Ultrafast pH-jump two-dimensional infrared spectroscopy

We present a pH-jump two-dimensional infrared (2D IR) spectrometer to probe pH-dependent conformational changes from nanoseconds to milliseconds. The design incorporates a nanosecond 355 nm source into a pulse-shaper-based 2D IR spectrometer to trigger dissociation of a caged proton prior to probing subsequent conformational changes with femtosecond 2D IR spectroscopy. We observe a blue shift in the amide I mode (C═O stretch) of diglycine induced by protonation of the terminal amine. This method combines the bond-specific structural sensitivity of ultrafast 2D IR with triggered conformational dynamics, providing structural access to multiscale biomolecular transformations such as protein folding.