Interactions of an acidic cyclooctapeptide with metal ions: microcalorimetric and fluorescence analyses
The metal-ion binding preferences of an acidic amphipathic cyclopeptide, cyclo[ D-Leu-Leu-D-Leu-Trp-(D-Glu-Glu)(2)](CP), was studied by isothermal titration calorimetry and steady-state fluorescence spectroscopy. CP adopted a partial beta structure, and variable temperature circular dichroism showed small secondary structural changes over the temperature range from 5 degrees C to 95 degrees C. The peptide did not bind alkali or alkaline earth metal ions but exhibited selectivity for some divalent transition metal ions (with association constants KCu 2+ 4.5x10(4) M-1, KZn 2+ 1.6x10(5) M-1, KCd 2+ 1.3x10(4) M-1, KHg 2+ (1) 2.2x10(6) M-1, and KHg 2+ (2) 6.5x10(3) M-1), for Pb2+ (2.0x10(5) M-1), and a trivalent Group III metal, Al3+ (1.6x10(5) M-1). The thermodynamic data show that the interaction between CP and these metal ions are spontaneous and entropically driven. A large range of binding enthalpies coupled with a smaller range of binding free energies of CP for these metal ions indicate an entropy-enthalpy compensation dependent on the ionic size of participating metal ions. The interaction of Pb2+, Hg2+, and Cu2+ with CP in aqueous solution specifically modulates the fluorescence emission properties of CP. The results of this study show that CP exhibits selectivity in metal-ion binding, which is reflected in its fluorescence spectra. The observed trends can be useful for the design of heavy-metal sensors based on fluorophore-tagged acidic cyclopeptides.