907 titrando automatic titrator

Potentiometric titration
of the Aβ_5–9 peptide and its Ni(II) complexes
was performed on a 907 Titrando automatic titrator (Metrohm, Herisau,
Switzerland) using a Biotrode combined glass electrode (Metrohm, Herisau,
Switzerland) calibrated daily by nitric acid titrations. 100 mM NaOH
(carbon dioxide-free) was used as a titrant. 1.5 mL samples were prepared
in a 96 mM KNO₃/4 mM HNO₃ solution. Complex
formation was studied using 1:0.33, 1:0.45, and 1:0.9 peptide-to-Ni(II)
molar ratios. All experiments were performed under argon at 25 °C
in the pH range of 2.7–11.5. The obtained data were analyzed
using the SUPERQUAD and HYPERQUAD programs.^{22 (link),23 (link)}

Potentiometric titrations were performed on a 907 Titrando Automatic Titrator (Metrohm, Herisau, Switzerland), using a Biotrode combined glass electrode (Metrohm), calibrated daily by nitric acid titrations [45 (link)]. One hundred millimolar NaOH (carbon dioxide-free) was used as a titrant. Samples (1.5 mL) were prepared by dissolving peptides in 4 mM HNO₃/96 mM KNO₃ to obtain 0.8–1.5 mM peptide concentrations. The Ni(II) complex formation was studied using samples in which the molar ratios of a peptide to the metal ion were 1:0.9, 1:0.45, and 1:0.3. The pH range for all potentiometric titrations was 2.7–11.6. All experiments were performed under argon at 25 °C. Three titrations were included simultaneously in calculations for protonation and five for Ni(II) complexation. The data were analyzed using the SUPERQUAD and HYPERQUAD programs [46 (link),47 (link)].

Potentiometric titrations were performed on a 907 Titrando Automatic Titrator (Metrohm, Herisau, Switzerland) with a Biotrode combined glass electrode (Metrohm). The electrode was calibrated daily by titrating an argon-bubbled 4 mM HNO₃/96 mM KNO₃ solution with 0.1 M NaOH. The solubility of the peptide in the HNO₃/KNO₃ solution was verified using UV/Vis spectroscopy. Pure peptide, stored anaerobically, was dissolved in 7.5 mL of 0.4 mM HNO₃/99.6 mM KNO₃ solution to a final concentration of 180 μM and incubated with 0–0.9 molar equivalents of Zn²⁺ (ZnCl₂) at 25 °C for 5 min before potentiometric titrations with 0.1 M NaOH. All experiments were performed under argon at 25 °C. The SUPERQUAD and HYPERQUAD programs [48 (link)] were used to analyze the data and generate the species distribution diagrams. The formation of complexes was characterized by the general equilibrium process: $pM+qH+rL \stackrel{\beta MpHqLr}{\to } MpHqLr$
$\beta MpHqLr =\frac{\left[MpHqLr\right]}{\left[M\right]p\left[H\right]q\left[L\right]r}$
where M represents metal, i.e., Zn²⁺, L represents deprotonated ligand, i.e., the 11-residue ZnT8 C-terminal peptide, H represents protons and β is the stability constant of a complex.