Molecular-Level Evidence of the Surface-Induced Transformation of Peptide Structures Revealed by Scanning Tunneling Microscopy

Molecular structures of polypeptide molecules (DELERRIRELEARIK) adsorbed at liquid−solid interfaces of graphite have been studied using scanning tunneling microscopy (STM). The polypeptide is originally stable with an α-helical conformation in solution and in its crystal states. STM observations reveal that the adsorbed polypeptides form homogeneous β-sheet-like assemblies on the graphite surface. The separation (4.7 ± 0.1 Å) between two neighboring polypeptides and the full lengths of the polypeptides determined from STM images suggest distinctively different molecular conformations from the α-helical structure. The N 1s peak in the X-ray photoelectron spectroscopy (XPS) spectrum confirmed the presence of polypeptides on the graphite surface. In addition, the circular dichroism (CD) results provide supporting evidence that the polypeptides would undergo a structural transformation to β-sheet secondary structure upon the addition of graphite particles to the peptide solution. Such conformational rearrangements upon adsorption on a hydrophobic surface could benefit the studies on protein−surface interactions.