Experimental and theoretical studies of potassium cation interactions with the acidic amino acids and their amide derivatives

The binding of K+ to aspartic acid (Asp), glutamic acid (Glu), asparagine (Asn), and glutamine (Gln) is examined in detail by studying the collision-induced dissociation (CID) of the four potassium cation-bound amino acid complexes with Xe using a guided ion beam tandem mass spectrometer (GIBMS). Formed by electrospray ionization, these complexes have energy-dependent CID cross sections that are analyzed to provide 0 K bond energies after accounting for unimolecular decay rates, internal energy of reactant ions, and multiple ion-molecule collisions. Quantum chemical calculations for a number of geometric conformations of each K+(L) complex are determined at the B3LYP/6-311+G(d,p) level with single-point energies calculated at B3LYP, B3P86, and MP2(full) levels using a 6-311+G(2d,2p) basis set. Theoretical bond dissociation energies are in good agreement with the experimental values. This coordinated examination of both experimental work and quantum chemical calculations allows for a comprehensive understanding of the molecular interactions of K+ with the Asx and Glx amino acids. K+ binding affinities for the amide complexes are systematically stronger than those for the acid complexes by 9 +/- 1 kJ/mol, which is attributed to an inductive effect of the OH group in the carboxylic acid side chain. Additionally, the K+ binding affinity for the longer-chain amino acids (Glx) is enhanced by 5 +/- 1 kJ/mol compared to the shorter-chain Asx because steric effects are reduced. Further, a detailed comparison between experimental and theoretical results reveals interesting differences in the binding of K+ and Na+ to these amino acids.