High yield methods for electrochemical preparation of cysteine and analogues

Amino acid free bases are prepared electrochemically without production of intermediate acid salts. Amino acids having reducible disulfide linkages and at least one basic nitrogen group are reduced at a high surface area, noncontaminating cathode to provide a current density of at least 50 mA/cm.sup.2, product yield of at least 90% and an operating current efficiency of at least 90%.

BACKGROUND OF THE INVENTION 
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 1-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: 
EQU (--S--CH.sub.2 CH(NH.sub.2)CO.sub.2 H) .sub.2 +2H.sup.+ +2e.fwdarw.2 
HSCH.sub.2 CH(NH.sub.2)CO.sub.2 H 
This reduction has been conducted chemically with reagents such as 
Na/liquid NH.sub.3, Zn, Al or Sn in aqueous HCl, or solutions of 
NaBH.sub.4 have been employed. However, 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. 
Heretofore, electrochemical reduction of cystine to cysteine was usually 
conducted in aqueous acid solution in which the cystine was dissolved in 
aqueous HCl or H.sub.2 SO.sub.4. Rambacher in U.S. Pat. No. 2,907,703 
(1959) described the electrochemical reduction of an aqueous suspension of 
cystine hydrochloride in 2N aqueous HCl solution, using an electrochemical 
cell containing a cathode of Sn, Cu, Ag, Ni or carbon, in which the anode 
compartment is separated from the cathode compartment by means of a porous 
diaphragm. If the cathode is a sheet of Cu or a carbon rod, SnCl.sub.2 is 
added to the catholyte, and if 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. Thus, with Rambacher's 
method, in order to prepare cysteine free-base electrochemically, it was 
necessary to first prepare the acid salt. 
Likewise, Wong and Wang, J. Chinese Chem. Soc., 25. 149 (1977) have 
described the electrochemical reduction of cystine in aqueous HCl solution 
at stainless steel electrodes in an electrochemical cell fitted with an 
anion-exchange membrane. The purpose of the anion-exchange membrane is to 
allow anions, such as chloride ion to pass through the membrane to the 
anode side of the cell but not allow cations, or the starting material or 
product through. The electrolysis product, after evaporation of the 
aqueous electrolyte solution, was cysteine as the HCl salt. The free amino 
acid cysteine was then prepared by dissolving the cysteine HCl in ethanol, 
carefully adding aqueous NH.sub.4 OH solution to pH 6.2, and filtering off 
and drying the free cysteine. Whereas, 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 
kg, hence losses of cysteine through precipitation steps or otherwise are 
costly. The Wong and Wang process is impractical on a longer-term 
production basis, since under these conditions, stainless steel anodes 
would soon corrode as Cl.sub.2 is evolved at the anode, and moreover 
Cl.sub.2 or HOCl generated thereby would eventually attack and destroy the 
kind of anion exchange membrane that was used (Asahi Glass Co., Selemion 
AMV). 
Mizuguchi et al, Bull. Tokyo Inst. Technol. No. 64, 1-6 (1965) conducted 
electrolyses of cystine in aqueous acid media (HCl or H.sub.2 SO.sub.4 and 
in aqueous alkaline media (NaOH, Na.sub.2 CO.sub.3 and NH.sub.4 OH), using 
a porous porcelain diaphragm in a first electrolysis cell to separate 
anode and cathode compartments. When the 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. In alkaline media, Mizuguchi showed 
that appreciable losses of cystine and cysteine occurred through the 
porous porcelain diaphragm. Mizuguchi's results with aqueous NH.sub.4 OH 
solution are particularly pertinent to the present invention. Electrolysis 
of cystine (12.lg) was conducted at a Pb cathode at a low current density 
of 25mA/cm.sup.2 using 3M NH.sub.4 OH (about 10% NH.sub.4 OH by weight) 
with added (NH.sub.4).sub.2 CO.sub.3, in a batch cell containing a porous 
porcelain diaphragm. After prolonged electrolysis the catholyte solution 
was evaporated to dryness leaving 9.0g of crude product containing 7.0g of 
cysteine and 2.0g of cystine. According to the authors, Pb was not 
detected in the product. 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 
H.sub.2 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/cm.sup.2. Like Mizuguchi et 
al, Hasaka's product yield using alkaline electrolyte was low, ie 75%. 
Although the Japanese patent (Hasaka) stresses that low cost metals can now 
be used with alkaline anolyte which could not be employed with acidic 
solutions, it has also been discovered that lead cathodes like those of 
Hasaka are capable of introducing unsafe, toxic levels of lead into the 
cysteine rendering the product unacceptable particularly as a food grade 
material for additives, nutritional supplements, an intermediate for 
synthesis of pharmaceuticals, and other products especially intended for 
internal as well as external use. 
