Detail:
Abstract: Ultrafast nonlinear spectroscopy and microscopy are powerful approaches for obtaining structural features, equilibrium dynamics, and spatial arrangements of molecular systems. In this talk, I will first describe the application of ultrafast two-dimensional infrared (2D-IR) spectroscopy to oligopeptides that serve as a paradigm for studies of backbone conformations in proteins. Using multiple pulse sequences and polarization configurations as well as strategic isotope substitutions, we demonstrate that 2D IR can provide diagnostic cross-peak patterns for distinguishing different secondary structure, probe the onset of secondary structure, and reveal complex vibrational couplings between the amide-I and amide-II modes. Experimental spectra are compared to simulations based on nonlinear response theory, vibrational eigenstates and couplings derived from DFT-optimized structures, and trajectories from molecular dynamics simulations. Our results show that 2D IR can provide a large set of spectral constraints that are useful for determining peptide structure and testing theoretical models. In the second part of my talk, I will describe a novel polarization- and phase-sensitive vibrationally-resonant sum frequency generation (SFG) microscope for rapid chemical imaging. The microscope can achieve submicrometer lateral resolution, multimodal compatibility, and heterodyne detection. We have applied it to image collagen fibers and revealed the structural orientation micro-domains within rat tail tendon tissues. Polarization and spectrally resolved analysis reveals the origin of higher contrast observed in SFG imaging, compared to SHG. The improved contrast and the ability to tune into specific molecular resonances make point-scanning SFG microscopy an attractive addition to the family of nonlinear optical imaging techniques.
