Vibrational sum frequency study on biological interfaces

Abstract

Vibrational sum frequency spectroscopy (VSFS) is a nonlinear optical process. The sum frequency signal is proportional to the square of second order nonlinear susceptibility, which is proportional to the average of polarizabilities of molecules, which is related to molecular orientation. Since the polarizabilities of molecules in bulk phase will be canceled out, a sum frequency signal can only be generated from interfaces where the inversion symmetry is broken. Because of its interfacial specificity, VSFS has been applied to study many interfacial phenomena. In this dissertation we investigated various biological interfaces with VSFS. Fibrinogen adsorption was studied at the protein/solid interface in combination with atomic force microscopy (AFM), immunoassay, and VSFS. Astonishing changes in the interfacial water orientation accompanied by the pH changes provided fibrinogen’s adsorption mechanism up to the amino acid level. Enzymatic fragmentation of fibrinogen revealed that the adsorption property of fibrinogen was mainly from the alpha C fragments of the protein. Mimicking of the fibrinogen binding site with polypeptides was successfully performed and showed very similar properties of fibrinogen adsorption. Protein stability is sensitive to the salts in solutions. The ability of ions to stabilize protein was ordered by Hofmeister in 1888 and the order is SO4 2- ≅ HPO4 2- > F- > Cl- > Br- > NO3 - > I- (≅ ClO4 -) > SCN-. Even though the phenomenon was observed in various biological systems, the origin of those ionic effects is still not well understood. We studied ion effects on alkyl chain ordering and interfacial water structure for octadecylamine, dimethyldidodecylammonium bromide, and dilauroylphosphotidyl choline monolayers. Because of its ability to probe a hydrophobic moiety and interfacial water at the same time, VSFS provided further information to understand the Hofmeister series. We found that the Hofmeister effect is a combinatorial effect of screening effects, ion binding, and dispersion forces.