Conformational heterogeneity of surface-grafted amyloidogenic fragments of alpha-synuclein dimers detected by atomic force microscopy.

A force-spectroscopy-based approach is used to characterize separation between amyloidogenic peptide fragments of alpha-synuclein. Interactions between individual molecules are studied using a scanning-force-microscopy-based technique. Alpha-synuclein fragments are attached to the solid surfaces via flexible long poly-(ethylene glycol) linkers removing aggregation state uncertainty of solution-based approaches and spurious surface effects. Tethering one fragment to the scanning probe tip and another fragment to the second surface ensures that interactions between tethered molecules are studied. Control experiments with only one tethered peptide indicate peptide-peptide interactions as the source of observed interaction forces in the double-tether experiment. The temperature dependence of rupture forces from 17.5 degrees C to 40 degrees C reveals similar molecular parameters indicating that no significant conformational changes occur in the associated molecules over this temperature range. Rate-dependent measurements indicate conformational heterogeneity of joined peptide molecules.