The three-dimensional structure of peptides and proteins and their fluctuations are essential properties responsible for the extremely high specificity of biological reactions. Progress in understanding biological processes such as enzyme reactions originates from the detailed knowledge of the secondary, tertiary and quaternary structures of the participating biomolecules. Two major spectroscopic techniques are responsible for this development x-ray scattering which can measure electron densities in a molecule and two-dimensional (2D) NMR which can measure the distances between pairs of protons in a molecule. The next step must be the determination of structures in motion over a wide range of time scales. The techniques we have developed at the RLBL, multidimensional IR spectroscopy can address this new challenge. Two-dimensional IR spectroscopy uses two ultrafast infrared pulses, a pump pulse which is tuned to a particular frequency and a broad band probe pulse which is dispersed and detected with a multichannel MCT array. The transient difference spectra are mapped as the pump pulse is tuned through the region. By plotting the observed spectrum versus the pump wavelength, 2D infrared correlations have been generated for the Amide I region of proteins and peptides.
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