| Abstrakt: |
Of the 20 well-known buffers proposed by Good, all but 3 form metal ion complexes which can result in serious interferences, particularly in protein analyses. The structural features responsible for such complex formation have been identified. Based on a mechanistic analysis of the metal complexation process, it is proposed that tertiary amine compounds, having N-substituents which are ethyl or larger, are sterically inaccessible for initial bond formation with solvated metal ions in aqueous solution. Thus, in the absence of donor atoms on the α-, β-, or γ-carbons, metal complexation cannot proceed. The proposed noncomplexing compounds include Good's 3 noncomplexing buffers (Mes, Mops, Pipes) plus six related species. Mixed-mode acid dissociation constants (hydrogen ion in terms of activity, conjugate acid–base species in molar concentrations) have been determined for all compounds in their protonated form at 25°C, μ = 0.10m(NaNO3). The values for four compounds in this series are reported here for the first time: viz.,N,N′-diethylpiperazine for which logKa1m= 4.67 ± 0.03 and logKa2m= 8.83 ± 0.02;N,N,N′,N′-tetraethylmethylenediamine for which logKa1m< 1 and logKa2m= 11.01 ± 0.03;N,N′-di ethyl-N,N′-bis(3-sulfopropyl)ethylenediamine for which logKa1m= 5.75 ± 0.03 and logKa2m= 9.37 ± 0.02; and piperazine-N,N′-bis(2-ethanesulfonic acid) (Pipes) for which logKa1m= 2.81 ± 0.01 (the value of logKa2mhaving been previously reported). |
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