Real-Time In Situ Secondary Structure Analysis of Protein Monolayer with Mid-Infrared Plasmonic Nanoantennas.

Dynamic detection of protein conformational changes at physiological conditions on a minute amount of samples is immensely important for understanding the structural determinants of protein function in health and disease and to develop assays and diagnostics for protein misfolding and protein aggregation diseases. Herein, we experimentally demonstrate the capabilities of a mid-infrared plasmonic biosensor for real-time and in situ protein secondary structure analysis in aqueous environment at nanoscale. We present label-free ultrasensitive dynamic monitoring of β-sheet to disordered conformational transitions in a monolayer of the disease-related α-synuclein protein under varying stimulus conditions. Our experiments show that the extracted secondary structure signals from plasmonically enhanced amide I signatures in the protein monolayer can be reliably and reproducibly acquired with second derivative analysis for dynamic monitoring. Furthermore, by using a polymer layer we show that our nanoplasmonic approach of extracting the frequency components of vibrational signatures matches with the results attained from gold-standard infrared transmission measurements. By facilitating conformational analysis on small quantities of immobilized proteins in response to external stimuli such as drugs, our plasmonic biosensor could be used to introduce platforms for screening small molecule modulators of protein misfolding and aggregation.