Hopkins–Cole reaction

{{Short description|Chemical reaction used to detect the presence of tryptophan in proteins}}

The Hopkins-Cole reaction, also known as the glyoxylic acid reaction, is a chemical test used for detecting the presence of tryptophan in proteins.{{cite book|author=R.A. Joshi|title=Question Bank of Biochemistry|url=https://books.google.com/books?id=Acf9BkEqJWYC&pg=PA64|year=2006|publisher=New Age International|isbn=978-81-224-1736-4|page=64}} A protein solution is mixed with Hopkins Cole reagent, which consists of glyoxylic acid. Concentrated sulfuric acid is slowly added to form two layers. A purple ring appears between the two layers if the test is positive for tryptophan.{{cite book|author=Debajyoti Das|title=Biochemistry|url=https://books.google.com/books?id=QqXpWJHICUYC&pg=PA56|year=1980|publisher=Academic Publishers|isbn=978-93-80599-17-5|page=56}}{{cite book|author=P. M. Swamy|title=Laboratory Manual on Biotechnology|url=https://books.google.com/books?id=_oFtJObOYTIC&pg=PA90|year=2008|publisher=Rastogi Publications|isbn=978-81-7133-918-1|page=90}} Nitrites, chlorates, nitrates and excess chlorides prevent the reaction from occurring.{{cite book|author=Chatterjea|title=Textbook of Biochemistry for Dental/Nursing/Pharmacy Students|url=https://books.google.com/books?id=xN0YYypnZVkC&pg=PA51|date=1 January 2004|publisher=Jaypee Brothers Publishers|isbn=978-81-8061-204-6|page=51}}

The reaction was first reported by Frederick Gowland Hopkins and Sydney W. Cole in 1901,{{cite journal |first1= Frederick Gowland |last1= Hopkins |author-link1= Frederick Gowland Hopkins |first2= Sydney W. |last2= Cole |title= On the proteid reaction of Adamkiewicz, with contributions to the chemistry of glyoxylic acid |year= 1901 |journal= Proceedings of the Royal Society |volume= 68 |issue= 442–450 |pages= 21–33 |doi= 10.1098/rspl.1901.0008 |s2cid= 84899015 }} as part of their work on the first isolation of tryptophan itself.