HEME GEOMETRY IN THE 10 K PHOTOPRODUCT FROM SPERM WHALE CARBONMONOXYMYOGLOBIN

We have measured the Soret band of the photoproduct obtained by comple te photolysis of sperm whale carbon-monoxymyoglobin at 10 K. The exper imental spectrum has been modeled with an analytical expression that t akes into account the homogeneous bandwidth, the coupling of the elect ronic transition with both high and low frequency vibrational modes, a nd the effects of static conformational heterogeneity. The comparison with deoxymyoglobin at low temperature reveals three main differences. In the photoproduct, the Soret band is shifted to red. The band is le ss asymmetric, and an enhanced coupling to the heme vibrational mode a t 674 cm(-1) is observed. These differences reflect incomplete relaxat ion of the active site after ligand dissociation. The smaller band asy mmetry of the photoproduct can be explained by a smaller displacement of the iron atom from the mean porphyrin plane, in quantitative agreem ent with the X-ray structure analysis, The enhanced vibrational coupli ng is attributed to a subtle heme distortion from the planar geometry that is barely detectable in the X-ray structure.