ESEEM and Mössbauer studies of the ferriheme model compound bis(3-aminopyrazole)tetraphenylporphyrinatoiron(III) chloride, [TPPFe(NH2PzH)2]Cl

Models of the membrane-bound cytochromes: mössbauer spectra of crystalline low-spin ferriheme complexes having axial ligand plane dihedral angles ranging from 0 degree to 90 degrees

Crystalline samples of four low-spin Fe(III) octaalkyltetraphenylporphyrinate and two low-spin Fe(III) tetramesitylporphyrinate complexes, all of which are models of the bis-histidine-coordinated cytochromes of mitochondrial complexes II, III, and IV and chloroplast complex b(6)f, and whose molecular structures and EPR spectra have been reported previously, have been investigated in detail by Mössbauer spectroscopy. The six complexes and the dihedral angles between axial ligand planes of each are [(TMP)Fe(1-MeIm)(2)]ClO(4) (0 degree), paral-[(OMTPP)Fe(1-MeIm)(2)]Cl (19.5 degrees), paral-[(TMP)Fe(5-MeHIm)(2)]ClO(4) (26 degrees, 30 degrees for two molecules in the unit cell whose EPR spectra overlap), [(OETPP)Fe(4-Me(2)NPy)(2)]Cl (70 degrees), perp-[(OETPP)Fe(1-MeIm)(2)]Cl (73 degrees), and perp-[(OMTPP)Fe(1-MeIm)(2)]Cl (90 degrees). Of these, the first three have been shown to exhibit normal rhombic EPR spectra, each with three clearly resolved g-values, while the last three have been shown to exhibit "large g(max)" EPR spectra at 4.2 K. It is found that the hyperfine coupling constants of the complexes are consistent with those reported previously for low-spin ferriheme systems, with the largest-magnitude hyperfine coupling constant, A(zz), being considerably smaller for the "parallel" complexes (400-540 kG) than for the strictly perpendicular complex (902 kG), A(xx) being negative for all six complexes, and A(zz) and A(xx) being of similar magnitude for the "parallel" complexes (for example, for [(TMP)Fe(1-MeIm)(2)]Cl, A(zz) = 400 kG, A(xx) = -400 kG). In all cases, A(yy) is small but difficult to estimate with accuracy. With results for six structurally characterized model systems, we find for the first time qualitative correlations of g(zz), A(zz), and DeltaE(Q) with axial ligand plane dihedral angle Deltavarphi.

Axial ligand complexes of the Rhodnius nitrophorins: reduction potentials, binding constants, EPR spectra, and structures of the 4-iodopyrazole and imidazole complexes of NP4

Previously, we utilized 4-iodopyrazole (4IPzH) as a heavy atom derivative for the initial solution of the crystal structure of the nitrophorin from Rhodnius prolixus, NP1, where it was found to bind to the heme with the iodo group disordered in two positions. We have now determined the structure of the 4IPzH complex of NP4 at pH 7.5 and find that the geometry and bond lengths at the iron center are extremely similar to those of the imidazole (ImH) complex of the same protein (structure determined at pH 5.6), except that the G-H loop is not in the closed conformation. 4IPzH binds to the heme of NP4 in an ordered manner, with the iodo substituent pointed toward the opening of the heme pocket, near the surface of the protein. In order to understand the solution chemistry in terms of the relative binding abilities of 4IPzH, ImH, and histamine (Hm, a physiological ligand for the nitrophorins), we have also investigated the equilibrium binding constants and reduction potentials of these three ligand complexes of the four Rhodnius nitrophorins as a function of pH. We have found that, unlike the other Lewis bases, 4IPzH forms less stable complexes with the Fe(III) than the Fe(II) oxidation states of NP1 and NP4, and similar stability for the two oxidation states of NP2 and NP3, suggesting that this ligand is a softer base than ImH or Hm, for both of which the Fe(III) complexes are more stable than those of Fe(II) for all four nitrophorins. Surprisingly, in spite of this and the much lower basicity of 4IPzH than imidazole and histamine, the EPR g-values of all three ligand complexes are very similar.

