Changes in the apparent quantum efficiency for photolysis of Hb(CO)1

Area of Science:

• Biochemistry
• Photochemistry
• Protein Dynamics

Background:

• The allosteric state of hemoglobin influences carbon monoxide (CO) geminate recombination and photolysis efficiency.
• Previous research indicates the protein environment around heme groups affects CO dynamics in hemoglobin.

Purpose of the Study:

• To investigate the time-dependent changes in the apparent quantum efficiency (AQE) for carboxyhemoglobin (Hb(CO)1) photolysis.
• To correlate observed AQE changes with conformational states of hemoglobin.

Main Methods:

• Utilized combined flow/flash experiments to measure AQE.
• Experiments were conducted at 20°C in 0.1 M phosphate buffer (pH 7.0) with low CO saturation (<10%).
• AQE was measured as a function of time delay after Hb(CO)1 formation.

Main Results:

• AQE for Hb(CO)1 photolysis decreased from approximately 1.0 at short times to ~0.6 after 2 seconds.
• The observed AQE values are influenced by the competition between geminate CO recombination and CO escape to solution.
• The decrease in AQE suggests conformational changes in Hb(CO)1 over time.

Conclusions:

• Observed changes in AQE reflect alterations in rapid reaction steps, likely due to hemoglobin conformational changes.
• The data is consistent with the conversion of one or more hemes to an R-like state.
• A simple two-state allosteric model could not fully describe these observed conformational changes.