Accordingly there is a need for a more economic, more reliable and 
efficient method of producing high purity cysteine and its analogues 
electrochemically from cystine and its corresponding analogues which 
minimizes losses of costly disulfide feed and sulfhydryl product, does not 
necessitate additional conductive salts, simplifies the separation of 
product as the free amino acid from the electrolyte solution, avoids the 
need for a second deacidification electrolyzer, and provides for a single 
improved electrolyzer which produces the product at higher current 
densities, in high yields, current efficiency and conversion. 
The present invention provides such improved methods for the 
electrochemical production of cysteine and its sulfhydryl analogues. 
SUMMARY OF THE INVENTION 
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/cm.sup.2 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. 
It is a further object of the present invention to provide basic 
nitrogenous electrolytes comprising inter-alia aqueous ammonia, anhydrous 
liquid ammonia with sufficient concentrations of the disulfide reactant to 
maintain the desired high product yield of at least 90% without loss of 
the valuable disulfide reactant. Accordingly, a still further object is to 
conduct the reaction in an electrochemical cell having a high efficiency 
divider, and in particular an ion-exchange type membrane for separating 
the catholyte from the anolyte without loss of reactant. 
It is yet a further object of the present invention to conduct the 
electrochemical reaction at consistently higher current efficiencies of at 
least 90% with improved high surface area electrodes preferably comprising 
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. 
These and other features and advantages will become more apparent from the 
following detailed description of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
The methods of the invention are primarily concerned with preparation of 
amino acids, II, such as in their d-, 1-, or dl-forms. The term 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: 
##STR1## 
where R.sub.1 and R.sub.2 are H, lower aliphatic (C.sub.1 to C.sub.6, 
aryl, aralkyl, or in which R.sub.1 and R.sub.2 taken together form a 
nitrogen heterocyclic ring of 3 to 7 atoms in which the nitrogen is basic. 
Thus, disulfide compounds of structure I may be considered to be alpha, 
beta, gamma or even omega-amino acids. Examples of disulfide amino acid 
analogues of structure I include: 
##STR2## 
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) according to the present invention 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.degree. 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. These 
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 NH.sub.3 and (CH.sub.3).sub.3 N in water 
and/or alcohol. Suitable amines are of general formula, R.sub.3 N 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. Amongst the nitrogenous solutions, 
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. Thus, 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. 
When aqueous solutions of ammonia are used, 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 NH.sub.3 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 1.0M or more. 
While 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. 
When an ion-exchange membrane is used as a component of the electrolyzer 
this should preferably be a cation exchange membrane to minimize transfer 
and loss of the negatively charged carboxylate anion of the disulfide 
substrate and/or the product through the membrane into the anode 
compartment. In membrane separated electrolyzers, 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 (NH.sub.4).sub.2 SO.sub.4, 
(NH.sub.4).sub.3 PO.sub.4, (NH.sub.4).sub.2 CO.sub.3, and ammonium salts 
of organic acids like acetate, formate, oxalate, etc. Other suitable 
anolytes may be aqueous H.sub.2 SO.sub.4 or aqueous H.sub.3 PO.sub.4. 
While halogen containing anolytes such as aqueous NH.sub.4 Cl and aqueous 
HCl may be used, these are not preferred, since provision must then be 
made for generation of Cl.sub.2 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.0. 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, PbO.sub.2 on Pb, PbO.sub.2 on Ti, or uncatalyzed or catalyzed 
ceramic, such as Ebonex.RTM. anodes (Ti.sub.4 O.sub.7) 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. 
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. Generally, for purposes of this invention 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. Thus, for pharmaceutically related products, 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. 
High surface area, carbonaceous materials are preferred 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. Amongst the highest product yields, 
conversions, and current efficiencies are found at these carbonaceous 
cathodes, compared to metal cathodes. However, carbonaceous cathodes of 
high surface area like particulate beds, porous carbons, felts, cloths, or 
reticulated vitreous carbon (manufactured by ERG Corp., California) 
provide even better performance. 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. For 
purposes of this invention, 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. Also included by the expression 
"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. In any event, the porous, high 
surface area carbonaceous cathodes of the present invention are intended 
to include these so called "graphite" materials. For larger electrode 
configurations, 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. Here, the anode side of a suitable cation-exchange membrane, 
eg Nafion.sup.R 117, manufactured by DuPont, U.S.A. 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.degree. to +50.degree. 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 throughout the electrolysis of at least 
50mA/cm.sup.2, and more preferably from 75 to about 250mA/cm.sup.2 without 
significant deterioration in current efficiency, until almost all of the 
disulfide substrate has been converted. 
