Planar-nonplanar conformational equilibrium in metal derivatives of octaethylporphyrin and meso-nitrooctaethylporphyrin

Abstract

The planar and nonplanar conformers of metal derivatives of 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) and 5-nitro-2,3,7,8,12,13,17,18-octaethylporphyrin (N02-OEP) are investigated using resonance Raman spectroscopy. The structural heterogeneity is assessed by analysis of the line shapes of the structure-sensitive Raman lines. First, heterogeneity in the conformation of the macrocycle has been detected in solutions of the nickel and cobalt derivatives of OEP, that is, both planar and nonplanar conformers are found to coexist at room temperature for these metal porphyrins but not for the Cu and Zn derivatives. The latter metals expand the porphryin core, shifting the equilibrium entirely to the planar conformer. Second, we find that substitution with a single NO2 group at one of the methine-bridge carbons shifts this planar-nonplanar equilibrium substantially toward the nonplanar conformer. Thus, both crowding of the peripheral substituents and contracting of the porphyrin core (Ni(I1) < Co(I1) < Cu(1I) < Zn(I1)) displace the equilibrium toward the nonplanar conformer. Finally, the frequencies of several Raman lines correlate with structural parameters such as core size (obtained either from molecular mechanics calculations or from X-ray crystallographic studies). The calculations predict and the marker line frequencies verify that a small expansion of the core results from the steric repulsion between the nitro and the ethyl groups. Core size dependence of the intensities and frequencies of the NO2 stretching vibrations suggests that the NO2 stretches are coupled to nearby vibrational modes of the porphyrin macrocycle.

Conformational control of cofactors in nature – the influence of protein-induced macrocycle distortion on the biological function of tetrapyrroles

“…26 It was shown that the distortion was predominantly induced via the CYS thioether linkages to the macrocycle and that this affected an asymmetric distribution of the pyrrole tilt angles with respect to the mean-plane. Additionally, on the basis of an earlier resonance Raman study that indicated a reluctance of metalloporphyrins with core-sizes ≥2 Å to ruffle, 198 they suggested that the distortion occurred at significant energetic cost to the protein. Together with the emerging knowledge that porphyrin conformation influenced redox potentials, these results were proposed to imply a functional significance of the conserved conformation.…”

The structure of coral allene oxide synthase reveals a catalase adapted for metabolism of a fatty acid hydroperoxide

“…Studies with synthetic hemes have shown that nonplanar heme groups are more easily oxidized than their planar counterparts (28), and the nonplanarity can affect not only the site of oxidation (metal or ring) but also the type of oxidation (two electrons vs. one electron) (29). Because distortion of heme planarity is energetically unfavorable (30), the difference in AOS and catalase heme shape is likely necessary for the difference in redox properties. In catalase, the nonplanar heme is oxidized with a two-electron oxidation removing electrons from the Fe and the porphyrin ring in the formation of compound I.…”

High-resolution structures of the oxidized and reduced states of cytochrome c554 from Nitrosomonas europaea

“…Although isolated porphyrins are essentially planar [27], distortions of heme porphyrins are common in proteins [28,29]. The deviations from planarity are thought to affect the electronic structure of the heme, which in turn can alter the redox potential and ligandbinding properties [30].…”