The monomer–dimer transition of insulin has been probed with two-dimensional infrared spectroscopy and related infrared spectroscopies to isolate spectral signatures of the conformational changes concomitant with dissociation. These experiments were atomistically interpreted using 2D IR spectra calculated from an ensemble of monomer and dimer structures including the effects of disorder, which provided a complement and a point of comparison to NMR and X-ray crystallography models. The amide I ν⊥ mode, which is delocalized over both monomer units through an intermolecular antiparallel β sheet, was lost upon dimer dissociation and shifts were observed in the ν∥ β-sheet and α-helix bands. These spectral changes provided a structurally sensitive probe of dimer dissociation, which was used to measure the binding constant, KD, and to parameterize a thermodynamic model for the dimer fraction. The solvent conditions surveyed the effects of ethanol and salt addition on the dimer fraction in acidic, deuterated water as a function of temperature. It was found that addition of ethanol had a significant destabilizing effect on the dimer state, and shifted KD from 70 μM in D2O to 7.0 mM in 20% EtOD at 22 °C. Simulation of the monomer 2D IR spectra indicates that the B-chain C terminus is partially disordered, although not fully solvated by water.
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