Fluoride binding to the cytochrome c ferric heme octapeptide. A model for anion binding to the active site of high spin ferric heme proteins

A ternary Fe(ii)-terpyridyl complex-based single platform for reversible multiple-ion recognition

Multiple ion-recognition activity by a ternary Fe(ii)-terpyridyl complex, [Fe(PhT)(PT)]2+ (1) (PhT = 4'-phenyl-2,2':6',2''-terpyridine; PT = 4'-pyridyl-2,2':6',2''-terpyridine), is demonstrated for cyanide (CN-), fluoride (F-) and hydroxide (OH-) ions in an aqueous medium with sufficient sensitivity, fast response, reproducibility and selectivity with a dual optical read-out. The sensing event was reversible with the "by-eye" visualization of back and forth colour changes. Three cyanide ions replaced PT from 1, as observed from the crystal structure of the 1 + CN- couple. Fluoride and hydroxide ions appeared to show multivariate interactions with 1. Observed structural and spectral changes correlated well with theoretical calculations. A string of cations at quantitative levels (Ag+/Hg2+/Fe2+/Fe3+) was used to decouple the 1 + anion complex to yield 1, which enabled the recognition of these cations while permitting the reuse of 1 for at least five set-reset cycles.

Insights into porphyrin chemistry provided by the microperoxidases, the haempeptides derived from cytochrome c

The water-soluble haem-containing peptides obtained by proteolytic digestion of cytochrome c, the microperoxidases, have been used to explore aspects of the chemistry of iron porphyrins, and as mimics for some reactions catalysed by the haemoproteins, including the reactions catalysed by the peroxidases and the cytochromes P450. The preparation of the microperoxidases, their physical and chemical properties including their electronic structure, the kinetics and thermodynamics of their reactions with ligands, electrochemical studies and examples of their uses as haemoproteins mimics, is reviewed.

The coordination of imidazole and substituted pyridines by the hemeoctapeptide N-acetyl-ferromicroperoxidase-8 (FeIINAcMP8)

The N-terminus acetylated ferric hemeoctapeptide from cytochrome c, N-acetylmicroperoxidase-8 (Fe(III)-NAcMP8) can be reduced by dithionite in aqueous solution to produce Fe(II)-NAcMP8. The UV-Vis spectrum has a broad Soret band and relatively poorly defined Q bands which is consistent with a mixture of a five-coordinate high spin species with His as the axial ligand and a six-coordinate, predominantly high spin species with His/H(2)O as axial ligands. There are two spectroscopically observable pK(a)s at 8.7+/-0.1 and 10.9+/-0.2 which are attributed to ionization of a heme propionic acid group and coordinated H(2)O, respectively; a pK(a) > or = 14 is due to ionization of the proximal His ligand. Equilibrium constants were determined by UV-Vis spectrophotometry at 25.0+/-0.2 degrees C and 0.5 M ionic strength (NaClO(4)) for the coordination of imidazole and a number of substituted pyridines, and complement available data for the ferric hemepeptide, allowing a comparison to be made of the affinity of an iron porphyrin with Fe in the +2 and +3 oxidation states towards these ligands. Imidazole is coordinated more strongly by the ferric porphyrin (log K=4.08) than by the ferrous porphyrin (log K=3.40). The equilibrium constants for coordination of pyridines by the ferric and ferrous porphyrins increase and decrease, respectively, with increasing ligand basicity. Values determined by cyclic voltammetry show the same dependence on the identity of the ligand. In the ferric porphyrin, the stability of the complex increases with the basicity of the ligand and hence its ability to donate electron density onto the metal. In the case of the more electron rich ferrous porphyrin, greater stability occurs with pyridine ligands that have an electron withdrawing group and hence can accept electron density from the metal. This is consistent with the midpoint reduction potentials E(1/2) of the pyridine complexes determined by cyclic voltammetry; E(1/2) is linearly dependent on, and becomes more negative with an increase in, ligand basicity. Log K for coordination of pyridines by the ferrous hemepeptide correlates well with the energy of the ligand frontier orbital with pi symmetry, suggesting that pi-bonding effects are significant in determining the strength of binding of pyridines by a ferrous porphyrin.

