Axial Coordination and Conformational Heterogeneity of Nickel(II) Tetraphenylporphyrin Complexes with Nitrogenous Bases † Song-Ling Jia, ‡,§ Walter Jentzen, ‡,§,| Mayou Shang, ⊥ Xing-Zhi Song, ‡,§ Jian-Guo Ma, ‡,§ W. Robert Scheidt, ⊥ and John A. Shelnutt* ,‡,§ Materials Theory and Computation Department, Sandia National Laboratories, Albuquerque, New Mexico 87185-1349, Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87131, Klinik und Poliklinik fu ¨r Nuklearmedizin, Universita ¨t GH Essen, Hufelandstrasse 55, D-45147 Essen, Germany, and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 ReceiVed March 13, 1998 Axial ligation of nickel(II) 5,10,15,20-tetraphenylporphyrin (NiTPP) with pyrrolidine or piperidine has been investigated using X-ray crystallography, UV-visible spectroscopy, resonance Raman spectroscopy, and molecular mechanics (MM) calculations. By varying the pyrrolidine concentration in dichloromethane, distinct ν 4 Raman lines are found for the four-, five-, and six-coordinate species of NiTPP. The equilibrium constants for addition of the first and second pyrrolidine axial ligands are 1.1 and 3.8 M -1 , respectively. The axial ligands and their orientations influence the type and magnitude of the calculated nonplanar distortion. The differences in the calculated energies of the conformers having different ligand rotational angles are small so they may coexist in solution. Because of the similarity in macrocyclic structural parameters of these conformers and the free rotation of the axial ligands, narrow and symmetric ν 2 and ν 8 Raman lines are observed. Nonetheless, the normal-coordinate structural-decomposition analysis of the nonplanar distortions of the calculated structures and the crystal structure of the bis(piperidine) complex reveals a relationship between the orientations of axial ligand(s) and the macrocyclic distortions. For the five-coordinate complex with the plane of the axial ligand bisecting the Ni-N pyrrole bonds, a primarily ruffled deformation results. With the ligand plane eclipsing the Ni-N pyrrole bonds, a mainly saddled deformation occurs. With the addition of the second axial ligand, the small doming of the five-coordinate complexes disappears, and ruffling or saddling deformations change depending on the relative orientation of the two axial ligands. The crystal structure of the NiTPP bis(piperidine) complex shows a macrocycle distortion composed of waV(x) and waV(y) symmetric deformations, but no ruffling, saddling, or doming. The difference in the calculated and observed distortions results partly from the phenyl group orientation imposed by crystal packing forces. MM calculations predict three stable conformers (ruf, sad, and planar) for four-coordinate NiTPP, and resonance Raman evidence for these conformers was given previously. Introduction The possible functional significance of nonplanar heme distortions in proteins is becoming a more and more compelling question. 1-11 This was first brought to light when it was noticed that the nonplanar distortions of the iron-porphyrin (heme) cofactor are conserved for all of the high-resolution crystal structures of mitochondrial cytochromes c. 1-4,10,11 Recently, an analysis using normal-coordinate structural decomposition (NSD) of the more than 800 heme groups in protein crystal structures convincingly showed that the nonplanarity of the porphyrin is conserved for many types of proteins. 10,11 It is also now evident that the protein must provide the distortion energy required to make the heme group nonplanar, since in solution the heme is nearly planar. 12 Furthermore, it is clear that the type and degree of nonplanar distortion depends on the fine points of the protein-heme interaction. Hydrogen bonds, † 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. Work at Notre Dame University was supported by NIH Grants GM-38401 and RR-06709. * To whom correspondence should be addressed. ‡ Sandia National Laboratories. § University of New Mexico | Universita ¨t GH Essen. ⊥ University of Notre Dame. (1) Shelnutt, J. A.; Song, X.-Z.; Ma, J.-G.; Jia, S.-L.; Jentzen, W.; Medforth, C. J. Chem. Soc. ReV. 1998, 27, 31. (2) Hobbs, J. D.; Shelnutt, J. A. J. Protein Chem. 1995, 14, 19. (3) Ma, J.-G.; Laberge, M.; Song, X.-Z.; Jentzen, W.; Jia, S.-L.; Zhang, J.; Vanderkooi, J. M.; Shelnutt, J. A. Biochemistry 1998, 37, 5118. (4) Martinez, S. E.; Smith, J. L.; Huang, D.; Szczepaniak, A.; Cramer, W. A. 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J.; Park, M.-S.; Chamberlain, J. R.; Ondrias, M. R.; Senge, M. O.; Smith, K. M.; Shelnutt, J. A. J. Am. Chem. Soc. 1993, 115, 581. 4402 Inorg. Chem. 1998, 37, 4402-4412 S0020-1669(98)00289-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/31/1998