Upon completion of the electrolysis the desired product is isolated, 
usually by removal of the nitrogenous solvent by distillation or 
evaporation under reduced pressure. For cysteine, 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. Alternatively, purification may be effected 
by crystallization from cold water, or water-alcohol. 
If desired, the amino acid free-base may be converted to an inorganic salt 
by conventional means. The hydrochloride, sulfate and phosphate salts are 
representative examples. 
The following specific examples demonstrate various aspects of the 
invention, however, it is to be understood that these examples are for 
illustrative purposes only and do not purport to be wholly definitive as 
to conditions and scope. 
EXAMPLE 1 
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 (100cm.sup.2) 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 0.42M 1-cystine in 
30% aqueous ammonia solution, and the anolyte 3M aqueous sulfuric acid 
solution. The catholyte solution was circulated at a rate of 4.7 
liters/minute and the temperature was maintained below 40.degree. C. while 
kept under a nitrogen gas blanket to prevent air oxidation. Table 1 
compares results for electrochemical reduction of 1-cystine at silver, 
graphite and carbon felt cathodes. The carbon felt cathode was constructed 
by bonding carbon felt (100cm.sup.2), Electrosynthesis Co. Inc. Cat. No. 
GF-S6 to a graphite plate, by means of graphite-filled epoxy resin. The 
cathode current density was maintained at 60mA/cm.sup.2 throughout the 
experiment, with electrolysis conducted to the extent of 100% of the 
theoretical charge passed required to convert 1-cystine to 1-cysteine. 
After electrolysis, the ammonia solvent was evaporated off to dryness and 
the product analyzed iodometrically. 
TABLE 1 
______________________________________ 
Flow Cell Experiments At Various Cathode Materials 
Cell Voltage 
Experiment 
Cathode Material 
Volts Yield*(%) 
______________________________________ 
1 Silver Plate 4.2-6.1 75.6 
2 Graphite Plate 
4.4-6.0 82.8 
3 Carbon Felt 4.4-4.8 96.6 
______________________________________ 
*The yield and current efficiency are the same here. 
The yields shown in Table 1 demonstrate that high surface area carbon felt 
is superior to low surface area silver or graphite plate cathodes in 
reducing the disulfide linkage. 
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 
1-cystine (0.42M) taken to the theoretical required number of coulombs to 
form 1-cysteine. The anolyte was 3M aqueous H.sub.2 SO.sub.4, except as 
noted. 
TABLE 2 
______________________________________ 
ELECTROLYSIS OF L-CYSTINE AT 
CARBON FELT IN AMMONIA SOLUTION 
Current Density 
Cell Voltage 
Yield % 
Expt mA/cm.sup.2 Volts At 100% Theory** 
______________________________________ 
3 60 4.4-4.8 96.6 
4 100 5.2-6.5 99.2 
5* 100 6.2-8.4 95.6 
6 150 6.4-7.8 96.5 
7 200 6.6-9.8 90.6 
8 250 6.2-8.4 94.6 
______________________________________ 
*The anolyte was 1M aqueous (NH.sub.4).sub.2 SO.sub.4 
**The yield and current efficiencies are the same here. 
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. 
EXAMPLE 3 
To exemplify the relative simplicity of product isolation and purification 
using nitrogenous catholyte solutions, the product of electrolysis 
experiment #3 of Example 1, was worked up. The crude product, after 
ammonia evaporation, was dissolved in 750ml of distilled water, the 
mixture filtered, and the solids washed with a little cold distilled 
water. The filtrate was evaporated to dryness in vacuo at 40.degree. C. 
leaving the purified material. Iodometric analysis showed this material 
was 99.6% 1-cysteine by weight. The specific rotation of a sample of 5.02g 
in 100ml of 1M aqueous HCl was +6.255, which corresponds to an assay for 
1-cysteine of 99.4%. Elemental analysis %: (observed) C, 29.69;H, 5.84;N, 
11.51;S, 26.41; (calculated) C, 29.74;H, 5.82;N, 11.56;S, 26.46. 
EXAMPLE 4 
L-Cysteine free base was prepared in a manner closely following the method 
outlined in Japanese Patent application No. 58-23450 (Hasaka) using 
aqueous NH.sub.4 OH containing (NH.sub.4).sub.2 CO.sub.3. 
The two compartments were separated by a cation exchange membrane 
(Nafion.RTM. 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% 1-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. 
While the invention has been described in conjunction with specific 
examples thereof, this is illustrative only. Accordingly, many 
alternatives, modifications and variations will be apparent to persons 
skilled in the art in light of the foregoing description, and it is 
therefore intended to embrace all such alternatives, modifications and 
variations as to fall within the spirit and broad scope of the appended 
claims.