Simultaneous Analysis of d,l-Amino Acids in Human Urine Using a Chirality-Switchable Biaryl Axial Tag and Liquid Chromatography Electrospray Ionization Tandem Mass Spectrometry

Although d,l-amino acids are symmetrical molecules, l-isomers are generally dominant in living organisms. However, it has been found that some d-amino acids also have biological functions. A new method for simultaneously analyzing d,l-amino acids in biological samples is required to allow unknown functions of d-amino acids to be investigated. d-Amino acids in urine are currently receiving increasing amounts of attention, particularly for screening for chronic kidney diseases. However, simultaneously analyzing d,l-amino acids in human urine is challenging because of interfering unknown compounds in urine. In this study, the axially chiral derivatizing agent (R)-4-nitrophenyl-N-[2-(diethylamino)-6,6-dimethyl-[1,1-biphenyl]-2-yl] carbamate hydrochloride was used to allow enantiomers of amino acids in human urine to be simultaneously determined by liquid chromatography electrospray ionization tandem mass spectrometry. The optimized method gave good linearities, precision results, and recoveries for 18 proteinogenic amino acids and their enantiomers and glycine. The chiral-switching method using (S)-4-nitrophenyl-N-[2-(diethylamino)-6, 6-dimethyl-[1,1-biphenyl]-2-yl]carbamate hydrochloride confirmed the expected concentrations of 32 of the 37 analytes. The method was successfully used to determine the concentrations of d-serine, d-alanine, d-asparagine, d-allothreonine, d-lysine, and the d-isomers of 10 other amino acids in five human volunteer urine samples.