Resonance Raman evidence for an unusually strong exogenous ligand—metal bond in a monomeric nitrosyl manganese hemoglobin

摘要

>Resonance Raman spectroscopy has been employed to determine the vibrational modes of monomeric nitrosyl manganese Chironomus thummi thummi hemoglobin (CTT IV). This insect hemoglobin has no distal histidine. By applying various isotope-labeled nitric oxides (14N16O, 15N16O, 14N18O), we have identified the Mn II-NO stretching mode at 628 cm−1, the MnII-N-O bending mode at 574 cm−1 and the N-O stretching mode at 1735 cm−1. The results suggest a strong MnII◀NO bond and a weak N◀O bond. The vinyl group substitution does not influence the ν(MnII-NO), δ(MnII-N-O) and ν(N◀O) vibrations. The MnII-NO stretching frequency is insensitive to distal histidine interactions with NO, whereas the N◀O stretching frequency is sensitive. Nitric oxide also binds to Met manganese CTT IV to form an MnIII·NO complex which undergoes a slow but complete autoreduction resulting in the MnII·NO species. In manganese meso-IX CTT IV, the MnIII·NO MnII·NO conversion alters the intensities of the porphyrin ring modes at 342, 360, 1587 and 1598 cm−1, but shifts the frequencies at 1504 and 1633 cm−1 (in MnIII·NO) to 1497 and 1630 cm−1 (in MnII·NO), respectively. The unshifted marker line at 1378 cm−1 reflects the fact that the π* electron densities of the porphyrin ring are the same in the two complexes.