Ultrafast 2D-IR spectroscopy of haemoproteins

Applications of ultrafast two-dimensional infrared (2D-IR) spectroscopy to study the structural dynamics of haem-containing proteins are reviewed. The 2D-IR experiments discussed exploit diatomic ligands bound to the haem as reporters on the dynamic protein environment in the electronic ground-state. This is possible because fluctuations of the protein give rise to inhomogeneous broadening of the ligand stretching vibrational mode that is manifest as spectral diffusion in a time-resolved 2D-IR measurement. Methods for measuring and quantifying spectral diffusion data are introduced, prior to a discussion of recent results focussing on the influence of protein structure, water ingress into the haem pocket and substrate binding on the measured dynamics. Particular emphasis will be placed on proteins featuring the ferric oxidation state of the haem ligated by a nitric oxide molecule, though comparisons with other haem systems will be drawn throughout.