Resonance Raman Spectra of Nitrosyl Heme Proteins and of Porphyrin Analogues

Article abstract of DOI:10.1021/ja00538a019

Addition of inositol hexaphosphate (IHP) to nitrosylhemoglobin (NOHb) produces a new resonance Raman (RR) band at 592 cm^-1, identifiable with Fe-NO stretching via its ¹⁵NO shift, in addition to the 553-cm^-1 Fe-NO band observed without IHP.This observation supports previous evidence that the R --> T switch induces the Fe-imidazole bonds of half the NOHb heme groups to break.The high-frequency RR spectra are similar to those reported by Szabo and Barron, although slight additional shifts on IHP binding are observed.A series of analogue heme complexes have been studied and the RR spectra effects of NO, CO, imidazole, and imidazolate are analyzed in terms of ? back-bonding and trans labilization.The IHP-induced shifts in NOHb are consistent with Fe-imidazole bond breaking, but additional protein influences are apparent.An alternate hypothesis, that the R --> T shift in NOHb involves protonation of bound imidazolate, is not supported by the RR spectra.