Solubility of Aβ40 peptide

In this work we measured, by using a direct approach, the equilibrium solubility of recombinant Aβ40 peptide to be S ​= ​0.36 ​± ​0.15 ​μM in aqueous solution of 20 ​mM sodium phosphate buffer at pH 7.4. Microfluidic diffusional sizing (MDS) and mass spectrometry (MALDI-TOF/TOF) with isotope standard were used to quantify the concentration of soluble Aβ40 species coexisting with formed fibrils. A sample set of Aβ40 at different concentrations was incubated at 37 ​°C under quiescent conditions and for different incubation times. After incubation, the samples were centrifuged and the concentration of Aβ40 in the supernatant was determined. This allowed us to also determine the metastable zone in supersaturated Aβ40 monomer solutions, as a function of the observation time, as well as the monomer concentration coexisting with formed fibrils. Moreover, the hydrodynamic radius (R_H) of the Aβ40 species detected in the supernatant at different time points was also measured, and showed a constant value of ∼1.8 ​nm, consistent with random coil Aβ40 monomers.