Automated 2D IR spectrometer mitigates the influence of high optical densities

Two-dimensional infrared spectroscopy is a powerful tool for studying molecular structure and kinetics. However, standard ways of implementing the technique are not amenable to samples with high optical densities. In this paper, we demonstrate that a shaper-based automated 2D IR spectrometer largely compensates for most of the distortions caused by high optical densities. By comparing a series of 2D IR spectra collected with varying concentrations and sample thicknesses, we find that high quality 2D IR spectra can be obtained at optical densities of >1.2 when these spectra are collected using a pulse shaping method recently developed in our lab. Furthermore, distortions due to high OD primarily appear along the pump axis and are largely absent along the probe axis. Using this knowledge, we have applied our approach to study a high optical density sample of a truncated form of the human islet amyloid peptide that is involved in Type 2 diabetes. Our methodology promises to aid in the interpretation of 2D IR lineshapes for systems where the optical density cannot be controlled, such as in protein folding or chemical reactions where large changes in optical density occur during the kinetics.