Resonance Raman characterization of the 7-ns photoproduct of (carbonmonoxy)hemoglobin: implications for hemoglobin...

Resonance Raman spectra are reported for deoxyhemoglobin (deoxyHb) and the (carbonmonoxy)hemoglobin (HbCO) photoproduct Hb by use of 7-ns YAG laser pulses at wavelengths of 416 and 532 nm, where enhancement is observed for totally symmetric and nontotally symmetric modes, respectively. The frequencies of the porphyrin skeletal modes v10, v2, v19, v11, and v3 have been determined to be 1602, 1559, 1553, 1542, and 1466 cm-1 in Hb. These frequencies are 2-3 cm-1 lower than the corresponding frequencies for deoxyHb. The v19 and v11 frequencies are at the expected values for a Ct-N distance of 2.057 A, the known core size for a 6-coordinate high-spin FeII-porphyrin complex. The remaining frequencies, however, deviate from the core size correlations for these modes in the same direction as do those of deoxyHb, suggesting that the porphyrin ring is domed in both species. Thus, the heme structure is similar for deoxyHb and Hb but is slightly expanded in the latter. The expanded heme in Hb implies a restraint on the full out-of-plane displacement of the Fe atom, by an estimated approximately 0.1 A relative to deoxyHb. This could result from a residual interaction with the CO molecule if the latter remains held by the protein against the Fe atom, in a high-spin 6-coordinate complex. The available spectroscopic evidence suggests that such a complex may be stabilized at 4 K but is unlikely to persist at room temperature beyond the electronic relaxation (0.35 ps) of the electronically excited heme.(ABSTRACT TRUNCATED AT 250 WORDS)