Photodissociation of (DMSO)₂Fe(II)PPIX to form a transient five‐coordinate complex as studied using transient resonance Raman spectroscopy

Abstract

Resonance Raman spectra of iron(1I) protoporphyrin IX [ Fe(II)PP] in dimethyl sulfoxide (DMSO) and deuterated DMSO were obtained at high and low photon densities using 10 ns laser pulses. At low photon density, comparison of the Raman spectra of the complex with those of carbonmonoxy-hemoglobin indicates that a lowspin, six-coordinate (DMSO),Fe(lI)PP complex exists; at high photon density, spectral comparisons with deoxyhemoglobin and the 2-methylimidazole heme complex (2-Melm)Fe(II)PP indicate that a transient high-spin, five-coordinate complex is formed by photodissociation of a DMSO ligand. This phenomenon is reversible and observable on a 10 ns time-scale.

Structure and Dynamics of CO−Iron(II) Protoporphyrin IX in Dimethyl Sulfoxide

“…The optical absorption of the (DMSO) 2 Fe II PPIX and (CO)(DMSO)Fe II PPIX complexes are displayed in Figure . The optical absorption spectrum of the (DMSO) 2 Fe II PPIX displays a Soret maximum at 424 nm with visible bands located at 556 nm (α-band) and 527 nm (β-band) consistent with previous results. , The positions of these bands are characteristic of six-coordinate, low-spin heme complexes (see Table ). Exposure of the (DMSO) 2 Fe II PPIX to CO results in a hypsochromic shift in the Soret band to 415 nm and bathochromic shifts in both the α- and β-bands to 568 and 535 nm, respectively (Figure , trace b).…”

“…Transient Absorption Studies of (DMSO) 2 Fe II PPIX and (CO)(DMSO) Fe II PPIX Complexes. Previous resonance Raman and transient absorption studies have demonstrated that the (DMSO) 2 Fe II PPIX complex is photolabile. , The photolysis product displays an absorption difference spectrum with an absorption maximum at 435 nm and a minimum at 423 nm. These studies have further shown that the decay of the (DMSO)Fe II PPIX transient occurs monophasically by rebinding a DMSO ligand to the vacant sixth-coordination site.…”

“…Nanosecond transient absorption and resonance Raman data were obtained with instrumentation described in detail elsewhere. , Nanosecond transient absorption data were obtained by exciting the sample in a 1-cm quartz optical cuvette with a 10 ns pulse from a frequency doubled Nd:YAG laser (Continium SureLite II, 532 nm, 3.0 mJ/pulse). The change in absorbance was monitored by focusing the the arc of a 75-W Xe arc lamp through the sample and overlapping with the excitation pulse.…”

“…It has recently been shown that the (DMSO) 2 Fe II PPIX complex forms a photolabile six-coordinate low-spin complex. , We have previously found that photolysis of this complex yields a five-coordinate, high-spin (DMSO)Fe II PPIX complex that exhibits a relatively slow rate of recombination ( k obs = ∼2.0 × 10 6 s -1 ) when compared to recombination rates of nitrogen-based ligands. This slow recombination rate was attributed to strong dipolar interactions between the DMSO ligand and the transient five-coordinate high-spin heme.…”

Dynamics and Equilibrium of Heme Axial Ligation in Mesoporous Nanocrystalline TiO2 Thin Films

“…Band-gap excitation of the thin films immersed in Ar-purged solvent resulted in the reduction of Fe III to Fe II and a bathochromic shift in the Soret band for all ligations (Figure and Table ) . In the ferrous state, the axial ligands were identified clearly as DMSO or py by examination of the Q-band region of the visible absorption spectrum. ,− The axial ligands for thin films immersed in methanol are unknown but for consistency are referred to as (MeOH) 2 Fe III/II PPIX/TiO 2 .…”