The metal-ligand stretching frequencies in metalloproteins fall in the far-infrared region. The iron atom in the hemeprotein, myoglobin, binds four nitrogen ligands from the heme group, one nitrogen ligand from a histidine residue in the protein, and the sixth coordination position is the oxygen-binding site. Since other ligands such as carbon monoxide and nitric oxide also competitively bind to the oxygen-binding site, it is important to characterize the structural and electronic intermediates that are involved in the ligand-binding process. Since the iron-ligand bond can be photolyzed at 10 K, we are Hable to use photolyzed/unphotolyzed difference FTIR to probe vibrational modes such as (1) the iron-ligand stretching frequency, (2) the iron-proximal histidine stretching frequency, and (3) heme doming modes, all of which differ in the ligand-bound and photolyzed states. Resonance Raman studies have also been used to identify some of these modes. However, the conditions by which several of them are resonance-enhanced are still unclear, making infrared spectroscopy a necessary alternative. In the past, we have been unsuccessful with myoglobin films in polyvinyl alcohol (PVA) on polyethylene due to (1) the temperature-dependence of the far infrared spectrum of PVA and (2) Hthe optically-opaque nature of polyethylene, making it difficult to Hphotolyze myoglobin at low temperature. Recently, we have shown that Hsolution samples in 75:25 glycerol:water (50 ?m pathlength) between Hsapphire windows are reasonably transparent in the far infrared region Hbelow 100 K and completely transparent in the visible. These findings Hwill greatly improve the ability to (1) photolyze the sample, (2) Hcollect high quality difference spectra, and (3) collect far-infrared Hprotein spectra in solution?a more biologically relevant state of the protein.
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