Resonance Raman spectra of photodissociated carbonmonoxy hemoglobin and deoxy hemoglobin at 10 K.

Resonance Raman data from deoxy hemoglobin and photodissociated carbonmonoxy hemoglobin (Hb*) at low temperature (10 K) are reported. At this temperature with 457.9 nm excitation, the iron-histidine (Fe-His) stretching mode in deoxy hemoglobin disappears, although with 441.6 nm excitation it is detected at 235 cm-1. The intensity change is interpreted as resulting from changes in energy of iron d orbitals induced by the shortening of the Fe-His bond on temperature reduction. The previously reported narrowing of the Fe-His mode with temperature reduction has been found to be artifact. In Hb*, the Fe-His stretching mode is present with 457.9 nm as well as 441.6 nm excitation and is at higher frequency (242-244 cm-1) than it is in the deoxy samples. Several porphyrin skeletal frequencies are also shifted in Hb* with respect to deoxy hemoglobin. These data are interpreted as resulting from perturbations on the ligand-free histidine-porphyrin complex by the liganded conformation of the heme pocket. We find no evidence that the dissociated CO molecule interacts with the heme in Hb*.