Structures and Dynamics using two-dimensional infrared and sum frequency generation spectroscopies
Two dimensional infrared (2D IR) spectroscopy, which is the infrared analog of 2D NMR, has the ability to resolve congested spectra better than linear infrared spectroscopy by adding an extra dimension. When multiple vibrational modes are present in the system of interest, the 2D IR spectrum depends critically on whether these modes are coupled to one another. One of the major strengths of 2D IR over conventional linear IR is that coupling between different modes is probed directly, and existence of cross-peaks in a 2D IR spectrum is evidence for coupling between different vibrations. In the event that there are chemically distinct species present in equilibrium, wherein fast chemical processes like hydrogen bond making and breaking interchange populations between the species, cross peaks appear on the time scales of the equilibrium kinetics of this chemical exchange. Examples of such exchange dynamics, observed for aromatic nitriles in methanol will be presented. Similar interpretations can be applied to the ultrafast dynamics of liquids, which can be thought of as a distribution of well-defined structures characterized by different solvation environments evolving spectrally on a range of time scales. Hence, the spectral diffusion seen in 2D IR experiments that derives from decay of the vibrational frequency correlations might be thought of as exchange among multiple solvent−solute configurations, and these underlying configurations are resolvable when the solvent dynamics are significantly slower than bulk. 2D IR spectra of amide vibrations in proteins and peptides exhibit solvent exchange under certain conditions, thus verifying the above hypothesis, and examples of the same will be discussed. The capabilities of 2D spectroscopy can be extended to surfaces through two-dimensional sum frequency generation (2D SFG) spectroscopy, which is a novel technique capable of measuring spectra analogous to 2D IR but with monolayer sensitivity and SFG selection rules. Applications of 2D SFG to exploring structures of a peptide segment FGAIL, a conserved sequence found in the islet amyloid polypeptide, will be presented. The 2D SFG spectra of FGAIL on model membranes reveal how hydrogen bonding interactions can play a vital role in the formation of aggregates on membranes, which is at the heart of understanding amyloid diseases such as type II diabetes. New technological advances that implement multi beam detection schemes in 2D IR experiments using mid-IR focal plane arrays will be shown. Details of future research plans will also be presented.