Complex and Heterogeneous Biophysical Systems Studied with 2DIR

Abstract: I will present three projects. In the first two, we used two-dimensional infrared (2DIR) spectroscopy to study properties of some complex peptide and protein assemblies 1: Proline-arginine (PR) dipeptide repeats are an example of a growing number of intrinsically disordered proteins (IDPs) known to assemble into membrane-less organelles by liquid-liquid phase separation (LLPS), and has also been implicated in the disease mechanism of Amyotrophic lateral sclerosis (ALS). We show that formation of droplets by PR20 accompanies changes in the Amide-I spectra consistent with folding into poly-proline helical structures, providing, to our knowledge, the first piece of evidence that LLPS can drive folding of IDPs. 2: The infrared absorption spectra of cross-α fibrils, recently discovered structures derived from the biofilms of virulent strains of Staphylococcus Aureus bacteria, are virtually indistinguishable from monomeric α-helices. As such, we performed a detailed study of the nonlinear infrared response of this system under a variety of experimental conditions and showed how polarization sensitive 2DIR, in combination with both broad-band time-domain and narrow-band frequency domain spectroscopy, together can reveal a coherent nonlinear signature unique to cross-α fibrils. In addition, spatial scanning of the sample revealed underlying polymorphism in the fibrils, demonstrating that this peptide is capable of adopting both cross-α and cross-β fibril structure in the same environment. 3: Finally, while vibrational strong coupling has received a great deal of recent interest as a potential means to drive chemistry, we instead explore a potential opportunity for using strong coupling as a probe of chemical dynamics. Namely, we explore the potential of strong coupling in a linear optical measurement to probe the type of chemical dynamics that were previously accessible only through nonlinear ultrafast measurements. By tuning the Fabry Perot cavity lifetime of a strongly coupled sample, we can vary the time scales over which the photonic modes are sensitive to dynamics, providing a window into ultrafast chemical dynamics using a comparatively simple experimental design.