SUBSTITUENT EFFECTS ON THE CHOICE OF THE ORBITAL PREFERRED FOR ELECTRON-SPIN DELOCALIZATION IN 2 PARAMAGNETIC LOW-SPIN IRON(III) PORPHYRINS- MAPPING THE SPIN-DENSITY DISTRIBUTION AT THE PYRROLE POSITIONS BY H-1 COSY AND NOESY TECHNIQUES

In order to elucidate the distribution of unpaired electron spin densi ty within the porphyrin Ir orbitals of two unsymmetrically substituted derivatives of (tetraphenylporphyrinato)iron(III), [(p-Cl)(p-NEt(2))( 3)TPPFe(N-MeIm)(2)]-Cl and [(p-NEt(2))(p-Cl)(3)TPPFe(N-MeIm)(2)]Cl(NEt (2) = diethylamino; N-MeIm = N-methylimidazole), H-1 COSY and NOESY sp ectra were acquired. The cross peak patterns observed in the 2-D maps of these complexes allow assignment of the proton resonances in the sa me pyrrole ring (H-a,H-b and H-c,H-d) and of those that are closest to each other in adjacent pyrrole rings (H-b,H-c). Using these assignmen ts, it is possible to delineate the pattern of unpaired electron spin delocalization in the porphyrin ring. This pattern can be unambiguousl y correlated to the relative electronic effects of the two types of ph enyl substituents present, thereby permitting the electron-donating or -withdrawing effects of other substituents to be predicted. In order to test if the pattern of delocalized spin density derived from the 2- D NMR studies can be modeled, simple Huckel molecular orbital calculat ions were performed. Although only intended to provide a qualitative u nderstanding of the effects of electron-donating and -withdrawing subs tituents, these most basic calculations provided significant results. Using well-established Huckel parameters, together with lowering or ra ising the energy of the p, orbital of one meso-carbon atom to account for the electronic nature of the unique substituent, an electron densi ty distribution was obtained that fully supports the H-1 resonance ass ignments based on the cross peaks observed in the COSY and NOESY maps of these complexes. This indicates that more sophisticated theoretical calculations may lead to a quantitative description of the effects of porphyrin substituents on the pattern of spin delocalization in synth etic and naturally occurring hemes and their byproducts. In support of this conclusion, it should be noted that even the simplest Huckel cal culations described herein have already proved valuable in explaining the pattern of spin delocalization observed in the substrate-bound for m of the enzyme heme oxygenase, which catalyzes the stereospecific alp ha-meso bridge cleavage of hemin to yield biliverdin-IX alpha (Hernand ez, G.; Wilks, A.; Paolesse, R.; Smith, K. M.; Ortiz de Montellano, P. R.; La Mar, G. N. Biochemistry, in press).