Resonance Raman spectroscopy of non-planar nickel porphyrins

Non-planar conformational distortions have recently been implicated in the biological activity of porphyrins and related tetrapyrroles in proteins, and several studies have used highly substituted porphyrins to model these nonplanar conformational distortions. Several aspects of non-planarity in the highly substituted metalloporphyrins are discussed, focusing on resonance Raman spectroscopy as a technique for investigating these structural issues. First, different non-planar distortions cause characteristic changes in the Raman spectrum. Specifically, the decreases in frequency of several Raman lines when compared with planar porphyrin analogs are shown to be similar for several classes of non-planar highly substituted porphyrins. Second, the effect of the central metal ion [ M = Ni(II), Co(II), Cu(II), Zn(II), Co(III), Fe(II1)I on the conformation of the sterically constrained non-planar porphyrin octaethyltetraphenylporphyrin is considered. Responding to the conformational adjustments resulting from different metal size, the frequency of the structure-sensitive Raman line v2 correlates with several structural parameters obtained from either mechanics calculations or crystallographic studies. The parameters include C,--C, bond length, core size and C,-N-Ca angle.. Finally, an effect of electron-withdrawing substituents on the Raman frequencies is evident for the different classes of highly substituted porphyrins.