Oxidation of Cysteine to Cystine Using Hydrogen Peroxide L-(+)-Cysteine
is
readily oxidized to L-{ —)-cystine by aqueous hydrogen peroxide 2
HSCH2CHCOO“ + H2O2 NH;,+
“OOCCHCHa—S—S—CH2CHCOO~ NH:i+
NH;}+
Downloaded via UNIV OF CAMBRIDGE on July 18, 2018 at 11:18:36 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
This synthesis has several advantages as an undergraduate laboratory exercise. First, the reactants are familiar substances that present no hazard beyond those likely to be encountered in everyday activities. Second, the starting amino acid is quite soluble in water, and the product amino acid is only slightly soluble; it crystallizes out as it is formed. Third, the product can be recrystallized from hot water, making the use of toxic or flammable solvents unnecessary. Fourth, the experiment can be done at the micro or macro level or as a lecture demonstration. Fifth, the experiment is useful in several different courses, from the freshman level to biochemistry. Finally, the reaction simulates the one involved in the crosslinking of proteins. It also is analogous to the “setting” step in the permanent waving of hair. Procedure (Macro Scale), Weigh out 2.2 g cysteine hydrochloride monohydrate.1 Add 5 mL of freshly made 10% aqueous sodium hydroxide solution.2 Next, add 20 mL of 3% aqueous hydrogen peroxide solution/1 The product crystallizes from the reaction mixture. When the reaction appears to be complete (the precipitate has settled to the bottom of the flask), filter with suction. Recrystallize from water. Wash the product with a small amount of ice-cold water. Yields average about 80% (range 43-98%). Procedure (Micro Scale). Weigh out 88 mg of cysteine hydrochloride in a small test tube. Add 0.20 mL of 10% aqueous NaOH solution and 0.80 mL of 3% aqueous hydrogen peroxide solution. The tube can be centrifuged to speed the settling of the precipitate. Pour off the supernatant liquid. Recrystallize from water. Filter (or centrifuge). Wash the product with a few drops of ice-cold water. Yields average about 75%. Lecture Demonstration, The macro scale synthesis can be demonstrated in the classroom. For large rooms, it can be scaled up for maximum visibility. Students see the precipitate formed as the reaction proceeds. Conclusions, The product can be characterized by infrared and nuclear magnetic resonance spectroscopy. Infrared spectra show the presence of small amounts of water after air drying. If desired, this water can be removed by drying for an hour in an oven at 110 °C. A sealed-tube melting-point determination shows decomposition at 235 °C. Acknowledgement We thank our organic chemistry laboratory students for their help and for their patience in developing this experiment.
We used cysteine hydrochloride because it is stable on the shelf whereas cysteine must be kept in converts the hydrochloride salt to free cysteine. 2 We found that old NaOH solutions with carbonate deposits led to inconsistent results. 3 Purchased from a drug store or supermarket. 1
a
refrigerator. The sodium hydroxide
John W. Hill Robert B. Coy Peter E. Lewandowski University of Wisconsin—River Falls River Falls, Wl 54022
172
Journal of Chemical Education