Models of the bis-histidine-coordinated ferricytochromes: Mössbauer and EPR spectroscopic studies of low-spin iron(III) tetrapyrroles of various electronic ground states and axial ligand orientations

The EPR and magnetic Mössbauer spectra of a series of axial ligand complexes of tetrakis(2,6-dimethoxyphenyl)porphyrinatoiron(III), [(2,6-(OMe)(2))(4)TPPFeL(2)](+), where L= N-methylimidazole, 2-methylimidazole, or 4-(dimethylamino)pyridine, of one axial ligand complex of tetraphenylporphyrin, the bis(4-cyanopyridine) complex [TPPFe(4-CNPy)(2)](+), and of one axial ligand complex of tetraphenylchlorin, [TPCFe(ImH)(2)](+), where ImH=imidazole, have been investigated and compared to those of low-spin Fe(III) porphyrinates and ferriheme proteins reported in the literature. On the basis of this and previous complementary spectroscopic investigations, three types of complexes have been identified: those having (d(xy))(2)(d(xz),d(yz))(3) electronic ground states with axial ligands aligned in perpendicular planes (Type I), those having (d(xy))(2)(d(xz),d(yz))(3) electronic ground states with axial ligands aligned in parallel planes (Type II), and those having the novel (d(xz),d(yz))(4)(d(xy))(1) electronic ground state (Type III). A subset of the latter type, with planar axial ligands aligned parallel to each other or strong macrocycle asymmetry that yield rhombic EPR spectra, cannot be created using the porphyrinate ligand. Type I centers are characterized by "large g(max)" EPR spectra with g>3.2 and well-resolved, widely spread magnetic Mössbauer spectra having A(zz)/ g(N)mu(N)>680 kG, with A(xx) negative in sign but much smaller in magnitude than A(zz), while Type II centers have well-resolved rhombic EPR spectra with g(zz)=2.4-3.1 and also less-resolved magnetic Mössbauer spectra, and usually have A(zz)/ g(Nmu(N) in the range of 440-660 kG (but in certain cases as small as 180 kG) and A(xx) again negative in sign but only somewhat smaller (but occasionally larger in magnitude) than A(zz), and Type III centers have axial EPR spectra with g( upper left and right quadrants ) approximately 2.6 or smaller and g( vertical line )<1.0-1.95, but often not resolved, and less-resolved magnetic Mössbauer spectra having A(zz)/ g(N)mu(N) in the range of 270-400 kG, and A(xx) again negative in sign but much smaller in magnitude than A(zz). An exception to this rule is [TPPFe(4-CNPy)(2)](+), which has A(xx)/ g(N)mu(N)=-565 kG, A(yy)/ g(N)mu(N)=629 kG, and A(zz)/ g(N)mu(N)=4 kG. A subset of Type II complexes (Type II') have rhombicities ( V/Delta) much greater than 0.67 and A(zz)/ g(N)mu(N) ranging from 320 to 170 kG, with A(xx) also negative but with the magnitude of A(xx) significantly larger than that of A(zz). These classifications are also observed for a variety of ferriheme proteins, and they lead to linear correlations between A(zz) and either A(xx), g(zz), or V/Delta for Types I and II (but not for A(zz) versus V/Delta for Type II'). Not enough data are yet available on Type III complexes to determine what, if any, correlations may be observed.

1H pulsed ENDOR and ESEEM evidence that the bis-imidazole complexes of iron(III) tetraphenylchlorin and tetraphenylporphyrin have the same order of g values, and the same electronic ground state

The electronic structures of the bis-imidazole complexes of iron(III) tetraphenylporphyrin ([(TPP)Fe(ImH)(2)](+)) and iron(III) tetraphenylchlorin ([(TPC)Fe(ImH)(2)](+)) in frozen glassy solutions have been studied by the pulsed electron nuclear double resonance (ENDOR) technique of Mims and by electron spin-echo envelope modulation (ESEEM) spectroscopy. ESEEM spectra have been used to determine the orientation of the imidazole ligand planes with respect to the g tensor axes. In the ENDOR spectra, the manifestations of the implicit TRIPLE effect described and explained earlier by Doan et al. (J. Am. Chem. Soc. 1996, 118, 7014) were seen. In this work, the explicit expressions describing this effect were derived for the first time and used to successfully simulate the proton ENDOR spectra at the low- (LF) and high-field (HF) edges of the EPR spectrum. Using pulsed ENDOR, we have been able to determine the spin density distributions in the pi-systems of both tetrapyrroles and show that [(TPC)Fe(ImH)(2)](+) has the electronic orbital ground state (d(xy)())(2)(d(xz)(),d(yz)())(3), the same as that known for [(TPP)Fe(ImH)(2)](+), and the largest principal g value corresponds to the g tensor axis 3, which is normal to the heme plane. For the TPP complex, the g tensor axis 1, corresponding to the smallest principal g value, was found to be at an angle phi(1) of 30-35 degrees from the N-Fe-N axis, with the ligand planes rotated by the angle of 20-25 degrees in the opposite direction. For the TPC complex, phi(1) was found to be about 25 degrees from the direction N(I)-Fe-N(III), where N(I) corresponds to the nitrogen of the saturated pyrrole ring. The ligand planes in this complex were found to be oriented at an angle of about 10 degrees in the opposite direction.