Probing the binding of Cu2+ ions to a fragment of the Aβ(1-42) polypeptide using fluorescence spectroscopy, isothermal titration calorimetry and molecular dynamics simulations

Steady-state and time-resolved fluorescence quenching measurements supported by isothermal titration calorimetry (ITC) and molecular dynamics simulations (MD), with the NMR-derived restraints, were used to investigate the interactions of Cu2+ ions with a fragment of the A beta((1-42)) polypeptide, A beta((5-16)) with the following sequence: Ac-Arg-His-Asp-Ser-Gly-Tyr-Glu-Val-His-His-Gln-Lys-NH2, denoted as HZ). The studies presented in this paper, when compared with our previous results (Makowska et al., Spectrochim. Acta A 153: 451-456), show that the affinity of the peptide to metal ions is conformation-dependent. AB the measurements were carried out in 20 mM 2-(N-morpholino)ethanesulfonic acid (MES) buffer solution, pH 6.0. The Stern-Volmer equations, along with spectroscopic observations, were used to determine the quenching and binding parameters. The obtained results unequivocally suggest that CuZ+ ions quench the fluorescence of HZ) only through a static quenching mechanism, in contrast to the fragment from the N-terminal part of the FPB28 protein, with sequence Ac-Tyr-Lys-Thr-Ala-Asp-Gly-Lys-Thr-Tyr-NH2 (D9) and its derivative with a single point mutation: Ac-Tyr-LysThr-Ala-Asn-Gly-Lys-Thr-Tyr- NH2 (D9_M), where dynamic quenching occurred. The thermodynamic parameters (Delta H-Pi C, Delta S-Pi C) for the interactions between Cu2+ ions and the HZ1 peptide were determined from the calorimetric data. The conditional thermodynamic parameters suggest that, under the experimental conditions, the formation of the Cu2+-HZ1 complex is both an enthalpy and entropy driven process. (C) 2016 Elsevier B.V. All rights reserved.