Oxidation of Cysteine to Cystine Using Hydrogen Peroxide L-(+)-Cysteine is readily oxidized to L-(-1-cystine by aqueous hydrogen peroxide

2 HSCHCHCOO- + H202 --OOCCHCH2-S-S-CH2CHC00 I I I

This synthesis has several advantages as an undergraduate laboratory exercise. First, the reactants are familiar substances that presentno hazard beyond those likely to he encountered in everyday activities. Second, the startingaminoicid is quite soluble in water, and the product amino acid is only slightly soluble; i t crystallizes out as it is farmed. Third, the product can be recrystallized from hot water, making the use of toxic or flammable solvents unnecessary. Fourth, the experiment can he 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 oroteins. I t also is analomus to the "settine" steo in the oermanent wavinr of hair. .. .

Procedure (Macro Scale). Wegh out 2.2 p, cysterne hydnwhlorrde munohydrate: Add 5 mL of freshly made 10% aqueouaadrum hgdruxide sulution: i\'rrt,add 20mLuf 39 aqucuus hvdngcn peroxiderdutim The product rrystallms 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 iee-cold water. Yields average about 80% (range 43-9870).

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 settline of the orecioitate. Pour off the suoernatant liouid. Recrvstallize from water. Filter (or centrifuee). Wash the - . produit with a Eew 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 he scaled up for maximum visihility. Students see the precipitate formed as the reaction proceeds.

Conclusions. The product can he 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 OC. A sealed-tube melting-point determination shows decomposition at 235 OC.

Acknowledgement We thank our organic chemistry laboratory students for their help and for their patience in developing this experiment.

' We used cysteine hydrochlwide because it is stable on Hm shelf whereas cyrtelne must be kept in a refrigerator. The sodium hydroxide converts me hydrochloride salt to tree cysteine.

We found that old NaOH solutions with carbonate deposits led to inconsistent results. Purchased from a drug store or supermarket.

John W. Hlll Robert 6. Coy

Peter E. Lewandowskl University of Wlsconsin-River Falls

River Falls, WI 54022

172 Journal of Chemical Education