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

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 Mn^II-N-O bending mode at 574 cm^-1 and the N-O stretching mode at 1735 cm^-1. The results suggest a strong Mn^IINO bond and a weak NO bond. The vinyl group substitution does not influence the ν(Mn^II-NO), δ(Mn^II-N-O) and ν(NO) vibrations. The Mn^II-NO stretching frequency is insensitive to distal histidine interactions with NO, whereas the NO stretching frequency is sensitive. Nitric oxide also binds to Met manganese CTT IV to form an Mn^III·NO complex which undergoes a slow but complete autoreduction resulting in the Mn^II·NO species. In manganese meso-IX CTT IV, the Mn^III·NO Mn^II·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 Mn^III·NO) to 1497 and 1630 cm^-1 (in Mn^II·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.