Direct observations of ligand dynamics in hemoglobin by subpicosecond infrared spectroscopy.
The photodissociation of CO from HbCO at ambient temperature is studied by means of a femtosecond IR technique. The bleaching of the FeCO absorption and the appearance of a new IR absorption near that of free CO are both observed at 300 fs after optical excitation. The bleach does not recover on the time scale of a few picoseconds but does recover by approximately 4% within 1 ns, which suggests that a barrier to recombination is formed within a few picoseconds. The CO spectrum does not change significantly between 300 fs and 1 ns, suggesting that the CO quickly finds some locations in the heme pocket that are not more than a few angstroms from the iron. The de-ligated CO appears in its ground vibrational level. There is evidence that 85 +/- 10% of this CO remains in the heme pocket at 1 ns; it probably resides there for 50 ns. The flow of excess vibrational energy from the heme to the solvent was directly observed in the IR experiments. The heme cools within 1-2 ps while thermal disruption of the surrounding solvent structure requires approximately 30 ps.
Anfinrud
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PA
Department of Chemistry, University of Pennsylvania, Philadelphia 19104.
Han
C
C
Hochstrasser
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RM
D002263:Carboxyhemoglobin / Q000378:metabolism*; D003903:Deuterium; D017666:Deuterium Oxide; D006454:Hemoglobins / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D007834:Lasers; D008024:Ligands; D010782:Photolysis; D013055:Spectrophotometry, Infrared / Q000379:methods; D013997:Time Factors; D014867:Water
D006454:Hemoglobins; D008024:Ligands; D014867:Water; D002263:Carboxyhemoglobin; D003903:Deuterium; D017666:Deuterium Oxide