Reactions of specific amino acids

It is often difficult to quantitate one particular amino acid in the presence of others because of chemical similarities. Interference from substances other than amino acids is also a problem in many reputedly specific methods. Ultraviolet spectroscopy is of little value in the detection of aromatic amino acids because they have similar absorbance maxima and considerably different molar absorption coefficients. 

The fluorimetric methods often offer improved specificity and sensitivity over colorimetric procedures and the quantitative assays for the aromatic amino acids tyrosine and phenylalanine illustrate this point. 

Phenylalanine reacts with ninhydrin in the presence of a dipeptide (usually glycyl-L-leucine or L-leucyl-L-alanine) to form a fluorescent product. The fluorescence is enhanced and stabilized by the addition of an alkaline copper reagent to adjust the pH to 5.8 and the resulting fluorescence is measured at 515 nm after excitation at 365 nm see Procedure 10.2. 

L-Leuc y I - l- alani ne (5 mmol I ) or Glycyl-i.-leucine (5 mmol I ) 

This reagent is hazardous and must be handled in accordance with approved procedures. 

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