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 X-ray crystal structures of synthetic and protein-bound metalloporphyrins are analyzed using a new normal structural decomposition method for classifying and quantifying their out-of-plane and in-plane distortions. These distortions are characterized in terms of equivalent displacements along the normal coordinates of the D 4h -symmetric porphyrin macrocycle (normal deformations) by using a computational procedure developed for this purpose. Often it turns out that the macrocyclic structure is, even in highly distorted porphyrins, accurately represented by displacements along only the lowest-frequency normal coordinates. Accordingly, the macrocyclic structure obtained from just the out-of-plane normal deformations of the saddling (sad, B 2u )-, ruffling (ruf, B 1u )-, doming (dom, A 2u )-, waving [waV(x), waV(y); E g ]-, and propellering (pro, A 1u )-type essentially simulates the out-of-plane distortion of the X-ray crystal structure. Similarly, the observed inplane distortions are decomposed into in-plane normal deformations corresponding to the lowest-frequency vibrational modes including macrocycle stretching in the direction of the meso-carbon atoms (meso-str, B 2g ), stretching in the direction of the nitrogen atoms (N-str, B 1g ), x and y pyrrole translations [trn(x), trn(y); E u ], macrocycle breathing (bre, A 1g ), and pyrrole rotation (rot, A 2g ). The finding that the displacements of the 24 atoms of the macrocycle primarily occur along the lowest-frequency normal coordinates is expected on physical grounds and is verified by structural decomposition of more than 100 synthetic and 150 protein-bound metalloporphyrin X-ray crystal structures. Because of the high resolution of the X-ray crystal structures of synthetic metalloporphyrins, the small displacements for other normal coordinates are also able to be discerned. However, for the heme groups in proteins, only the displacements along the lowest-frequency modes are detectable because of the large uncertainties in the atomic positions. The heme groups in the four X-ray crystal structures of deoxyhemoglobin are used to evaluate the structural decomposition method. We find that the corresponding heme groups in different X-ray crystal structures are similar. Furthermore, the outof-plane distortions for the heme groups in the R-and -chains are found to be inequivalent, that is, the two

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The influence of substituents with increasing steric demands on the structure of nickel(II) 5,15-disubstituted porphyrins has been investigated with X-ray crystallography, UV−visible absorption spectroscopy, resonance Raman spectroscopy, molecular energy optimization calculations, and INDO/s molecular orbital calculations. Nickel 5,15-diphenylporphyrin is predicted by molecular mechanics calculations to be a mixture of planar and nonplanar conformers. All of the nickel dialkylporphyrins (where the alkyl group is propyl, isopropyl, and tert-butyl) are calculated to be in a predominantly gabled (gab) conformation resulting from an αα orientation of the substituents with respect to the macrocycle. This nonplanar gab distortion is made up of a linear combination of distortions along the lowest-frequency out-of-plane macrocycle normal coordinates of A2u (doming) and B1u (ruffling) symmetry types. A higher energy stable αβ conformer is also predicted for dialkylporphyrins, and its nonplanar structure can be represented as an equal combination of distortions along the x- and y-components of the lowest-frequency Eg (waving) normal coordinate. The nonplanar structures calculated by using molecular mechanics have been structurally decomposed into the displacements along the lowest-frequency normal coordinate of each symmetry type, and the contributions of each type to the total distortion in the calculated structures agree well with contributions obtained from structural decompositions of the available X-ray crystal structures. The predicted gab distortion is confirmed most convincingly by the X-ray crystal structure of [5,15-di-tert-butylporphinato]nickel(II) which is found to be in a gab αα conformation. Finally, INDO/s calculations show that the red shifts in the absorption spectra of the nickel disubstituted porphyrins are caused by the increasing nonplanarity resulting from increasing steric crowding within the series.