Dipeptide interactions with Zn(II)–cyclen artificial model for molecular recognition

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 ¹H NMR spectroscopy. Especially, the ¹H 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)(HL^1,2)]²⁺ and [Zn(L)(L^1,2)]⁺, in both the systems. In the [Zn(L)(HL^1,2)]²⁺ complex, presence of carbonyl‐carboxylate chelate was confirmed, and in the [Zn(L)(L^1,2)]⁺ 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^1,2)]⁺ 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. Copyright © 2015 John Wiley & Sons, Ltd.