Figure 3

Mass spectra of the titration of α-syn with CaCl₂. The dashed lines indicate the theoretical m/z values of the 8+ charge state of α-syn with a specific number of Ca²⁺ ions bound, at the top of the dotted line. The control shows only the unbound peak, indicated by ‘0’ and with one Na⁺ adduct still remaining after thorough purification steps, next to a contaminant (peak indicated with an arrow), which also seems to bind Ca²⁺ with a similar peak distribution as α-syn itself. With increasing CaCl₂ concentration more Ca²⁺ ions bind with a maximum of nine, defined by a S/N ≥ 3 of the peak at the correct m/z value. Grey dotted lines indicate theoretical values of the 8+ charge state of α-syn with more than nine Ca²⁺ ions bound and do not align with the peaks in the spectrum, which can be attributed to a lower number of Ca²⁺ bound to the contaminant.