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16
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0004226423
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Special Publication No. 17, The Chemical Society, London “Best” values near 25° and moderate ionic strengths are chosen
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“Stability Constants,” Special Publication No. 17, The Chemical Society, London, 1964. “Best” values near 25° and moderate ionic strengths are chosen.
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18
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0000617905
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Y. Nozaki, F. R. N. Gurd, R. F. Chen, and J. T. Edsall, J. Amer. Chem. Soc., 79, 2123 (1957).
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33947338546
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2+ species is no longer the major one above pH 4, as demonstrated graphically in the figure of ref 17
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2+ species is no longer the major one above pH 4, as demonstrated graphically in the figure of ref 17.
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0006402862
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The formation constants reported in this reference were determined at high molar ratios of amide to metal ion and refer to simple coordination. The constants may be less than those determined at lower ratios where chelation by a ligand molecule might assume more importance
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R. B. Martin and J. T. Edsall, J. Amer. Chem. Soc., 82, 1107 (1960). The formation constants reported in this reference were determined at high molar ratios of amide to metal ion and refer to simple coordination. The constants may be less than those determined at lower ratios where chelation by a ligand molecule might assume more importance.
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J. Amer. Chem. Soc.
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These authors propose a glycine-like binding mode for histidine and copper(II) at all pH >5.5. Their main argument rests upon an analysis of “discontinuous” titrations, where the observed average number of protons displaced from each ligand molecule at a given pH is compared with that calculated on the basis of an assumed binding mode. The “observed” values are evaluated on the assumption “that the reaction between metal ion and ligand is complete.” This assumption is false, and as a result their argument is destroyed. In the histamine, alanine, and imidazole examples offered, their observed values at low pH are always less (up to 43% less) than the calculated values with these ligands of presumed known complex structure, because of their faulty assumption of negligible quantities of unbound ligand. Indeed, the evaluation of this quantity is part of standard procedures for determination of formation constants. For the histidine complex their evaluation of the presumed bidentate binding sites, whether through bidentate nitrogen or glycine-like modes, depends upon comparisons of calculated values for each mode and observed values assuming complete 2:1 complex formation and no protonated complexes. The two calculated values differ significantly only at low pH, precisely where the authors' faulty assumptions are most seriously in error. Even so, the fact that their observed values are greater than those calculated for the glycine-like mode demands that at least some ligands are not bound in this way but instead through the rejected bidentate nitrogen mode (they consider tridentate binding unlikely on other grounds). Features of this and other arguments by these authors are considered implicitly in this paper
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B. Sarkar and Y. Wigfield, J. Biol. Chem. 242, 5572 (1967). These authors propose a glycine-like binding mode for histidine and copper(II) at all pH >5.5. Their main argument rests upon an analysis of “discontinuous” titrations, where the observed average number of protons displaced from each ligand molecule at a given pH is compared with that calculated on the basis of an assumed binding mode. The “observed” values are evaluated on the assumption “that the reaction between metal ion and ligand is complete.” This assumption is false, and as a result their argument is destroyed. In the histamine, alanine, and imidazole examples offered, their observed values at low pH are always less (up to 43% less) than the calculated values with these ligands of presumed known complex structure, because of their faulty assumption of negligible quantities of unbound ligand. Indeed, the evaluation of this quantity is part of standard procedures for determination of formation constants. For the histidine complex their evaluation of the presumed bidentate binding sites, whether through bidentate nitrogen or glycine-like modes, depends upon comparisons of calculated values for each mode and observed values assuming complete 2:1 complex formation and no protonated complexes. The two calculated values differ significantly only at low pH, precisely where the authors' faulty assumptions are most seriously in error. Even so, the fact that their observed values are greater than those calculated for the glycine-like mode demands that at least some ligands are not bound in this way but instead through the rejected bidentate nitrogen mode (they consider tridentate binding unlikely on other grounds). Features of this and other arguments by these authors are considered implicitly in this paper.
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J. Biol. Chem.
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Sarkar, B.1
Wigfield, Y.2
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43
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H. C. Freeman, J. M. Guss, M. J. Healy, R. P. Martin, and C. E. Nockolds, Chem. Commun., 225 (1969).
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Freeman, H.C.1
Guss, J.M.2
Healy, M.J.3
Martin, R.P.4
Nockolds, C.E.5
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