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 alpha-helical conformation in solution and in its crystal states. STM observations reveal that the adsorbed polypeptides form homogeneous beta-sheet-like assemblies on the graphite surface. The separation (4.7+/-0.1 A) between two neighboring polypeptides and the full lengths of the polypeptides determined from STM images suggest distinctively different molecular conformations from the alpha-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 beta-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.