Electrospray ionization mass spectrometry (ESI-MS) was used to study the binding of selected group II and divalent transition-metal ions by cyclo(Pro-Gly) 3 (CPG3), a model ion carrier peptide. Metal salts (CatX n ) were combined with the peptide (M) at a molar ratio of 1:10 M/Cat in aqueous solvents containing 50% vol/vol acetonitrile or methanol and 1 or 10 mM ammonium acetate (NH 4 Ac). T he biogeochemistry of a particular metal ion is strongly influenced by its ability to form stable complexes with dissolved organic compounds under ambient conditions [1][2][3]. Identification and characterization of these complexes is critical to understanding and predicting the bioactivity and distribution of metals in biological systems and the environment. Peptides constitute one of the most important classes of compounds involved in the binding and transportation of metal ions [4,5]. Cysteine-containing peptides such as phytochelatins (PCs) and metallothioneins (MTs) have long been implicated in the binding, detoxification, and sequestration of heavy metals [5]. These compounds have been found in bacteria, plants, and animals, suggesting a universal role in metal detoxification and homeostasis. Alteration of MT expression and PC biosynthesis has been investigated as a means of increasing metal tolerance and uptake in plants, with the intention of using them for remediation of metalcontaminated sites [5][6][7][8]. However, this approach currently is limited by the low specificity with which metals such as cadmium, copper, and zinc bind to and induce expression of these peptides. Moreover, interactions between these metals and MTs or PCs in vivo remain poorly understood, while apparent localization of MT-metal complexes in roots limits uptake, translocation, and recovery of metals from aerial tissues [5,6].Biosynthesis and release of peptides and related compounds is another mechanism by which microorganisms such as soil bacteria and marine phytoplankton are able to regulate uptake of essential and nonessential metals from their surroundings [9,10]. Such compounds include cysteine-containing tripeptides similar to glutathione and PCs [11] and cyclic peptides such as rhodotorulic acid, a bacterial siderophore [12]. Many cyclic peptides have antibiotic or antifungal properties that arise from the ability to bind and transport metals across biological membranes [4,13,14]. These peptides show a high degree of specificity, forming stable complexes with metal ions of a particular charge and ionic radius. Metal binding specificity is related to the size and conformation of the peptide and the type and orientation of groups involved in metal binding [14]. It also is affected by solvent-metal and solvent-ligand interactions [4,14]. Biosynthesis of cyclic peptides in plants could, in principle, be used to promote uptake of specific metals, thereby enhancing the phytoremediative properties of metal (hyper)accumulators or the
