Biomolecular structure at solid-liquid interfaces as revealed by nonlinear optical spectroscopy

Abstract

In this review, we have presented examples of biomolecules at the solid-liquid interface in order of increasing complexity, from amino acids to peptides and proteins to membrane embedded protein complexes. SHG and SFG offer insight into several aspects of the adsorption process and dynamics of the molecules. Structural insight is generally provided through studies that vary the polarization of the incident beam(s) and select an output polarization for detection. Partially due to the reliance on the electronic properties of the molecules, the most detailed quantitative studies are performed for small molecules. The advantage here is 2-fold. First, with less than ∼100 atoms electronic structure calculations are tractable. Second, the orientation of the molecular hyperpolarizability within the nuclear framework is well-defined. When the molecular response (elements of α(2)) is related to the measured response (elements of χ(2)), essentially solving for parameters in the presumed orientation distribution function, the orientation of chemical bonds with respect to the surface is obtained. In the case of peptides and proteins, a local mode approximation enables sections of the molecule to be treated in much the same way as for small molecules, particularly for high frequency vibrational modes. When the systems become more complicated, as in the study of live cells, the modality becomes more focused on observing and accounting for changes in the response. The combination of experimental advances and complementary simulations is pushing studies of molecules of all sizes forward. In the next 5-10 years, we expect to see the equivalent level of structural detail as is currently possible for small molecules available for proteins at surfaces.