FEMTOSECOND MID-IR SPECTROSCOPY USED TO PROBE THE ULTRAFAST DYNAMICS OF LIGAND MOTION SUBSEQUENT TO PHOTODISSOCIATION OF CARBONMONOXY-MYOGLOBIN

Author Institution: Department of Chemistry, Harvard UniversityLigand association to and dissociation from the heme prosthetic group [iron (II)-containing porphyrin] of myoglobin (Mb) is essential to its function as an oxygen carting/storing protein. Elucidation of the motion of the ligand within the hydrophobic interior of the protein and particularly near the iron (called the heme pocket) is important in order to understand the impact that ligand dynamics has on the function of the protein We have developed a time-resolved mid-IR absorption spectrometer that provides $< 250$ fs time-resolution, $\approx 3.3 - 5.5 \mu m$ spectral coverage. $2 cm^{-1}$ spectral resolution, and sufficient sensitivity to probe the vibrational absorption spectrum of photodissociated CO which provides a sensitive probe of the ligand’s external environment. The absorption spectrum of photodissociated CO within the heme pocket of Mb (1) consists of two narrow ($\approx 8 cm^{-1}$) features suggesting the CO is well localized, (2) grows with a time constant of $\approx$ 1 ps suggesting the motion of the CO is initially diffusive Followed by localization near the iron, and (3) remains for $\approx$ 200 ns suggesting CO is localized in a very stable ``docking site”. This ``docking site” allows the ligand to bind metastably with protein and may provide n holding bay for the ligand until the protein adopts a conformation favorable for ligand binding or escape