Picosecond Resonance Raman Spectroscopic Evidence for Excited-State Spin Conversion in Carbonmonoxy-Hemoglobin Photolysis

The structure of the carbonmonoxy-hemoglobin (COHb) photoproduct has been studied on the picosecond time scale with resonance Raman spectroscopy, by tightly focusing the 30-ps pulses of a synchronously pumped mode-locked cavitydumped dye laser on a jet stream of COHb solution. The spectrum of the photoproduct is similar to that of deoxy Hb, but the frequencies 1603 cm-1(depolarized), 1552 cm-1(anomalously polarized), and 1542 cm-1(depolarized) are 2-4 cm-1lower than those of deoxy Hb. Similar low frequencies are observed for a species believed to be the bis-tetrahydrofuran adduct of FeIIoctaethylporphyrin, containing in-plane high-spin FeII. These results indicate that in the COHb photoproduct the FeIIis already high-spin but is closer to the heme plane than in deoxy Hb. Photodissociation from a quintet ligand-field excited state of COHb is suggested. The frequency shifts relative to deoxy Hb persist when the laser pulses are lengthened to 20 ns. The apparently slow relaxation to the fully out-of-plane heme conformation of deoxy Hb is suggested to be associated with change of the globin tertiary structure.