Cystine
(sis' teen).
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C6H12N2O4S2 240.30
l-Cystine;    
3,3¢-Disulfanediylbis [(2R)-2-aminopropanoic acid]     [56-89-3].
DEFINITION
Cystine contains NLT 98.5% and NMT 101.5% of C6H12N2O4S2, as l-Cystine, calculated on the dried basis.
IDENTIFICATION
•  B. Optical Rotation, Specific Rotation 781S: 215 to 225, determined at 20
Sample solution:  20 mg/mL, in 1 N hydrochloric acid
•  C. The RF value of the principal spot of the Sample solution in the test for Organic Impurities corresponds to that of the Standard solution.
ASSAY
•  Procedure
Sample solution:  Transfer about 0.1 g of Cystine to a glass-stoppered flask, and dissolve in a mixture of 2 mL of dilute sodium hydroxide (1 in 20) and 10 mL of water. Add 10 mL of potassium bromide solution (200 g/L in water), 50.0 mL of 0.1 N potassium bromate VS, and 15 mL of dilute hydrochloric acid (17 in 100). Immediately insert the stopper into the flask, and cool in an ice water bath. Allow to stand protected from light for 10 min.
Titrimetric system 
Mode:  Residual titration
Titrant:  0.1 N potassium bromate VS
Back-titrant:  0.1 N sodium thiosulfate VS
Endpoint detection:  Colorimetric
Equivalency:  Each mL of 0.1 N potassium bromate VS is equivalent to 2.403 mg of C6H12N2O4S2 on the dried basis.
Analysis:  Add 1.5 g of potassium iodide, and after 1 min, titrate with 0.1 N sodium thiosulfate VS, using starch TS as the indicator. Perform a blank determination, and make any necessary correction.
Acceptance criteria:  98.5%–101.5% on the dried basis
IMPURITIES
Inorganic Impurities 
•  Residue on Ignition 281: NMT 0.1%
•  Chloride and Sulfate, Chloride 221: NMT 200 ppm. A 0.7-g portion shows no more chloride than corresponds to 0.40 mL of 0.01 N hydrochloric acid.
•  Chloride and Sulfate, Sulfate 221: NMT 200 ppm. A 1.2-g portion shows no more sulfate than corresponds to 0.25 mL of 0.020 N sulfuric acid.
•  Iron 241: NMT 10 ppm
•  Heavy Metals, Method I 231: NMT 10 ppm
Organic Impurities 
•  Procedure
Standard solution:  Dissolve a quantity of USP Cystine RS in 1 N hydrochloric acid, and dilute with water to obtain a solution having a known concentration of about 0.02 mg/mL.
Sample solution:  Dissolve a quantity of Cystine in 1 N hydrochloric acid, and dilute with water to obtain a solution having a known concentration of about 10 mg/mL.
System suitability solution:  Dissolve quantities of USP Cystine RS and USP Arginine Hydrochloride RS in 1 N hydrochloric acid, and dilute with water to obtain a solution having a known concentration of about 0.4 mg/mL each.
Chromatographic system 
Mode:  TLC
Adsorbent:  A 0.25-mm layer of chromatographic silica gel mixture
Application volume:  5 µL
Developing solvent system:  A mixture of ammonia and 2-propanol (3:7)
Spray reagent:  Dissolve 0.2 g of ninhydrin in 100 mL of a mixture of butanol and 2 N acetic acid (95:5).
Analysis 
Samples:  Standard solution, Sample solution, and System suitability solution
Proceed as directed for Chromatography 621, Thin-Layer Chromatography. After air-drying the plate, spray with Spray reagent, and heat between 100 and 105 for about 15 min. Examine the plate. The chromatogram from the System suitability solution exhibits two clearly separated spots.
Acceptance criteria:  Any secondary spot from the Sample solution is not larger or more intense than the principal spot from the Standard solution.
Individual impurities:  NMT 0.2%
Total impurities:  NMT 2.0%
SPECIFIC TESTS
•  Loss on Drying 731: Dry a sample at 105 for 3 h: it loses NMT 0.2% of its weight.
ADDITIONAL REQUIREMENTS
•  Packaging and Storage: Preserve in well-closed containers, and store at a controlled room temperature.
•  USP Reference Standards 11
USP Arginine Hydrochloride RS Click to View Structure
USP Cystine RS Click to View Structure
Auxiliary Information— Please check for your question in the FAQs before contacting USP.
Topic/Question Contact Expert Committee
Monograph Huy T. Dinh, M.S.
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USP35–NF30 Page 1264
Pharmacopeial Forum: Volume No. 35(1) Page 122