Fluorogenic Cyclooctapeptides and Metal Ion Sensing

Abstract

Introduction Optical sensors for the detection of heavy metal ions have wide applications in environmental chemistry, and are directly relevant for the development of a more accurate and rapid identification of harmful metal ions in the environment. Although the design of chemical sensors for such metal ions has been reported [1,2] we are especially interested in developing cyclopeptide-based motifs for selective metal ion binding and sensing. Recent studies have shown that a model acidic cyclopeptide, c[D-Glu-Glu-D-Glu-Glu-D-Leu-Leu-D-Leu-Trp], (CP1), exhibited selective binding and specific fluorescent responses towards some divalent transition metal ions among the surveyed alkali, alkaline earth, transition and heavy metal ions [3]. The present study was undertaken to develop a better understanding of the underlying bimolecular interactions between cyclooctapeptides with acidic groups, (CP1) and c[Glu-D-Leu-Glu-D-Leu-Glu-D-Leu-D-Glu-D-Trp] (CP2), or sulfur containing residues, c[D-Leu-Leu-D-Phe-Met-D-His-Met-D-His-Lys(Dansyl)], (CP3), with Hg, Pb and Cd. We report here the thermodynamic parameters for the interactions between these cyclopeptides with these metal ions as determined by Isothermal Titration Calorimetry (ITC). In order to evaluate the signal transduced upon peptide-metal ion interactions, the fluorescence emission spectral changes following titration of the cyclopeptide with these metal ions was compared. Standard Stern-Volmer formalism and the dependence of fluorescence intensity on quencher concentrations are presented.