No. 58-23450 to Hasaka first laid open on June 7, 1962 also discloses a process for the electrochemical reduction of cystine to cysteine in aqueous alkaline solutions of ammonia, ammonium carbonate, ammonium chloride, pyridine HCl or piperidine HCl.
• Hasaka conducted his reaction with a cathode in the form of a low surface area bidimensional plate. Current density was only 10 to 30 mA/cm2.
• Hasaka's product yield using alkaline electrolyte was low, ie 75%.
• the present invention provides such improved methods for the electrochemical production of cysteine and its sulfhydryl analogues.
• It is a principal object of the present invention to provide a high yield, economic method for the electrochemical preparation of amino acid free-bases directly without preparing intermediate acid salts which comprises the steps of providing an electrochemical cell having an anode and a high surface area, noncontaminating cathode.
• a basic nitrogenous electrolyte solution comprising a disulfide compound is introduced into the cell as the catholyte.
• Product is generated by impressing a voltage across the anode and cathode sufficient to reduce the disulfide compound at the cathode.
• a high yield of the amino acid free-base is produced upon removal of the basic nitrogenous electrolyte.
• the concentration of the disulfide compound in the electrolyte and the high surface area of the cathode are sufficient to provide a current density of at least 50 mA/cm2 and a product yield of at least 90%, such product being virtually free of potentially toxic trace metals and other contaminants emanating from the cell electrodes.
• the amino acid free base materials are characterized as being sufficiently free of contaminants that it is suitable for use as a food grade material or additive, or as a intermediate for synthesis of food grade materials or additives, as well as pharmaceuticals.
• a carbonaceous material either amorphous or crystalline types, including amorphous carbons which are only partially graphitized, vitreous or glassy carbons, as well as fluorinated carbons, and especially high surface area three-dimensional carbonaceous cathodes having length, width and also depth.
• Methods contemplated herein also include step(s) for purifying the free-base materials with aqueous media, removing any insoluble residue from aqueous mixtures including unreacted disulfide compound, and recovering amino acid free base material by removing the aqueous solvent. This method also allows for recovery of any unreacted disulfide reactant.
• the present invention also includes the step of converting the amino acid free-base material to a salt of an inorganic acid, if so desired.
• disulfide analogues includes synthesis electrochemically of cysteine and related compounds containing a reducible disulfide linkage, at least one basic nitrogen group and a carboxylic acid function of the general formula, I: where R1 and R2 are H, lower aliphatic (C1 to C6), aryl, aralkyl, or in which R1 and R2 taken together form a nitrogen heterocyclic ring of 3 to 7 atoms in which the nitrogen is basic.
• R1 and R2 are H, lower aliphatic (C1 to C6), aryl, aralkyl, or in which R1 and R2 taken together form a nitrogen heterocyclic ring of 3 to 7 atoms in which the nitrogen is basic.
• disulfide compounds of structure I may be considered to be alpha, beta, gamma or even omega-amino acids.
• disulfide amino acid analogues of structure I include: Likewise examples of mercapto amino acids of structure (II) include cysteine, homocysteine, isocysteine, penicillamine, 2-­mercaptonicotinic acid and 2-amino-3-mercapto-benzoic acid. Other examples of mercapto amino acids will be apparent to persons of ordinary skill in this art from the amino acid analogues disclosed above.
• Basic nitrogenous catholytes for the electrochemical production of cysteine and its analogues (II) include aqueous ammonia, anhydrous liquid ammonia and aqueous amine solutions.
• the amines are lower aliphatic and preferably have boiling points at atmospheric pressure below that of water, but not higher than about 130°C at atmospheric pressure to facilitate separation from the desired products.
• An important feature in the selection of the amine nitrogenous-catholyte solution is that upon distillation or evaporation, the amine completely evolves from solution leaving the salt-free disulfide substrate and/or sulfhydryl product, without any or substantially, any racemization or undesirable reaction occuring.
• Nitrogenous catholytes may also contain certain volatile organic cosolvents to assist solution of some otherwise insoluble disulfide substrates.
