Bonding in HNO-Myoglobin as Characterized by X-ray Absorption and Resonance Raman Spectroscopies

Abstract

The EXAFS and resonance Raman spectra on the HNO-myoglobin adduct, 1, are consistent with the presence of HNO bound to a heme center. The three-dimensional structure about the heme center of 1 obtained from multiple-scattering (MS) analysis of the EXAFS of the heme protein yielded an Fe-N-O bond angle of 131 degrees and an Fe-N bond length of 1.82 A, which compare well with published values for model complexes containing RNO ligands. Resonance Raman spectra identified the nu(N=O) stretch at 1385 cm-1 (confirmed by 15N labeling), which corresponds well with those reported for small molecule HNO complexes. The wavelength of the nu(Fe-N) at 636 cm-1 of 1 is significantly higher than those of MbIINO and MbIIINO (554 and 595 cm-1, respectively). The XAFS, XANES, and resonance Raman data are all consistent with the structure deduced from the NMR experiments, providing more detail on the bonding between HNO and the metal center.

Synthesis, properties, and reactivity of a series of non-heme {FeNO}7/8 complexes: Implications for Fe-nitroxyl coordination

“…Farmer's group, however, was successful in obtaining structural information on the {FeHNO} 8 complex with myoglobin, which is apparently very stable [38][39][40]. The X-ray absorption experiments revealed Fe-N and N-O lengths of 1.82 and 1.24 Å, respectively, with a severely bent Fe-N-O angle of 131° [38]. These values were largely in agreement with theoretical calculations for small molecules with diamagnetic ground states.…”

“…Additional theoretical work by Lehnert and Rodgers has also helped establish the structural and spectroscopic features of heme {Fe(H)NO} 8 complexes although no small molecule has been characterized by any structural methods [36,37]. Farmer's group, however, was successful in obtaining structural information on the {FeHNO} 8 complex with myoglobin, which is apparently very stable [38][39][40]. The X-ray absorption experiments revealed Fe-N and N-O lengths of 1.82 and 1.24 Å, respectively, with a severely bent Fe-N-O angle of 131° [38].…”

“…These results contrast those obtained for [Fe(cyclam-ac)(NO)] due to the triplet ground state of TauD-{FeNO} 8 that has been assigned as a highspin (HS) Fe(II) (S = 2) antiferromagnetically coupled to triplet 3 NO − (S = 1). This small collection of reported {Fe(H)NO} 8 complexes has led to some general properties of {FeNO} 8 : (i) the preference of S = 0 (diamagnetic) ground states (TauD being the only outlier) suggestive of a coordinated singlet nitroxyl or nitroxyl anion ( 1 HNO or 1 NO − ); (ii) ν NO values of~1500 cm −1 for heme; 1300 cm −1 for non-heme with the exception of MbHNO (ν NO : 1385 cm −1 ) [38,45] [43]. Seeking to add to the list of isolable {FeNO} 8 complexes, we describe in this paper new {FeNO} 8 complexes based on similar ligand backbones with subtle electronic/structural differences as a logical extension of our previous study.…”

Three Redox States of Metallonitrosyls in Aqueous Solution

“…Addition of NO led to the nitroxyl (NO À ) derivative, which in turn was treated with acetic acid to yield [Fe(3,5-Me-BAFP)(HNO)], the first ferrous heme-HNO model complex (116). The latter has an electronic spectrum that resembles Farmer's Mb(II)-HNO complex (156).…”

Nitrogen Monoxide and Nitrous Oxide Binding and Reduction

“…Extended X-ray absorption fine structure (EXAFS) and resonance Raman spectroscopic data [131] further support the compound formulation. EXAFS data analysis yielded an Fe-N bond length of 1.82 Å, while the Fe-NO bond angle was determined to be 1318, comparing well with complexes possessing RNO ligands for which X-ray structures are known.…”