Resonance Raman enhancement of the Mn-N-O bending mode in nitrosyl manganese "strapped" and "open" heme complexes

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.

Crystal structures of manganese- and cobalt-substituted myoglobin in complex with NO and nitrite reveal unusual ligand conformations

“…The expected and observed linearity of the MnNO bonds is supported by the spectroscopic characterization of several low-spin (por)Mn(NO)-containing compounds [44,46,128,136]. Yu et al employed resonance Raman spectroscopy to demonstrate that steric constraints within the distal face of a synthetic ''strapped'' (por)Mn(NO) complex enhances the Mn-N-O bending mode, and they attributed this observation to a tilting of the trans Mn-N(N-base) bond [136].…”

Resonance Raman Spectroscopic Study of Nitrophorin 1, a Nitric Oxide-Binding Heme Protein from Rhodnius prolixus, and Its Nitrosyl and Cyano Adducts

“…In the absence of steric hindrance, the Fe III NO moiety is known to adopt a linear configuration normal to the heme plane on the basis of the total number of electrons in the metal d and ligand π * orbitals, {FeNO} 6 . No bending mode of the Fe−N−O linkage has been observed for unhindered NO complexes of {FeNO} 6 heme proteins. , Detection of the δ (Fe−N−O) mode in the NP1(NO) RR spectrum suggests some type of off-axis influence that induces a slightly distorted Fe III −N−O configuration. , We carried out normal coordinate analysis (NCA) calculations based on the model (imidazole)−Fe(N p ) 4 −N−O to verify the assignment of the observed ν (Fe−NO) and δ (Fe−N−O) RR bands and to examine the effect of the Fe−N−O angle on their vibrational frequencies. The results are summarized in Table .…”

Synthesis, characterization and molecular structures of six-coordinate manganese nitrosyl porphyrinsElectronic supplementary information (ESI) available: Molecular structure of (TTP)Mn(NO)(1-MeIm). See http://www.rsc.org/suppdata/dt/b3/b308143p/

“…Prior to this work, the (por)Mn(NO)(imidazole) compounds reported in the literature were prepared in situ from exposure of NO to solutions of Mn II -porphyrins containing an excess of the imidazole ligand, and the success of this synthetic procedure was very limited. 16,31 We find that the preparative route described by eqn. ( 2) provides a convenient method by which (por)MnCl ϩ NO (excess) ϩ pip (por)Mn(NO)(pip) (1)…”