Nonplanar distortions of tetrapyrroles are prevalent in the hemes of hemoproteins, the pigments of photosynthetic proteins, and cofactor F 430 of methylreductase. The nonplanarity of these porphyrin cofactors is currently believed to influence factors in the biological activity of the proteins, in part, because the porphyrin deformations are often conserved within functional classes of proteins. The occurrence, classification, and study of nonplanar porphyrins in proteins and synthetic nonplanar porphyrin analogs are reviewed. † Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.

The out-of-plane distortions of porphyrins in hemoproteins are characterized by displacements along the lowest-frequency out-of-plane normal coordinates of the D4h-symmetric macrocycle. X-ray crystal structures are analyzed using a computational procedure developed for determining these orthogonal displacements. The x-ray crystal structures of the heme groups are described within experimental error, using the set composed of only the lowest frequency normal coordinate of each out-of-plane symmetry type. That is, the distortion is accurately simulated by a linear combination of these orthonormal deformations, which include saddling (B2u), ruffling (B1u), doming (A2u), waving (Eg), and propellering (A1u). For example, orthonormal structural decomposition of the hemes in deoxymyoglobins reveals a predominantly dom heme deformation combined with a smaller wav(y) deformation. Generally, the heme conformation is remarkably similar for proteins from different species. For cytochromes c, the conformation is conserved as long as the amino acids between the cysteine linkages to the heme are homologous. Differences occur if this short segment varies in the number or type of residues, suggesting that this small segment causes the nonplanar distortion. Some noncovalently linked hemes like those in the peroxidases also have highly conserved characteristic distortions. Conservation occurs even for some proteins with a large natural variation in the amino acid sequence.

The effects of ruffling on the axial ligation properties of a series of nickel(II) tetra(alkyl)porphyrins have been investigated with UV-visible absorption spectroscopy, resonance Raman spectroscopy, X-ray crystallography, classical molecular mechanics calculations, and normal-coordinate structural decomposition analysis. For the modestly nonplanar porphyrins, porphyrin ruffling is found to cause a decrease in binding affinity for pyrrolidine and piperidine, mainly caused by a decrease in the binding constant for addition of the first axial ligand; ligand binding is completely inhibited for the more nonplanar porphyrins. The lowered affinity, resulting from the large energies required to expand the core and flatten the porphyrin to accommodate the large high-spin nickel(II) ion, has implications for nickel porphyrin-based molecular devices and the function of heme proteins and methyl-coenzyme M reductase.

Electronic absorption and resonance Raman (RR) spectra of the ferric form of barley grain peroxidase (BP 1) at various pH values, at both room temperature and 20 K, are reported, together with electron paramagnetic resonance spectra at 10 K. The ferrous forms and the ferric complex with fluoride have also been studied. A quantum mechanically mixed-spin (QS) state has been identified. The QS heme species coexists with 6- and 5-cHS hemes; the relative populations of these three spin states are found to be dependent on pH and temperature. However, the QS species remains in all cases the dominant heme spin species. Barley peroxidase appears to be further characterized by a splitting of the two vinyl stretching modes, indicating that the vinyl groups are differently conjugated with the porphyrin. An analysis of the currently available spectroscopic data for proteins from all three peroxidase classes suggests that the simultaneous occurrence of the QS heme state as well as the splitting of the two vinyl stretching modes is confined to class III enzymes. The former point is discussed in terms of the possible influences of heme deformations on heme spin state. It is found that moderate saddling alone is probably not enough to cause the QS state, although some saddling may be necessary for the QS state.