Ultrafast relaxation in picosecond photolysis of nitrosylhemoglobin

We report the successful observation of a picosecond transient difference spectrum in human nitrosylhemoglobin. The sample (23 degrees C) is excited with an approximately 8 ps, approximately 10 microJ pulse at 353 nm that generates a prompt transient having a two-component decay: the first is approximately exponential with tau = 17 +/- 4 ps, whereas the second is much weaker with an approximate tau = 100 ps. At slightly lower temperature (4 degrees C), the spectrum and time dependence are essentially unchanged. In contrast to our previous observations on carboxyhemoglobin and oxyhemoglobin, we find no long-lived photoproduct in nitrosylhemoglobin. We tentatively attribute the 17 +/- 4 ps decay to geminate recombination. These results, in conjunction with our previous work in HbO2 and HbCO, show that the rate of geminate recombination for 5 ns greater than tau greater than 5 ps increases through the series HbCO less than HbO2 less than HbNO. We note that trends are also seen for microsecond recombination rates HbCO less than HbO2 approximately equal to HbNO and for the kinetic co-operativity ratio HbCO greater than HbO2 greater than HbNO. A "critical on-barrier" model is presented that provides a consistent representation of these results. We suggest that spin-orbit effects could be a major contribution to the different recombination characteristics exhibited by the three ligands.