Paper BI-WeA12
Probing Orientation and Conformation of α-Helix and β-Sheet Model Peptides on Self-Assembled Monolayers with SFG and NEXAFS Spectroscopy

Wednesday, October 22, 2008, 5:20 pm, Room 202

Understanding the interaction of proteins and peptides with engineered surfaces from first principles is essential for the design of biomaterials which are applicable in antifouling, implant technology and immunosensors. Controlled immobilization of peptides onto artificial biointerfaces plays a key role in these technologies and it is of crucial importance to develop tools to examine interfacial properties of adsorbed peptides such as orientation, and secondary structure. In this study we used sum frequency generation (SFG) vibrational spectroscopy and near edge X-ray absorption fine structure (NEXAFS) spectroscopy to characterize the structure of α-helix and β-strand model peptides on self-assembled monolayers (SAMs). The formation of peptide monolayers was confirmed using X-ray photoelectron spectroscopy. The α-helix peptide is a 14-mer and the β-strand is a 15-mer of hydrophilic lysine (K) and hydrophobic leucine (L) residues with a hydrophobic periodicity of 3.5 and 2, respectively. Both peptides have the hydrophobic side-chains on one side and the hydrophilic on the other. The SAMs used as hydrophobic and hydrophilic model surfaces were prepared from alkane thiols on gold having either charged COOH or hydrophobic CH₃ units as terminal groups. For SFG studies we used the deuterated analog of the latter SAM. SFG spectra collected in the C-H region exhibit strong peaks near 2965 cm^-1, 2940 cm^-1 and 2875 cm^-1 related to ordered leucine side chains on both surface chemistries. The relative phase of these features revealed the orientation of the leucine side chains. On COOH a relative phase of 1.4 and 1.6 rad for α-helix and β-strand, respectively, showed that the leucine was oriented away from the surface while a phase of 0 rad for both peptides on CH₃ proved that the leucines are oriented towards the interface. Amide I peaks observed at 1656 cm^-1 for the α-helix peptide confirm that the secondary structure is preserved on both SAMs. A strong linear dichroism related to the amide π* orbital at 400.1 eV was observed in the nitrogen K-edge NEXAFS spectra for the β-strand peptides on both surfaces, suggesting that the peptides are oriented parallel to the surface with the side-chains normal to the interface. For the α-helix the dichroism of the amide π* is weak, probably due to the broad distribution of amide bond orientations for this secondary structure.