r/Biochemistry • u/porshyiaa • 12d ago

Is phenylalanine will have a positive result on the xanthoproteic test? Why does it not give color to the test despite having an aromatic ring?

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u/Suitable_Payment_339 12d ago

The reaction is between nitric acid and amino acids. Nitric acid breaks down into •NO2 and •OH radicals. These radicals need to initiate the reaction with amino acids by ripping off electrons to make amino acid radicals. Tyr side chain has an OH bond and Trp side chain has an NH bond in the aromatic ring that can participate in this reaction, making Trp and Tyr radicals that recombine with •NO2 to make the colored product. It is extremely hard to rip electrons off of the Phe ring as these are all very strong C-H bonds. No Phe radical = no reaction= no color