| Abstract
| - Single-walled carbon nanohorns (SWNHs) are interesting carbon nanostructures that have applicationsto science and technology. Using M13 phage display technology, polypeptides directed again SWNHssurfaces have been created for a number of nanotechnology and pharmaceutical purposes, yet the molecularmechanism of polypeptide sequence interaction and binding to SWNHs surfaces is not known. Recently,we identified a linear 12-AA M13 phage pIII sequence, NH-12-5-2 (DYFSSPYYEQLF), that binds withhigh affinity to SWNHs surfaces. To probe the structure of this pIII tail polypeptide further, we investigatedthe conformation of a model peptide representing the 12 AA NH-12-5-2 sequence. At neutral pH, theNH-12-5-2 model polypeptide is conformationally labile and exhibits two-state conformational exchangeinvolving the D1−S5 N-terminal segment. Simultaneous with this conformational exchange process is theobservation that the P6 residue exhibits imido ring conformational variation. In the presence of the structure-stabilizing solvent, TFE, or at pH 2.5, both the exchange process and Pro ring motion phenomena disappear,indicating that the structure of this peptide sequence can be stabilized by extrinsic factors. Interestingly,we observe NMR parameters (ROEs, 3J coupling constants) for NH-12-5-2 in 90% v/v TFE that are consistentwith the presence of a partial helical structure, similar to what was observed at low pH in our earlier CDexperiments. We conclude that the NH-12-5-2 model polypeptide sequence possesses an inherentconformational instability that involves the D1−S5 sequence segment and the P6 residue but that thisinstability can be offset by extrinsic factors (e.g., charge neutralization, imido ring interconversion, andhydrophobic−hydrophobic interactions). These nonbonding interactions may play a role in the recognitionand binding of this phage sequence region to SWNHs surfaces.
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