Unraveling the structure and dynamics of biomolecular systems requires spectroscopic methods that are able to probe fast processes. Vibrational spectroscopy is an attractive tool for such purposes, as -in contrast to X-ray diffraction or NMR spectroscopy -it potentially offers femtosecond time resolution and is directly applicable to biomolecules in aqueous solution . In particular two-dimensional infrared (2D-IR) spectroscopy is a versatile spectroscopic method that can probe the secondary and tertiary structure of biomolecules and that can be used to obtain a detailed picture of the ultrafast dynamics in proteins .