A study of methionine dipeptide stationary phase for gas chromatographic resolution of amino acid enantiomers



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The synthesis of N-trifluoroacetyl-L-methionyl-L-methionine cyclohexyl ester has been successfully completed. A new method has been used to synthesize the dipeptide stationary phases. Direct coupling between L-amino acid esters and Li-trifluoroacetyl-L-amino acid, using 1,1 carbonyl di-imidazole as a coupling reagent, gives the dipeptide stationary phase in only three steps in good yield. In previous work, five or seven steps were required to prepare the dipeptide stationary phases. The sulfoxide and the sulfone form of the methionine phase were successfully synthesized. Two methods were used to prepare the sulfoxide and the sulfone phases. In the first method direct coupling between N-trifluoroacetyl-L-methionine sulfoxide or methionine sulfone to L-methionine sulfoxide or sulfone cyclohexyl ester was carried out. In the second method, N-trifluoroacetyl-L-methionyl-L-methionine cyclohexyl ester was oxidized to the sulfoxide and sulfone form. The properties of the new stationary phases were examined, and it was found that the methionine phase exhibited improved properties as a gas chromatographic stationary phase when compared to the previous Dipeptide stationary phases. For the first time high and low boiling point derivatives were analyzed on a 300 ft x 0.02 in ID column in a short time and with good resolution. Further, a dipeptide stationary phase was used in conjunction with temperature programming. When the sulfoxide phase was examined, it was found that it was incapable of resolving amino acid enantiomeric derivatives probably due to intra hydrogen bonding between the sulfoxide and the amide. The sulfone phase was found to be an inadequate stationary phase due to its high melting point (181-184[degrees]C) and its insolubility in conventional low boiling solvents.