Studies of interactions between peptides/proteins and lipid bilayers using sum frequency generation vibrational spectroscopy

The orientation of peptides and proteins on surfaces can have drastic implications on the function of these interfacial molecules. Interfacial proteins and peptides can play crucial roles in biological applications and processes such as antimicrobial selectivity, membrane protein activity, biocompatibility, and biosensing performance. The alpha-helical and beta-sheet structures are the most widely encountered secondary structures in peptides and proteins. The orientation of interfacial alpha-helical and beta-sheet structure can be determined using a combination of linear and second order nonlinear optical vibrational spectroscopies, namely Attenuated Total Reflectance Fourier Transformation Infrared Spectroscopy (ATR-FTIR) and Sum Frequency Generation (SFG) vibrational spectroscopy. Here in this dissertation, orientation determination methods of the interfacial alpha-helical, 3-10 helical and beta-sheet structures, using the combined ATR-FTIR and SFG spectroscopic techniques, have been systematically developed. SFG was used to probe multiple amide I vibrational modes, which are related to their respective molecular hyperpolarizability tensor components through the orientation of the studied secondary structures. By implementing the bond additivity model along with group theory, the molecular hyperpolarizability tensor was determined for the SFG active vibrational modes of the secondary structures from the calculated IR transition dipole moment and the Raman polarizability tensor. The SFG susceptibility ratio of the signals collected in different polarization combinations, together with polarized ATR-FTIR amide I signals, can be used to determine the orientation angles of the interfacial secondary structures being studied. As an illustration of the methodology, the orientations of magainin 2 (an alpha-helical peptide), Cytochrome b5 (an alpha-helical structure containing protein), tachyplesin I (a beta-sheet peptide), at various interfaces were determined.