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

A model heme complex, bis(3-aminopyrazole)tetraphenylporphinatoiron(III) ch loride, [TPPFe (NH(2)PzH)(2)]Cl, for which the EPR g-values lead to a rhomb icity V/Delta=1.2 if g(zz) is the largest g-value, have been investigated b y electron spin echo envelope modulation (ESEEM) and Mossbauer spectroscopi es. The ESEEM studies focus on the proton sum frequency peaks at near twice the proton Larmor frequency. Analysis of the distant proton peak (mainly d ue to the pyrrole-H) at exactly twice the proton Larmor frequency shows con clusively that g(zz) is aligned along the normal to the porphyrin plane, an d thus the electron configuration is (d(xy))(2)(d(xz),d(yz))(3), With g(zz) >g(yy)>g(xx). This system is thus another violation to Taylor's "proper axi s system" rule. The near proton (the alpha-H and N-H of the axial ligands) peaks provide distance information for those protons from the metal. Magnet ic Mossbauer studies of the same complex confirm the (d(xy))(2)(d(xz),d(yz) )(3) ground state and indicate that, as is the case for cytochrome P350(cam ), A(xx) is the largest magnitude A-value, and is negative in sign. Other l ow-spin iron(III) porphyrinates also have A(xx) of negative sign, but usual ly the magnitude is only about half that of A(zz), which is always positive in sign.