Iron-carbon bond lengths in carbonmonoxy and cyanomet complexes of the monomeric hemoglobin III from Chironomus thummi thummi: a critical comparison between resonance Raman and x-ray diffraction studies.

Abstract

Soret-excited resonance Raman spectroscopy yields direct information regarding the Fe-C bonding interactions in the cyanomet and carbonmonoxy complexes of Hb III from C. t. thummi (CTT III) in solution. Isotope exchange in cyanide (13CN-, C15N- and 13C15N-) and carbon monoxide (13CO, C18CO and 13C18O), assigned the Fe(III).sbd.CN- stretching at 453 cm-1, the Fe(III).sbd.C.sbd.N- bending at 412 cm-1, the Fe(II).sbd.CO stretching at 500 cm-2, the Fe(II).sbd.C.sbd.O bending at 574 cm-1, and the C.sbd.O stretching at 1960 cm-1. The resonance Raman data, in conjunction with those obtained from heme model complexes with well-known Fe.sbd.C bond distances, strongly suggest that the Fe(III).sbd.CN- bond (.apprxeq. 1.91 .ANG.) is longer (hence weaker) than the Fe(II).sbd.CO bond (.apprxeq. 1.80 .ANG.). This result disagrees with those of X-ray crystallographic studies in which the Fe.sbd.C bond lengths were reported a 2.2 .ANG. in cyanomet and 2.4 .ANG. in carbonmonoxy CTT III. Based on Badger''s rule and normal mode calculations, the X-ray data would lead to the prediction of 279 cm-1 for the Fe(II).sbd.CO stretching frequency in CTT III .cntdot. CO, which was not observed. The Fe.sbd.CO bond was estimated as .apprxeq. 1.82 .ANG., which is very similar to the 1.80-.ANG. value in human Hb-CO crystals. Isotope shift data were used to estimate the FE.sbd.C.sbd.O angle as 169 .+-. 5.degree., somewhat larger than the 161.degree. value found by Steigemann and Weber. There must be errors in the X-ray crystallographic refinement for the ligand geometry in carbonmonoxy and cyanomet CTT III.