Guanidinium chloride (GdmCl), LiCl, KCl, KSCN, KI, and l -alanyl-l -alanine (2Ala) were used without further purification. Solutions for DRS and NMR experiments were prepared using H2O with a specific resistivity >18 MΩcm−1 from a Millipore MILLI-Q purification unit or using D2O for infrared experiments. All samples were prepared by weighing the appropriate amount of salt and peptide into a 1 mL volumetric flask using an analytical balance. The peptide concentration was kept constant at 250 mM for all samples, while the salt concentration was varied from 0.05 M to 1.05 M for DRS and NMR experiments. For infrared experiments, we used salts concentrations of 1.05, 3, and 5 M.
For DRS experiments, 1 mL of each sample was brought in contact with the coaxial probes. For NMR experiments, 0.5 mL of the samples were placed in an NMR tube together with a capillary filled with DMSO-d6 for referencing. For all infrared experiments, solutions were contained between two CaF2 windows (2 mm thickness, 2.54 cm diameter) separated by a 25 µm Teflon spacer. The infrared absorption spectra were measured right before collecting the 2D-IR data.
For DRS experiments, 1 mL of each sample was brought in contact with the coaxial probes. For NMR experiments, 0.5 mL of the samples were placed in an NMR tube together with a capillary filled with DMSO-d6 for referencing. For all infrared experiments, solutions were contained between two CaF2 windows (2 mm thickness, 2.54 cm diameter) separated by a 25 µm Teflon spacer. The infrared absorption spectra were measured right before collecting the 2D-IR data.
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