CHM 266 Handout

Oxidation of Cysteine to Cystine

Spring 2008

 

 

                        Cysteine                                                                     Cystine

 

Reference: J. W. Hill, R. B. Coy, P. E. Lewandowski, J. Chem. Ed. (Journal of Chemical Education), 67, 172, (1990).1

 

L-(+)-Cysteine is readily oxidized to L-(-)-cystine by aqueous hydrogen peroxide.

 

Procedure:

 

            Weigh out 1.1 g of cysteine hydrochloride monohydrate.  Add 2.5 ml of freshly made 10% aqueous sodium hydroxide solution.2  Next add 10 ml of 3% aqueous hydrogen peroxide solution.3  The product crystallizes from the reaction mixture.  When the reaction appears to be complete (the precipitate has settled to the bottom of the flask), collect by vacuum filtration.  Recrystallize from water (approximately 1 liter hot water per gram product).4  Collect by vacuum filtration and wash with a small amount of cold water.  Once the product has dried, obtain a sealed tube m.p.5 (decomposes at 235°C). Obtain a Nujol Mull IR.6

 

 

Notes

 

1.      This is the reference you should use for your notebook.

2.      You need to figure out how to make this solution. It is a mass percentage of solute or weight percent.

3.      This should be in the refrigerator in the prep room.

4.      The day you intend to recrystallize the product, you should place approximately 1L. of water and a magnetic stirring bar in an Erlenmeyer flask before the lab lecture starts.  Heat the water to boiling on a hotplate with stirring.  The product requires ~ 1 L. of boiling water to dissolve.  It takes awhile to dissolve so it will help if you break up the any large chunks of solid with a stirring rod.  You must start heating the water immediately because it takes time to heat up this much water (remember that stuff you learned about heat capacity in general chemistry).  If no crystals have formed by the following week, evaporate the solution to 2/3's of its original volume in an open beaker.  Use the hot plates for heating.  Let the solution then cool to room temperature.  If the crystals have formed, cool them in an ice bath and then collect.

5.      Sealed tube m.p., decomposition, and Nujol mull IR will be described in the notes.

6.      Perform m.w. and # moles used calculations based on this structure.

 

cysteine hydrochloride monohydrate