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

Abstract

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.

Assignment of the Ferriheme Resonances of the Low-Spin Complexes of Nitrophorins 1 and 4 by 1H and 13C NMR Spectroscopy: Comparison to Structural Data Obtained from X-ray Crystallography

“…NOESY and HMQC spectra were obtained on a Bruker DRX-500 spectrometer operating at 500.03 MHz proton Larmor frequency. The 1 H- 13 C HMQC experiments were recorded using a 5 mm inverse-detection probe with decoupling during acquisition. A recycle delay of 200 ms and refocusing time of 2.5 ms (J = 200) were used.…”

Nitric oxide interaction with insect nitrophorins and thoughts on the electron configuration of the FeNO complex

“…The d xy ground state is also believed to exist for some reduced hemes in biological systems, including heme d and d 1 and siroheme, but this appears to depend upon the nature of the axial ligands [253][254][255]. A highly ruffled heme macrocycle is one of the hallmarks of d xy ground state complexes, although many d p heme centers are also ruffled.…”

Assignment of the Ferriheme Resonances of the High-Spin Forms of Nitrophorins 1 and 4 by 1H NMR Spectroscopy: Comparison to Structural Data Obtained from X-ray Crystallography

“…18,20 Binding of even-electron donor ligands, such as histamine, imidazoles, pyrazoles or cyanide, to the NOfree forms of these proteins produces low-spin (S = 1/2) Fe(III) complexes. 15,[18][19][20][21]39 The unpaired electron(s) on the metal of both S = 5/2 and S = 1/2 ferriheme proteins cause shifts (called hyperfine, isotropic or paramagnetic shifts) of the resonances from those observed in a diamagnetic protein. The two contributions to the paramagnetic shifts are the contact (through bonds) and electron-nuclear dipolar or pseudocontact (through space) contributions; these are discussed in considerable detail elsewhere.…”