The structural limitation of positive compounds in the Voges-Proskauer Reaction was investigated with guanidines, ureas, thioureas, formamidine and formaldoxime, using the α-naphthol alkaline and diacetyl solution. Only guanidino derivatives showed positive reaction among compounds tested above. Strongly positive compounds were asym-1, 1-dialkylguanidines. Especially, 1, 1-dimethylguanidine, 1-methyl-1-phenylguanidine and 1, 1-pentamethyl-eneguanidine had the highest absorbance at 535 mμ. Creatine which was usually used in this reaction, showed second higher absorbance. Monosubstituted guanidine gave always moderately positive reaction, and unsubstituted one was weakly positive. sym-1, 3-Dialkylguanidines such as 1, 3-dimethylguanidine was negative, suggesting that a free amidino group [chemical formula] would be necessary for the positive V-P reaction. This is further supported by the fact that tri-, tetra- and pentamethylguanidine also gave negative reaction. 8-Hydroxyquinoline could be used in place of α-naphthol in this reaction. Conditions for color development, stability of colored solution and the effect of pH on the wave length of λmax were studied.