• volatile cosolvents may include solvents, such as lower alcohols like methanol, ethanol and isopropanol, as well as acetonitrile, tetrahydrofuran, dioxane and other volatile solvents, or mixtures of nitrogeneous catholytes such as NH3 and (CH3)3N in water and/or alcohol.
• Suitable amines are of general formula, R3N where the R groups are H, methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec -­butyl or t -butyl or mixtures of alkyl groups.
• Other amines are also useful like pyrrolidine, isoamylamine, n-amylamine, piperidine, ethylenediamine, and morpholine.
• aqueous or anhydrous ammonia solutions are preferred because of their high solubilizing ability for substrates and products, low boiling point, good ionic conductivity in combination with dissolved substrates and/or products, ease of separation and low cost.
• the nitrogenous solution component may be present in the electrolyte in concentrations which partially or totally neutralizes the disulfide, or may be present in slight or even large excess.
• the preferable concentrations of the nitrogenous component will be such that its solution with the disulfide reactant results in satisfactory electrolyte conductivity and sufficient solubility of the disulfide which leads to high yields and current efficiencies, at high current density levels, of the mercaptan product.
• ammonia is preferably present in greater than about 5% by weight, more preferably above 10% by weight and optimally above 20% by weight to enable solution of higher concentrations of substrate(I).
• Even higher effective concentrations of ammonia than the 30% commercially available solution may be prepared by slurrying a saturated mixture of disulfide substrate and 30% aqueous ammonia solution while bubbling in NH3 gas until solution of substrate occurs to the desired concentration.
• These increased disulfide substrate concentrations permit electrolysis at higher current density, often with lower cell voltage and higher yield and current efficiency of product than heretofore attainable. Distillation or evaporation costs are thereby reduced, for removal of less solvent.
• the starting concentration of the disulfide substrate(I) in the nitrogenous catholyte should be greater than about 0. 001M and preferably greater than about 0. 1M , but most preferably in the range of about 0.2 to 1.0 M or more.
• conductive salts like carbonates and bicarbonates of the nitrogenous component may be added to raise the effective nitrogeneous component concentration, and while these salts are decomposed in the workup steps, these added salts are usually unnecessary and often undesirable since they add additional complexity to the process and cost to the economics.
• the anolyte solution may be a suitably conducting solution which preferably generates protons at the anode on electrolysis.
• Such anolytes may be various ammonium salts dissolved in aqueous media such as (NH4)2SO4, (NH4)3PO4, (NH4)2CO3, and ammonium salts of organic acids like acetate, formate, oxalate, etc.
• Suitable anolytes may be aqueous H2SO4 or aqueous H3PO4. While halogen containing anolytes such as aqueous NH4Cl and aqueous HCl may be used, these are not preferred, since provision must then be made for generation of Cl2 and possible undesirable and dangerous chlorinated nitrogen byproducts such as nitrogen trichloride.
• Anodes may be carbonaceous, such as carbon, graphite, vitreous carbon, or specifically fluorinated carbon, graphite or vitreous carbon.
• Specifically fluorinated carbons are soft fluorinated carbons manufactured and sold by The Electrosynthesis Company, Inc. P. O. Box 430, East Amherst, N. Y. 14051 and are readily available under the trademark "SFC" carbon. SFC materials tend to increase the corrosion stability of these carbons and impart useful catalytic properties.
• Anodes may also be metallic like Pt on Ti, Pt/Ir on Ti, PbO2 on Pb, PbO2 on Ti, or uncatalyzed or catalyzed ceramic, such as Ebonex R anodes (Ti4O7).
• Ebonex anodes When uncatalyzed by Pt or other noble metals, Ebonex anodes have been found to possess a high overpotential for oxidation of the sulfhydryl products to the corresponding disulfides, compared to oxidation of the nitrogenous electrolyte solution. Although some reoxidation occurs of the product to the disulfide substrate at the anode, use of Ebonex anodes allows removal of the ion-exchange membrane from the electrolyzer design, thereby saving considerable capital and operating costs.
