Abstract
The Zn(II)-cyclen-dipeptide ternary systems (where cyclen is abbreviated as L and dipeptide is glycylglycine (HL1) or glycyl-(S)-alanine (HL2)) were investigated by potentiometry applying both out-of-cell and direct titrations and by (1)H NMR spectroscopy. Especially, the (1)H NMR study was found to be very efficient to estimate speciation in the systems.
The results obtained under full equilibria indicated two main species, [Zn(L)(HL1,2)](2+) and [Zn(L)(L-1,L-2)](+), in both the systems. In the [Zn(L)(HL1,2)](2+) complex, presence of carbonyl-carboxylate chelate was confirmed, and in the [Zn(L)(L-1,L-2)](+) species, the peptide coordination is re-organized to carbonyl-amine chelate or only terminal amino group is coordinated.
Equilibrium constants describing [Zn(L)](2+)-dipeptide interaction are relatively low, log K=3.4 for Gly-Gly and 4.1 for Gly-(S)-Ala, respectively. Nevertheless, the values are slightly higher than stability constants for interaction of Zn(II) with the dipeptides (i.e. [Zn(L-1,L-2)](+) species) where a chelate formation is expected.
It indicates that interaction between Zn(II) ion in [Zn(L)](2+) and the dipeptides should be supported by some additional interactions. Potentiometry carried out under non-equilibrum condition showed different species where these additional stabilizing forces play more important role.