Avoiding interferences from Good's buffers: A contiguous series of noncomplexing tertiary amine buffers covering the...

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 alpha-, beta-, or gamma-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 degrees C, mu = 0.10 M (NaNO3). The values for four compounds in this series are reported here for the first time: viz., N,N'-diethylpiperazine for which log Ka1m = 4.67 +/- 0. 03 and log Ka2m = 8.83 +/- 0.02; N,N,N',N'-tetraethylmethylenediamine for which log Ka1m < 1 and log Ka2m = 11.01 +/- 0.03; N,N'-diethyl-N,N'-bis(3-sulfopropyl)ethylenediamine for which log Ka1m = 5.75 +/- 0.03 and log Ka2m = 9.37 +/- 0.02; and piperazine-N,N'-bis(2-ethanesulfonic acid) (Pipes) for which log Ka1m = 2.81 +/- 0.01 (the value of log Ka2m having been previously reported).