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

An improved method for the preparation of the heme octapeptide acetyl-MP8, obtained by proteolysis of horse heart cytochrome c, is described. AcMP8 obeys Beer's law at pH 7.0 in aqueous solution up to a concentration of 3 x 10(-)(5) M. The self-association constant measured at 25 degrees C (log K(D) = 4.04) is an order of magnitude lower than that for MP8, reflecting the role of the N-acetyl protecting group in abolishing intermolecular coordination. However, AcMP8 does form pi-stacked dimers in aqueous solution with increasing ionic strength. A more weakly packed pi-pi dimer reaches a maximum abundance at approximately 3 M ionic strength, but a more tightly packed dimer is favored at &mgr; > 3 M. An equilibrium model based on charge neutralization by specific binding of Na(+) ions gives a total molecular charge of 3- for AcMP8 at pH 7.0 and a self-association constant log K(D) = 4.20. AcMP8 exhibits six spectroscopically active pH-dependent transitions. The Glu-21 c-terminal carboxylate binds to the heme iron at low pH (pK(a) = 2.1) but is substituted by His-18 (pK(a) = 3.12) as the pH increases. The two heme propanoic acid substituents ionize with pK(a)'s of 4.95 and 6.1. This is followed by ionization of iron-bound water with a pK(a) = 9.59, DeltaH = 48 +/- 1 kJ mol(-)(1), and DeltaS = -22 +/- 3 J K(-)(1) mol(-)(1). The electronic spectra indicate that AcMP8 is predominantly in the S = (5)/(2) state at pH 7.0, while the hydroxo complex at pH 10.5 corresponds to an equilibrium mixture of S = (5)/(2) and S = (1)/(2) states at 25 degrees C. In the final transition, His-18 ionizes to form the S = (1)/(2) histidinate complex with a pK(a) of 12.71. AcMP8 is relatively stable under alkaline conditions, dimerizing slowly at high pH (k = 2.59 +/- 0.14 M(-)(1) s(-)(1)) to form a high-spin &mgr;-oxo-bridged species. The pH-dependent behavior of AcMP8 in the presence of excess 3-cyanopyridine, however, is markedly different. At low pH, AcMP8 simultaneously binds the exogenous ligand and the Glu-21 c-terminal carboxylate with a pK(a) < 2. His-18 replaces the carboxylate ligand at higher pH (pK(a) = 2.60), and both heme propanoic acid groups ionize with a mean pK(a) = 5.10. Unlike AcMP8.OH(-), the axial histidine of the 3-CNPy complex ionizes at near neutral pH (pK(a) = 7.83), prior to being replaced by OH(-) (pK(a) = 10.13). The sixth transition in the AcMP8/3-CNPy system produces the bis(hydroxo) complex (pK(a) > 13).

Azide, cyanide, fluoride, imidazole and pyridine binding to ferric and ferrous native horse heart cytochrome c and to its carboxymethylated derivative: a comparative study

Azide, cyanide, fluoride, imidazole, and pyridine binding to ferric and ferrous native horse heart cytochrome c and to its carboxymethylated derivative has been investigated, from the thermodynamic viewpoint, at pH 7.5 and 25.0 degrees C. Ligand affinity for ferric and ferrous carboxymethylated cytochrome c is higher by about 30- and 400-fold, respectively, than that observed for the native protein. The results here reported: (i) allow the estimation, for the first time, of the ligand-independent free energy associated with the heme-iron sixth coordination bond in ferric and ferrous native cytochrome c, which turns out to be +8.4 kJ mol-1 and +14.6 kJ mol-1, at 25.0 degrees C, respectively, and (ii) suggest an interplay between redox, structural, ligand binding, and recognition properties of cytochrome c.