L- (CAS NO.: ), with other names as cystine, could be produced through the following synthetic routes.

Human hair is freed from foreign matter, washed well with a lukewarm solution of soap, then twice with cold distilled water, and dried. This washing removes the natural oils from the hair. Two kilos of the dry, washed hair is pushed into a 12-l. round-bottomed Pyrex flask, and 4 l. of 20 per cent hydrochloric acid is added. An air-cooled reflux condenser, consisting of a wide glass tube, is attached to the flask. The hair is hydrolyzed by heating on the steam bath until the biuret reaction is entirely negative; this requires one hundred twenty to one hundred forty-four hours.

The mixture is filtered hot, and the insoluble residue is washed with distilled water. The total filtrate is now partially neutralized with 300 cc. of 40 per cent sodium hydroxide solution, while the mixture is well stirred and cooled, and then a saturated solution of 3750 g. of crystallized sodium acetate is added. The test for mineral acid should then be entirely negative. Care must be taken not to make the solution alkaline with sodium hydroxide. After standing for three days at room temperature, the precipitated cystine is filtered on a suction funnel. This crude material, containing, in addition to the cystine, some "humin" pigments and tyrosine, is dissolved in 3 l. of 3 per cent hydrochloric acid. The solution is filtered and completely decolorized by two to five treatments with 20–25 g. portions of decolorizing carbon (Norite) which has been completely freed from calcium phosphate by boiling with dilute hydrochloric acid and washing with cold water. The filtrate after decolorizing should be water-clear, or at the most only slightly yellow. If it shows more color, the treatment with decolorizing carbon should be carried out again. The solution should finally be filtered once by gravity to remove traces of a solid impurity which is apt to pass through the suction funnel.

The cystine is precipitated from the clear solution by adding a filtered saturated solution of 900–1000 g. of crystallized sodium acetate until the Congo red reaction is negative. After standing five to six hours, the cystine is filtered and washed twice with 100–200 cc. portions of hot, distilled water to remove the last traces of tyrosine. By this method the typical colorless hexagonal plates of cystine are obtained. The yield is 100–106 g. (5.0–5.3 per cent of the weight of the starting material).