Unfolding process of a single peptide molecule on a substrate was investigated by atomic force microscope

The unfolding process of a peptide on a gold substrate on the scale of a single molecule was investigated by measuring force curves using an atomic force microscope (AFM). The sequence of the synthesized peptide was AAKA (AEAAKA)5A-cysteine that formed an alpha-helix in a buffer. A single peptide molecule was stretched between the AFM tip and the substrate during the force curve measurement. In the force curves, many abrupt changes or jumps in cantilever deflection were observed during the peptide stretching. It seems likely that these discontinuities were due to the breaking of some hydrogen bonds of an alpha-helix as helical parts of the peptide were elongated to a stretched form. The distribution of lengths of the peptide elongation, as calculated from the discontinuity spacing, had two peaks around 8–11 and 18–21 Å. This suggests that 1.5 or 3 turns of the helical parts of the peptide were ruptured simultaneously.