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Experimental validation of theoretical potassium and sodium cation affinities of amides by mass spectrometric kinetic method measurements

Authors

  • Yuet Tsang,

    1. Department of Applied Biology and Chemical Technology and Central Laboratory of the Institute of Molecular Technology for Drug Discovery and Synthesis, Hong Kong Polytechnic University, Hung Hom, Hong Kong
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  • Fung Ming Siu,

    1. Department of Applied Biology and Chemical Technology and Central Laboratory of the Institute of Molecular Technology for Drug Discovery and Synthesis, Hong Kong Polytechnic University, Hung Hom, Hong Kong
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  • Chung Shun Ho,

    1. Department of Chemical Pathology, Chinese University of Hong Kong, Shatin, New Territories, Hong Kong
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  • Ngai Ling Ma,

    Corresponding author
    1. Materials and Industrial Chemistry Program, Institute of High Performance Computing, 1 Science Park Road, #01-01, The Capricorn, Singapore Science Park II, Singapore 117528
    • Materials and Industrial Chemistry Program, Institute of High Performance Computing, 1 Science Park Road, #01-01, The Capricorn, Singapore Science Park II, Singapore 117528.
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  • Chun Wai Tsang

    Corresponding author
    1. Department of Applied Biology and Chemical Technology and Central Laboratory of the Institute of Molecular Technology for Drug Discovery and Synthesis, Hong Kong Polytechnic University, Hung Hom, Hong Kong
    • Department of Applied Biology and Chemical Technology and Central Laboratory of the Institute of Molecular Technology for Drug Discovery and Synthesis, Hong Kong Polytechnic University, Hung Hom, Hong Kong.
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  • An Area of Excellence of the University Grants Committee (Hong Kong)

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. Copyright © 2004 John Wiley & Sons, Ltd.

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