title: Experimental validation of theoretical potassium and sodium cation affinities of amides by mass spectrometric kinetic method measurements.
abstract: In this study the theoretical Gaussian-2 K(+)/Na(+) binding affinities (enthalpies) at 0 K (in kJ mol(-1)) for six amides in the order: formamide (109.2/138.5) < N-methylformamide (117.7/148.6) < acetamide (118.7/149.5) < N,N-dimethylformamide (123.9/156.4) < N-methylacetamide (125.6/157.7) < N,N-dimethylacetamide (129.2/162.6), reported previously (Siu et al., J. Chem. Phys. 2001; 114: 7045-7051), were validated experimentally by mass spectrometric kinetic method measurements. By monitoring the collision-induced dissociation (CID) of K(+)/Na(+)-bound heterodimers of the amides, the relative affinities were shown to be accurate to within +/-2 kJ mol(-1). With these six theoretical K(+)/Na(+) binding affinities as reference values, the absolute K(+)/Na(+) affinities of imidazole, 1-methylimidazole, pyridazine and 1,2-dimethoxyethane were determined by the extended kinetic method, and found to be consistent (to within +/-9 kJ mol(-1)) with literature experimental values obtained by threshold-CID, equilibrium high-pressure mass spectrometry, and Fourier transform ion cyclotron resonance/ligand-exchange equilibrium methods. A self-consistent resolution is proposed for the inconsistencies in the relative order of K(+)/Na(+) affinities of amides reported in the literature. These two sets of validated K(+) and Na(+) affinity values are useful as reference values in kinetic method measurements of K(+)/Na(+) affinity of model biological ligands, such as the K(+) affinities of aliphatic amino acids.
authors.affiliation: Department of Applied Biology and Chemical Technology, Hong Kong Polytechnic University, Hung Hom, Hong Kong.
authors.forename: Yuet
authors.initials: Y
authors.lastname: Tsang
authors.forename: Fung Ming
authors.initials: FM
authors.lastname: Siu
authors.forename: Chung Shun
authors.initials: CS
authors.lastname: Ho
authors.forename: Ngai Ling
authors.initials: NL
authors.lastname: Ma
authors.forename: Chun Wai
authors.initials: CW
authors.lastname: Tsang
mesh_terms: D000577:Amides / Q000737:chemistry*; D002414:Cations, Monovalent; D007700:Kinetics; D013058:Mass Spectrometry / Q000379:methods*; D011188:Potassium / Q000737:chemistry*; D015203:Reproducibility of Results; D012964:Sodium / Q000737:chemistry*; D017550:Spectroscopy, Fourier Transform Infrared / Q000379:methods*
chemical_list: D000577:Amides; D002414:Cations, Monovalent; D012964:Sodium; D011188:Potassium