• cathode material Careful selection of the cathode material is of crucial importance to the high yield reduction of cystine and its disulfide analogues.
• Conventional metal cathodes comprised particularly of Pb, Hg and their alloys can introduce trace amounts to appreciable quantities of potentially toxic metals into the final product, rendering the product unsuitable for some applications.
• the expression -- noncontaminating cathode -- is intended to mean a cathode material which does not introduce potentially toxic substances into the product, but provides product which is food, drug, and cosmetic grade material, wherein the levels of heavy metals and other adulterants present are within the limits set forth by the United States Food, Drug and Cosmetic Act.
• no toxic heavy metals such as lead are acceptable, whereas for some external uses trace amounts of heavy metals may be permissible, to the extent that their presence does not violate regulatory laws pertaining to adulterants.
• carbonaceous materials are prefered since the amount of adulterant metals in the final product is usually minimal, or almost non-existent.
• the most preferred carbonaceous cathode materials are the porous and multidimensional types and include amorphous carbon and graphitic carbons, vitreous carbon, fluorinated carbons, and particularly soft fluorinated materials.
• amorphous carbon and graphitic carbons include amorphous carbon and graphitic carbons, vitreous carbon, fluorinated carbons, and particularly soft fluorinated materials.
• Carbon felts for example, provide near quantitative yield, conversion and current efficiency on electrolysis of cystine in ammonia solution, with passage of the theoretical current.
• expressions like “carbon felts” "carbon cloth” include both high surface area amorphous carbons, graphitic carbons and amorphous carbons which are partially graphitized. Representative examples of such materials are those available from The Electrosynthesis Company, Inc., East Amherst, N.Y. under the designation GF-S5 and GF-S6 which are 1/8" and 1/4" thick materials, respectively.
• Thin, high surface area porous carbonaceous materials represented by carbon fabrics include fabrics having plain and jersey knit construction. Carbon cloth is also intended to include carbon fiber fabrics.
• carbon felts are the so-called -- graphite felts -- which in many instances are predominantly amorphous type carbons which were carbonized to convert only part of the carbon to graphite.
• the porous, high surface area carbonaceous cathodes of the present invention are intended to include these so called “graphite” materials.
• these high surface area felts, cloths and reticulated vitreous carbons may be bonded for example, by means of suitable conductive epoxy to inert, more conductive current carriers such as graphite, Ebonex, or Ti to improve the current density distribution by making the current density more uniform over the entire available electrode surface.
• Solid polymer electrolyte technology can be employed in these electrolyses to advantage.
• the anode side of a suitable cation-exchange membrane eg Nafion R 117, manufactured by DuPont, USA is coated with a layer of Pt or Au, for example by electroless deposition , and then an anode screen of Pt on Ti is mechanically pressed against this deposited layer.
• the anolyte feed is then water without any additional conductive ions since the polymeric ionomeric membrane itself provides the ionic conductivity required for electrolysis.
• Use of solid polymer electrolyte technology has other advantages in terms of lower cell voltage and simpler cell design.
• the electrolysis of disulfide substrates should be preferably conducted at lower temperatures, usually -10 to + 50°C to avoid racemization of optically active substrates and products as well as other undesirable reactions, but may be conducted at higher temperatures, even up to near the boiling of the nitrogenous solution if racemization or side-reaction is not a concern and there is little or no opportunity for other undesirable reactions such as polymerization or decomposition occuring. Since reoxidation of the sulfhydryl product to the disulfide form can occur in presence of oxygen or air, especially in alkaline media, electrolyses are generally conducted under an inert atmosphere, usually nitrogen.
• the electrolysis cell design should provide for adequate turbulent circulation of the nitrogenous electrolyte solution containing the disulfide substrate to minimize mass transfer limitations.
• Plate-and-frame cells such as those manufactured by ElectroCell Systems AB (Sweden) are suitable for this purpose, and are sufficiently flexible in design to permit use of solid electrodes, particulate bed electrodes, and other porous electrodes such as carbonaeous felts and cloths, as well as reticulated vitreous carbon.
