Resonance Raman investigation of pH-dependent equilibria of water-soluble iron porphyrins

Abstract

Resonance Raman spectroscopy has been used to probe the structures of; tetrakis(1-methylpyridinium-4-yl)porphi natoi ron( II I), Fel I '(T4MPyP) ; tetrakis( 1 -methyl pyridium-2-y1)porphi natoi ron(iii), Fell '(T2MPyP) ; tetrakis(4sulphonatophenyl)porphinatoiron(iii), Fe'"(TSPP); and tetrakis(4-carboxylatophenyl)porphinatoiron(iii), Fe"'(TCPP), over a wide pH range. The anionic complexes Fe"'(TSPP) and Fe"'(TCPP) contain high-spin iron(iii) at all pHs. Both these complexes exhibit marked spectral changes at ca. pH 6 which correspond to conversion from the diaquo species, in acid solution, to hydroxy-or p o x 0 dimer complexes. Both cationic complexes show similar diaquo to high-spin hydroxy, or p-0x0 dimer, transitions at ca. pH 6. However, at pH > 11.5 for Fe'"(T4MPyP) and pH > 9 for Fel"(T2MPyP) a second equilibrium process is observed, leading to two new species. One of these is readily assigned as the low-spin iron(iii) dihydroxy complex by analogy with spectra of the dicyano complex. The second species is assigned to the hydroxy iron(ii) complex by comparison with photochemically generated FeI1(T4MPyP)(OH). The formation of iron(\\) species in alkaline solutions of Fe'"(T4MPyP) and Fe"'(T2MPyP) is entirely unexpected and the significance of the observation to previous investigations of the pH-dependent behaviour of these complexes is discussed.

Heme−Peptide Models for Hemoproteins. 1. Solution Chemistry of N-Acetylmicroperoxidase-8

“…The visible band at ∼600 nm in AcMP8···OH - has been assigned to the charge-transfer transition (b 2u (π) → e g (dπ)) of the high-spin component of the equilibrium mixture ( S = 1 / 2 ⇄ S = 5 / 2 ) and appears to reflect axial ligation by an anionic oxygen donor ligand ( vide supra ). On the basis of the similarity between the electronic spectrum of AcMP8 at pH 14 (after 106 h) and that of alkaline [Fe(PPIX)], which readily forms μ-oxo dimers as the pH is raised, 9c,− and the kinetically determined orders of 2 and 0 with respect to [Fe] and [OH - ], respectively, we propose that AcMP8 undergoes slow μ-oxo dimerization at high pH (mechanism in Scheme S1). Importantly, these results demonstrate that, on the time scale of the pH titration (Figure ), the spectroscopic changes due to μ-oxo dimerization of AcMP8 are insignificant and that the high-pH (>12) optical transition is a true ionization equilibrium rather than a spurious base-catalyzed side reaction.…”

Photo-decarboxylation of iron(III) porphyrin–amino acid complexes in aqueous solution

“…17-19 At pH > 11 a further change occurs to give a low-spin bis-ligated hydroxy species, 19-21 and possibly a high-spin hydroxyiron() species. 19 In the presence of a carboxylic acid, as discussed earlier, 11 the diaquo species is replaced by the carboxylate ligated iron porphyrin at pH 2-6, (3). However, in the pH range 6-11, the hydroxy complex still dominates since the carboxylate anion cannot compete successfully with the hydroxide anion as a ligand for the iron porphyrin.…”

Porphyrin-Encapsulated Metal–Organic Frameworks as Mimetic Catalysts for Electrochemical DNA Sensing via Allosteric Switch of Hairpin DNA

“…33 Moreover, the porous structure of HKUST-1(Cu) framework could accelerate the diffusion of o-PD in the electrochemical system. 64 When the FeTCPP@MOF-SA was immobilized on the GCE (curve e), the current response is slightly smaller than that of FeTCPP@MOF due to the inhibition of electron transfer by SA protein.…”