The biological importance of nitric oxide has changed from that of toxic gas to that of an essential cellular signaling agent. 1 In many of these processes, the binding of NO to a heme protein and the labilization of the ligand trans to NO or another rearrangement is the significant signaling event. Proposed mechanisms for the heme proteins cytochrome c′ and soluble guanylate cyclase have presented a scenario in which the coordination to heme changes during the physiological cycle. 2 Understanding how these coordination events impact already coordinated ligands (and vice versa) is an important step in understanding the heme-NO interaction. At the literal center of these studies is the heme iron, whose motion along the coordinate axes is associated with reactive modes such as the ν Fe -Im , ν Fe-NO /δ Fe-N-O and heme doming.In this communication, we examine whether ligand orientation and bond distance changes substantially modulate iron dynamics in six-coordinate [Fe(Porph)(1-MeIm)(NO)] 3 derivatives. This study has been facilitated by the isolation of two crystalline polymorphs of [Fe(TpFPP)(1-MeIm)(NO)] that display differing room temperature solid-state ν N-O (1631 cm -1 and 1640 cm -1 ). Additional vibrational data for the polymorphic forms is available from nuclear resonance vibrational measurements (NRVS). Briefly, NRVS is a novel technique that provides information on all vibrational frequencies for which there is iron motion. 4 Spectra were obtained on powder samples of the two species and are displayed in Figure S1. The powder data clearly shows differences in the overall iron vibrational modes. We have also obtained oriented single crystal NRVS data on both species that provides detailed information on the character of the modes, which allows for a detailed examination of the differences between the two. An analysis of their molecular structures and vibrational data yields a detailed view of how different molecular structure features affect the dynamics of the iron atom.The crystalline polymorphs are in the triclinic and monoclinic crystal systems; both are isolated from the same crystallization experiments. The two crystal types are illustrated in Figure S2. Figure 1 illustrates the two molecular structures. The FeNO and imidazole planes are within 1° of coplanarity (monoclinic form) and within 25° (triclinic form). The relative orientation of the imidazole N-CH 3 bond and the bent FeNO group are of the opposite sense in the two species; the monoclinic form has a ″cisoid″ arrangement and the triclinic a ″transoid″ one. The two NO ligands make angles of 38.5° and 43.2° with the closest Fe-N p vector, so that when the four porphyrin nitrogen atoms are superimposed, the NO ligands are almost superimposed (see Figure S3). Major differences include relative rotations of the two imidazole ligands, the relative sense of NO and imidazole directions, small differences in the trans Fe-N Im bond distance, core conformations and positions of the peripheral p-fluorophenyl groups. The changing peripheral group direct...

Keywords iron porphyrinates; Mössbauer spectroscopy; nuclear resonance vibrational spectra NO bonding π vs. σ; NO complexes One of the important issues of nitrosyl (nitric oxide, NO) iron porphyrinate derivatives (hemes) is to develop a detailed understanding of the molecular basis for selectivity (recognition) between the diatomic ligands NO, CO and O 2 . The sensing of these gaseous molecules is predominantly by heme-based proteins,[1] and heme-and heme proteindiatomic interactions continues to be an active area of research

Recent reports of potential physiological roles of hydrogen sulfide have prompted interest in hemesulfide interactions. Heme-H 2 S and/or heme-HS − interactions could potentially occur during endogenous production, transport, signaling events, and catabolism of H 2 S. We have investigated the interaction of the hydrosulfide ion (HS − ) with iron porphyrinates. UV-vis spectral studies show the formation of [Fe(Por) . Characterization by single-crystal Xray analysis, mass spectrometry, and Mössbauer and IR spectroscopy are all consistent with that of known sulfur-bound high-spin iron(II) compounds. The Fe-S distances of 2.3929(5) and 2.3887(13) Å are longer than all reported values of [Fe II (Por)(SR)] − species. An analysis of porphyrin nonplanarity for these derivatives and for all five-coordinate high-spin iron(II) porphyrinate derivatives with an axial anion ligand is presented. In our hands, attempts to synthesize iron(III) HS − derivatives led to iron(II) species.