- Effect of hydroxide ion on the kinetics of triethylenetetramine displacement of tripeptides from copper(II) complexes
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Triethylenetetramine (trien) reacts rapidly with doubly deprotonated (tripeptido)cuprate(II) complexes, Cu(H-2L)-, to give Cu(trien)2+ and the free tripeptide, where L = GAG, GGA, GAibG, and Aib3 (G, glycyl; A, L-alanyl; Aib, α-aminoisobutyryl). The reactions are first order in each reactant, and the second-order rate constants (M-1 s-1, 25.0°C, μ = 1.0 M) decrease greatly with the number of methyl groups in the second and third residues: 8.4 × 104 (GAG), 7.9 × 104 (GGA), 43 (GAibG), and 0.13 (Aib3). When L is GAG, GGA, and GAibG, the values of the second-order rate constants (for [trien]T and [Cu(H-2L)-]T) increase above p[H+] 9 and reach maximum values at p[H+] 10.7 ± 0.7. The increase occurs because trien and Htrien+ are more reactive than H2trien2+. The rate constants decrease in higher base due to the formation of Cu(H-2L)(OH)2-, which is less reactive than Cu(H-2L)- with trien. Values for the stability constants of the hydroxide adduct, KOH (M-1, 25.0°C, μ = 1.0 M), are 41, 10, and 4 for L = GAG, GAibG, and GGA, respectively. A hydroxide adduct is not observed for the Aib3 complex (KOH -1). The Cu(H-2Aib3)- complex behaves differently because the reaction with trien is more sterically hindered. An acid-assisted H2trien2+ path contributes to the observed rate below p[H+] 10, and the rate begins to increase below p[H+] 9.5. However, a proton-transfer step between H3O+ and Cu(H-2Aib3)- (kH = 2.5 × 106 M-1 s-1) can limit the reaction so that it no longer depends on the [trien]T concentration. Above p[H+] 10, the reaction is first order in both [trien]T and [Cu(H-2Aib3)-], and the rate increases to a plateau at p[H+] 12. At higher p[H+] levels the rate is assisted by OH-, in contrast to the inhibition found for the other tripeptides. A third-order rate constant (0.41 M-2 s-1) is found for the path due to [trien][OH-][Cu(H-2Aib3)-].
- Schwederski, Brigitte E.,Margerum, Dale W.
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p. 3472 - 3476
(2008/10/08)
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