Porphyrins conformational dynamics

Rotation of the oxygen around the Fe-O bond involves a small energy barrier ( 2 kcal mol ), suggesting that several rotational conformations could be available at room temperature. Indeed, our molecular dynamics simulations show that the 02 ligand undergoes large-amplitude oscillations within one porphyrin quadrant, jumping to another quadrant on the picosecond timescale. The dynamics of the FeCO unit are characterized by rapid mo-... 

A zinc(n) meso-meso linked porphyrin oligomer 57 exists in a nonhelical conformation in solution, but may adopt a dynamic helical conformation upon complexation with an achiral urea 58 through complementary hydrogen bonding interactions [115]. In the presence of the chiral diamine (S)-59, the 57-58 complex forms a predominantly one-handed helical conformation, thus showing a characteristic ICD in the absorption region of the porphyrin chromophore. This system may be used to sense the chirality of chiral diamines. 

The exponential in Eq. 2.14 represents the average over the system described by the hamiltonian Hx, and the corresponding series of conformers and configurational isomers is usually created by molecular dynamics or Monte Carlo methods. When the two systems X and Y are very similar, the exponential term vanishes, leading to a very slow convergence of the average in Eq. 2.14. A number of techniques have been described to overcome this problem 43 441. One of the few applications of this method to coordination compounds is the investigation of O2 and CO affinities to iron porphyrins[45]. 

The incorporation of a porphyrin group in a catenane raises the prospect that the former might serve as a centre at which photochemical, electrochemical or chemical processes are used to control the dynamic and/or conformational behaviour of the assembly as a whole. In addition, the tuning of such processes should be possible through the choice of suitable metallation as well as through the ability to vary substituents on the periphery of the porphyrin ring. 

Myriad metalloproteins bind iron-protoporphyrin IX, known as heme (Fig. 15). Heme protein properties are determined by a variety of factors within the inner coordination sphere and without. These include chemical modifications to the porphyrin macrocycle, different axial ligation, perturbations to conformation, and protein dynamics surrounding the cofactor. Because of the extensive proliferation of heme proteins, we will limit ourselves to a small subset. These will include the cytochromes c, myoglobins, heme oxygenases and peroxidases, and a heme-based chemical sensor. 

Finally, a DEER study on models for molecular wires made from butadiyne-linked zinc porphyrin oligomers, end-labeled with nitroxide radicals, was performed by Lovett, Anderson, and coworkers [107]. Unlike in [104—106], one can control the conformations of these metalloporphyrin-based strucmres by self-assembly with multidentate amine ligands, which bend the rigid oligomeric structure. The experimentally found end-to-end distances in these complexes match the predictions from molecular dynamics calculations. This study thus presents a proof-of-principle that DEER spectroscopy is also well suited for understanding more complex supramolecular stmctures. 

TR data for Ni and Zn porphine in the Soret band were analysed in terms of changes in C -Cm and Cp -Cp force constants on changing the core size. The molecular dynamics of the S2 state of nickel(II) porphyrin were probed by studying resonance Raman intensities Picosecond-timescale TR was used to monitor the vibrational energy relaxation in the (d,d)-excited state of Ni(OEP) Resonance Raman spectra of NF(L), where L = 5-NO2- or 5,15-(N02)2-octaethylporphyrin, show that each exists as at least three conformers in solution. Marker bands show that these exhibit differing degrees of non-planarity in the ligands. ... 

It is abundantly clear, however, that the solvent can play an important role in governing the dynamics of electron transfer and the degree of exciton coupling in face-to-face porphyrin dimers. For the examples selected, at least part of this solvent dependence may arise from conformational changes which modulate inter-porphyrin separation distances. However, an improved understanding of these materials can be acquired only by employing rigidly-linked porphyrinic systems. 

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