The role of peptides in the copper transport of aquatic ecosystems

Abstract

The flux of copper through an aquatic ecosystem is controlled by three types of process, namely 1. hydrodynamic mixing and particle transport (physical process) 2. abiotic transfers from dissolved to particulate phases and vice versa (purely chemical processes) 3. biological process (e.g. assimilation, excretion, decay) The study of metal balances in lakes has shown that within the system the phytoplankton are mainly responsible for copper transfer from the dissolved to the particulate phase [1]. In aerobic systems the process of type b) is negligible [2]. In anaerobic systems however copper can be co-precipitated on hydrous ferric oxides formed at the interface of the system [3, 4]. In all three phenomena natural organic ligands play a crucial role. Their copper complexes are not available for phytoplankton. The thermodynamic and physicochemical properties of the natural copper complexing ligands (ligands conc.: ~5 × 10-7 mol/mg dissolved organic carbon, copper conc.: ~1 × 10-8 M) lead to the following structural hypothesis [5]: 1. a molecule of the fulvic acid type (molecular size: 1-3 × 103 daltons) is folded to form the most stable copper complex, e.g. a CuN4 or CuN2O2 chromophore 2. any peptide chain has one preferential site for copper at the N-terminal end. The second hypothesis was investigated with model peptides having different chain lengths (from tri- to heptapeptides) and different side chains [6, 7]. Penta- or higher peptides form more stable complexes of the type Cu(H-3L) (the dominant form together with Cu(H-2L) at pH = 8) than shorter peptides. This effect ([Delta]G = 6 kJ/mol) is due to the formation of a fifth chelate ring to the axial position (Fig. 1). The side chain in position 5 (r5) can destabilize the complex because of steric (chirality and volume) and electrostatic (charge of R5) reasons. It is concluded that certain peptide fragments can compete successfully with inorganic ligands forming copper complexes under natural water conditions at pH> 8. However the model compounds do not reflect the observed characteristics of natural ligands in the pH range from 8 to 9.