UC San Diego

The configurational analysis of amino acids (AAs) in natural product peptides, often containing nonproteinogenic AAs, is mostly carried out by the venerable Marfey's method using a chiral derivatizing agent (CDA) 1-fluoro-2,4-dinitrophenyl-5-l-alaninamide (l-FDAA)─Marfey's reagent─which undergoes S_NAr reaction of the 1° amino group. The resulting AA-DAA derivatives are mostly well-separated by reversed-phase HPLC, but some DAA derivatives resist resolution. Here, we report the synthesis and characterization of two CDAs: l-FDTA (4) in which the l-alanine-derived auxiliary is replaced by l-tryptophanamide and (S)-FDNE (3) where the auxiliary is S-(6-methoxynaphth-2-yl)-1-ethylamine. Side-by-side comparisons of the two reagents were carried out by AA derivatization and reversed-phase HPLC analysis with variables such as organic solvent, additives, and the ionic strength of the mobile phase. l-DTA derivatives of l- and d-AAs were found to show superior HPLC performance and an improvement in resolutions. When incorporated into the mobile phase, the ammonium ion (NH₄⁺, 0-100 mM) showed a dramatic influence on differential retention times [Δt_R = Δt_Rd - Δt_Rl] of several key AAs. We attributed the effect to π-cation interactions between the indole ring of DTA and the NH₄⁺ counterion in the analyte, a hypothesis supported by ¹H NMR titrations and DFT calculations.