PROTON NMR-STUDY OF HIGH-SPIN IRON(III) ISOBACTERIOCHLORINS - THE EFFECTS OF 5-COORDINATION VS 6-COORDINATION AND COMPARISONS WITH HOMOLOGOUS PORPHYRIN AND CHLORIN COMPLEXES

The two stereoisomers of free base 2,3,7,8-tetrahydro-2,3,7,8,12,13,17,18-octaethylporphyrin (octaethylisobacteriochlorin, H2(OEiBC)), H2(ttt-OEiBC) and H2(tct-OEiBC), were separated to > 90% purity with medium pressure liquid chromatography. These two macrocycles and trans-7,8-dihydro-2,3,7,8,12,13,17,18-octaethylporphyrin (trans-octaethylchlorin, H2(t-OEC)) were used to prepare or generate in solution the following high-spin (S = 5/2) Fe(III) compounds: Fe(ttt-OEiBC)Cl, Fe(tct-OEiBC)Cl, [Fe(ttt-OEiBC)(DMSO)2]+[CF3SO3]-, [Fe(tct-OEiBC)(DMSO)2]+[CF3SO3]-, Fe(t-OEC)Cl, and (Fe(t-OEC)-(DMSO)2]+[CF3SO3]-. The isobacteriochlorin complexes are models for the siroheme active sites in the resting states of E. coli NADPH sulfite reductase and spinach ferredoxin nitrite reductase. Along with several specifically deuterated derivatives, the model complexes were studied by H-1 and H-2 NMR spectroscopy. (The six-coordinate complexes, reported here for the first time, were also characterized by UV-vis and EPR spectroscopy.) The OEiBC spectra are well enough resolved to show, for example, separate resonances for nearly all of the individual protons (not counting methyl protons) of the two C, stereoisomers of Fe(tct-OEiBC)Cl. For both ligation states, the change from ttt-OEiBC to tct-OEiBC results in only minor changes in contact shift patterns. The range of contact shifts exhibited by pyrrole methylene protons, pyrroline protons, and meso protons is far greater for the OEiBC complexes than for the corresponding t-OEC complexes. Nevertheless, the estimated amount of unpaired spin density in pyrrole rings is essentially the same, at parity of axial ligation, across the series OEP, t-OEC, ttt-OEiBC. For both types of hydroporphyrin complexes, the change from five- to six-coordination induces diagnostic changes in meso and pyrroline proton isotropic shifts. Thus, pyrroline H-1 NMR resonances, which are relatively sharp, can be used to elucidate whether high-spin Fe(III) hydroporphyrin prosthetic groups in green heme and siroheme enzymes are five- or six-coordinate. T1 relaxation times at 293 K for pyrrole methylene protons (approximately 2 ms) and for meso protons (approximately 0.3 ms) are qualitatively the same across the series OEP, t-OEC, ttt-OEiBC. For the hydroporphyrin complexes, the order of decreasing T1 relaxation times is pyrrole methylene > pyrroline H approximately pyrroline methylene > meso. The pyrroline H and pyrroline methylene T1's are measurably longer for the six-coordinate derivatives.