Stability constants and relaxation spectra of L-proline and L-hydroxyproline metal complexes

Abstract

Complex formation between L-proline and L-hydroxyproline and the ions Co²⁺, Ni²⁺, and Cu²⁺ has been studied at 25 °C and ionic strength 0·1 M(KNO₃) from equilibrium and kinetic standpoints. A potentiometric method was used to determine the following stoicheiometric stability constants: proline: Ni²⁺, log K₁= 6·15, log K₂= 5·13; Co²⁺, log K₁= 5·05, log K₂= 4·22; hydroxyproline: Co²⁺, logK₂= 4·81. log K₂= 3·81. (Literature values for Cu²⁺–proline and Cu²⁺– and Ni²⁺–hydroxyproline complexes were confirmed.) Temperature-jump studies were used to determine rate constants for the process ML_{n– 1}+ L ⇌ ML_n, where M and L are metal ion and ligand respectively and n= 1 or 2. The dissociation rate constants (i.e., the rate constant for ML_n→ ML_{n– 1}+ L), k_–n, can be calculated from the relation K_n=k_n/k_–n; the directly measured k_n/l mol^–1 s^–1 for the process ML_{n– 1}+ L → ML_nare as follows: proline: Cu²⁺, k₁=(2·5 ± 0·2)× 10⁹, k₂=(2·7 ± 0·6)× 10⁸; Ni²⁺, k₁=(3·4 ± 0·6)× 10⁴, k₂=(8·7 ± 1·3)× 10³; Co²⁺, K₁=(3·5 ± 0·5)× 10⁵, k₂=(9·6 ± 0·7)× 10⁵; hydroxyproline: Cu²⁺, k₁=(7·4 ± 0·4)× 10⁸, k₂=(2·8 ± 0·6)× 10⁸; Ni²⁺,K₁=(1·2 ± 0·3)× 10⁴, k₂=(1·8 ± 0·1)× 10⁴; Co²⁺, k₁=(7·0 ± 1·4)× 10⁴, k₂=(9·0 ± 0·4)× 10⁵. It is concluded that proline behaves ds a ‘normal’ amino-acid; hydroxyproline forms complexes less readily. As these results cannot be explained on the basis of the inductive effect of the substituted hydroxy-group, another effect must play a role in these reactions. Hydrogen-bond formation between co-ordinated water molecules of the metal ion and the hydroxy-group of the ligand could provide an explanation for these observations.