IDENTIFICATION OF THE ELUSIVE AMINO ACID 
TABLE I 
COLORIMETRIC DETECTION OF AMINO ACIDS ON PAPER 



Amino acid Developing agent Color 
Arginine Sakaguchi reaction Red 
Asparagine Ninhydrin Orange-brown 
Aspartic acid Ninhydrin Bright blue 
Citrulline Dimethylaminobenzaldehyde Yellow 
Cysteine Nitroprusside Red 
Cystine Ninhydrin—cyclohexylamine Orange 
Nitroprusside—cyanide Red 
Glycine Ninhydrin Grayish-purple 
Ninhydrin—cyclohexylamine Reddish-brow 
Naphthoquinone-4-sulfonate Green 
o-Phthalaldehyde Green 
Histidine Ninhydrin—cyclohexylamine Grayish-green 
Pauly reaction Red 
o-Phthalaldehyde Bluish-green 
Sakaguchi reaction Yellow 
Hydroxyproline Isatin Blue 
Naphthoquinone-4-sulfonate Red 
Ninhydrin Brownish-yellow 
Ninhydrin—cyclohexylamine Carmine 
Phenylalanine Ninhydrin—cyclohexylamine Bluish-gray 
Ninhydrin—alkali Intense blue 
Proline Tsatin Blue 
Naphthoquinone-4-sulfonate Red 
Ninhydrin Yellow 
Ninhydrin—cyclohexylamine Yellow 
Serine Hypochlorite—dinitrobenzene Violet 
Sulfur amino acids 
Periodate—Nessler 
Platinic iodide 
Reddish-brown 
Bleaching action 
Threonine Hypochlorite—dinitrobenzene Violet 
Ninhydrin—cyclohexylamine Grayish-purple 
Periodate—Nessler Reddish-brown 
Tryptophan Dimethylaminobenzaldehyde Yellow 
o-Phthalaldehyde Gray 
Sakaguchi reaction Brown 
Tyrosine Ninhydrin Greenish-purple 
Ninhydrin-cyclohexylamine 
Pauly reaction 
Gray 
Reddish-brown 

acids. Thus, histidine and tyrosine may be selectively detected by the Pauly reaction 
whereby the paper is treated with freshly diazotized sulfamilamide followed by sodium 
carbonate solution, with the histidine revealing a red color, ty rosine a reddish-brown, 
and other amino acids a yellow color!, 2°, Proline and hydroxyproline, by reaction 
with isatin in butanol-acetic acid, followed by heating, reveal a blue color, with all 
other common amino acids yielding a weak rose”!~*8; subsequent treatment with nor- 
mal hydrochloric acid causes the blue spot of proline to deepen in color whereas the 
spots due to the other amino acids fade away”4. Arginine may be qualitatively identified 
as a red spot by the Sakaguchi reaction whereby the paper is sprayed with an alkaline 
a-naphthol solution, followed by a sodium hypochlorite or hypobromite solution; 
employing this process, the spot for histidine becomes yellow and that for tryptophan 
brown, 26. Tryptophan and citrulline, by reaction with p-dimethylaminobenzalde- 
hyde in hydrochloric acid, yield a yellow color?’, the reaction with citrulline serving 
References p. 22/24 
