Electrochemical oxidation of 5-hydroxytryptophan in acid solution.

The electrochemical oxidation of 5-hydroxytryptophan (5-HTPP) in acid solution proceeds by an initial 1e-, 1H+ reaction to a radical intermediate. This radical can dimerize and three diastereomers of 4,4'-bis(5-hydroxytryptophan) have been isolated and characterized. The radical can also undergo further electrochemical oxidation (1e-, 1H+) to a quinoneimine intermediate. Nucleophilic attack by water on this quinoneimine, followed by further oxidation, gives tryptophan-4,5-dione. Nucleophilic attack by 5-HTPP on the quinoneimine gives a dimeric indolenine which undergoes a complex series of chemical and electrochemical reactions leading ultimately to 4-[4-(6-hydroxyquinolyl)]-5-hydroxytryptophan. Two diastereomers of the latter compound have been isolated and characterized.