Abstract
Resonance Raman spectra of the MnII-NO moiety in synthetic nitrosyl manganese heme complexes with and without steric hindrance are reported. The "strapped" hemes having a hydrocarbon strap (variable length) across one face of the heme hinder the perpendicular bonding of a linear ligand. These complexes were employed to investigate the effects of ligand distortion (primarily tilting) on Mn-NO stretching, Mn-N-O bending, and N-O stretching modes. It is demonstrated that ligand distortion in the MnII-NO system is a valid mechanism for causing the resonance enhancement of the Mn-N-O bending mode, similar to that observed in the FeII-CO system (Yu, N.-T., E. A. Kerr, B. Ward, and C. K. Chang. 1983. Biochemistry. 22:4534-4540). More interesting is the observation of the delta(Mn-N-O) enhancement caused by the tilting of the trans Mn-N epsilon bond in the "open" heme complexes (e.g., heme-5 and proto-1X dimethylester) with 1,2-dimethylimidazole or piperidine as a base. The nu(Mn-NO) and nu(N-O) modes exhibit an increase and a decrease, respectively, as the strap length decreases (hence the steric hindrance increases). Both nu(Mn-NO) and nu(N-O) frequencies are insensitive to the strength of the trans base. The results from "strapped" and "open" model heme systems imply that the Mn-N-O geometry is essentially linear and perpendicular in the nitrosyl complexes of monomeric manganese insect hemoglobin CTT IV and sperm whale myoglobin. The unusually low nu(N-O) frequency in the manganese myoglobin complex may be caused by the distal histidine-NO interaction. The delta(Mn-N-O) enhancement in both nitrosyl manganese CTT IV and nitrosyl manganese myoglobin may be caused by a tilting of the Mn"-Nf (proximal histidine) bond.