Paper BI-TuM2
Sum Frequency Generation Vibrational Spectroscopic Studies in the C-H, O-H, N-H, and Amide I Regions of Model Peptides at Solid-Liquid Interfaces

Tuesday, October 16, 2007, 8:20 am, Room 609

We have developed a library of small, model peptides and have examined their interfacial structure at model hydrophobic and hydrophilic surfaces using surface-specific sum frequency generation vibrational spectroscopy (SFG). A fourteen amino acid peptide containing hydrophobic leucine (L) and hydrophilic lysine (K) residues was synthesized and characterized. This amphiphilic α-helical peptide has sequence Ac-LKKLLKLLKKLLKL-NH2 (LK14). SFG spectra in the C-H, O-H, and N-H region reveal that at a hydrophobic deuterated polystyrene surface, methyl groups from the hydrophobic leucine residues are ordered at the interface, while the hydrophilic lysine residues adopt a random orientation (presumably due to lack of interaction with the surface). When adsorbed onto a hydrophilic silica surface, the SFG spectra reveal a completely different molecular orientation: the methyl groups now appear to have a random orientation, and the N-H groups of the lysine side chains and/or peptide backbone are now ordered. A study of the influence of the ionic strength of the solution on the structure of LK14 revealed the following results: the LK14 peptide was α-helical in solution at high ionic strength but random coil at low ionic strength. Furthermore, leucine side chain ordering on hydrophobic surfaces was not perturbed by ionic strength changes, but N-H ordering on hydrophilic surfaces had a strong dependence on ionic strength. More recently, we have developed a new optical parametric amplifier (OPA) utilizing lithium thioindate (LIS) to study the Amide I mode of the peptide backbone. LIS provides high IR energy (~175 µJ at 1500 cm-1, ~375 µJ at 2000 cm-1), a high damage threshold, and good beam quality. The high energy output of LIS allows for the study of interfacial peptide structure without having to use a total internal reflection geometry. Using this new OPA, we have seen evidence for α-helical peptide structure at both hydrophobic and hydrophilic surfaces (at high ionic strengths). Additionally, there appears to be evidence for α-helical structure of the LK14 peptide at hydrophobic surfaces in low ionic strength solutions. We are currently examining lysine homopeptides, collagen-like peptides, new experimental geometries, more biologically relevant surfaces (such as HEMA and polymers with Surface Modifying Endgroups) and molecular dynamics simulations of peptides at interfaces to aid interpretation of experimental data.