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 MLn– 1+ L ⇌ MLn, 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 MLn→ MLn– 1+ L), k–n, can be calculated from the relation Kn=kn/k–n; the directly measured kn/l mol–1 s–1 for the process MLn– 1+ L → MLnare 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.