• Other suitable cell designs are possible including cylindrical configurations, and packed or fluidized bed electrolyzers. Suitable cell designs including monopolar and bipolar designs are described in various texts, for example Industrial Electrochemistry , by D. Pletcher, published by Chapman and Hall, 1982.
• Electrolysis may be conducted to 80 to 150% of the theoretical number of coulombs required for conversion of disulfides to sulfhydryl products, but more preferably 100 to 110% of theoretical to ensure high conversions yet minimize hydrogen evolution.
• the cathode current density for these electrolyses is usually in the range of 50 to 500mA/cm2, with the higher effective cathode current densities being more appropriate near the outset of electrolysis and diminishing in value as the electrolysis proceeds toward complete conversion.
• An advantage of the above mentioned high surface area carbonaceous cathodes is that higher effective current densities may be maintained thoughout the electrolysis of at least 50mA/cm2, and more preferably from 75 to about 250mA/cm2 without significant deterioration in current efficiency, until almost all of the disulfide substrate has been converted.
• cysteine Upon completion of the electrolysis the desired product is isolated, usually by removal of the nitrogenous solvent by distillation or evaporation under reduced pressure.
• this solid product can be used as is for a number of applications since it can be as high as 98% or better in purity, but may be further easily purified mainly of cystine, by taking the product up in cold water sufficient to dissolve most of the initial product and filtering off the undissolved cystine and any insoluble material. Recovered cystine can be recycled and employed as feedstock. The filtrate is then evaporated to obtain cysteine with a purity of up to 99.5% or more.
• purification may be effected by crystallization from cold water, or water-alcohol.
• amino acid free-base may be converted to an inorganic salt by conventional means.
• the hydrochloride, sulfate and phosphate salts are representative examples.
• a two compartment electrochemical flow cell system was employed using an ElectroCell Systems AB (Sweden) MP Flow Cell, reservoirs for anolyte and catholyte solutions, magnetic drive pumps, Sorensen Model-DCR-45B Power Supply, and ESC Model 640 digital coulometer.
• the MP Flow Cell was constructed of polypropylene frames, EPDM gaskets, anode (100cm2) of titanium with a Pt/Ir coating, various cathode materials, and a DuPont Nafion 423 cation exchange membrane.
• Catholyte and anolyte volumes were initially about 1 liter, with the catholyte containing 0.42 M l-cystine in 30% aqueous ammonia solution, and the anolyte 3 M aqueous sulfuric acid solution.
• the catholyte solution was circulated at a rate of 4.7 liters/minute and the temperature was maintained below 40°C while kept under a nitrogen gas blanket to prevent air oxidation.
• Table 1 compares results for electrochemical reduction of l-cystine at silver, graphite and carbon felt cathodes.
• the carbon felt cathode was constructed by bonding carbon felt (100cm2), Electrosynthesis Co. Inc. Cat. No.
• Example 2 The experimental flow cell equipment described in Example 1 was used, containing a carbon felt cathode, with electrolyses conducted over a range of current densities. Table 2 lists the results of electrolysis of l-cystine (0.42 M ) taken to the theoretical required number of coulombs to form l-cysteine.
• the anolyte was 3 M aqueous H2SO4, except as noted.
• Table 2 demonstrates that carbon felt cathodes can be used very effectively to reduce the disulfide linkage in yields in excess of 90% even at considerably higher, more practical current densities of operation than heretofore reported.
• L-Cysteine free base was prepared in a manner closely following the method outlined in Japanese Patent application No. 58-23450 (Hasaka) using aqueous NH4OH containing (NH4)2CO3.
• the two compartments were separated by a cation exchange membrane (Nafion R 324).
• the cathode was a lead sheet.
• After electrolysis the catholyte was evaporated to dryness and the product dried under vacuum.
• the product was 89.1% l-cysteine by weight and was found to contain 43ppm lead, as shown by atomic adsorption analysis. For many applications, especially in food and pharmaceutical uses, this high lead level would be unacceptable in the product.

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• 1990
  • 1990-01-04 EP EP90100151A patent/EP0436055A1/fr not_active Withdrawn

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