Photo-lability of CO bound to Mo-nitrogenase from Azotobacter vinelandii

In the presence of CO and under turnover conditions, Mo-nitrogenase generates three different electron paramagnetic resonance (EPR) signals. One of the signals, lo-CO, is an S=1/2 signal and occurs under low CO concentrations. The other two signals, hi-CO ( S=1/2) and hi(5)-CO ( S=3/2) displace the lo-CO as the CO concentration is raised above 0.05 atm. Irradiation of hi-CO with visible light at 12 K converts it into lo-CO. Using a series of color filters, the corrected action spectrum is determined and shown to contain 2–3 broad maxima in the region 350–730 nm. The conversion of lo-CO back into hi-CO is accomplished by warming the sample to 77 K for 5 min. Using this temperature cycle, the rate constant for the re-association of CO with lo-CO to form hi-CO is determined in the range 12–90 K. From these data, the activation energy for this reaction is calculated to be 3.9 kJ/mol. Identical irradiation of either lo-CO or hi(5)-CO induces no spectral change, showing that both of these states are photo-stable. The photo-stability of hi(5)-CO demonstrates that it is structurally different from hi-CO.