ABSTRACT
The non-covalent interactions between 18-crown-6 (18c6) and 20 common types of protonated amino acids were explored by electrospray ionization mass spectrometry (ESI-MS).The mass spectra showed the formation of 1:1 stoichiometric non-covalent complexes between 18c6 and amino acids.The calibration curves and linear equations for the complexes of L-Phe,L-Tyr,L-Lys and L-Asp with 18c6 were established by mass spectrometric titration and used as reference values for competitive ESI-MS.Through competitive equilibrium,the binding constants for the complexes of 18c6 with other L-amino acids and their D-isomers were derived.It was found,as a general trend,lgKa for the complexes of 18c6 with the basic amino acid and the amino acid with alkyl side chain were larger than other complexes,and among the amino acids with alkyl side chain,Gly and Ala exhibited greater 18c6 binding affinities.As for Ser and Thr,the intramolecular hydrogen bond between the nitrogen atom from terminal NH2and the oxygen atom from carboxyl may impede their protonated amino-group to attack the 18c6.Furthermore,Gln and Asn exhibited lower 18c6 binding affinities probably due to effects of electron-withdrawing group of acylamide.Finally,the chiral selectivity of 18c6 for 19 L-,or D-amino acids was measured by ESI-MS,indicating 18c6 could only recognize some neutral amino acid isomers.