Under this condition, oxidation is expected to involve the formyl group.
A pronounced change in the IR spectrum was observed with evidences of the conversion of the formyl group to the carboxylic group.
The fact that mixing the reactants in a 2 : 3 molar ratio leads to oxidation of the formyl group confirms that oxidation will involve the formyl group in presence of large excess of the two acids as implemented in this study.
In addition, different one-carbon substituents can be present, including methyl and formimino groups at position N-5, formyl groups
at either N-5 or N-10, or a methenyl or methylene bridge that connects these 2 positions (1).