The electronic structure of porphyrins

When organic chemists say that a compound is aromatic , they are describing that compound s structure and properties. When August Kekule first coined the term in 1865, to describe the peculiar stability of benzene and its derivatives, he could hardly have guessed that 130 years later his concept would still be fascinating chemists world-wide. 

The first so-called aromatic compounds to be studied seriously, such as vanillin (derived from vanilla), had two obvious properties. They had a sweet smell and were remarkably stable. This last property was the reef on which many of the early theories of chemical bonding foundered. Consider benzene. Kekule knew that its molecular formula was C H. The only way he could rationalise this formula with the known properties of benzene, was to imagine the six carbon atoms joined in a ring and connected by three alternate double bonds. This is where the trouble started because double bonds are supposed to confer reactivity on an organic molecule benzene is stable. Double bonds can readily be added to for example, they will undergo fast reactions with bromine and sulphuric acid to give simple addition compounds. The reagents simply add across the double bond. 

This structure, however, was flawed. For example, if there really were three alternating double bonds, then the benzene hexagon should be severely distorted. This is because double bonds are shorter than single bonds. In fact, the benzene hexagon is perfectly regular. Kekule s critics scoffed and proceeded to publish alternative structures that to us now, make little chemical sense. 

Unhybridised 2p arbitals on each carbon overlap sideways to produce a k bond. 

Not all cyclic molecules with alternating double bonds are stabilised in this way. For example, cyclooctatetraene has four alternating double bonds 

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Porphyrin is a multi-detectable molecule, that is, a number of its properties are detectable by many physical methods. Not only the most popular nuclear magnetic resonance and light absorption and emission spectroscopic methods, but also the electron spin resonance method for paramagnetic metallopor-phyrins and Mossbauer spectroscopy for iron and tin porphyrins are frequently used to estimate the electronic structure of porphyrins. By using these multi-detectable properties of the porphyrins of CPOs, a novel physical phenomenon is expected to be found. In particular, the topology of the cyclic shape is an ideal one-dimensional state of the materials used in quantum physics [ 16]. The concept of aromaticity found in fuUerenes, spherical aromaticity, will be revised using TT-conjugated CPOs [17]. 

Porphyrins have been found to bind the tetravalent ions of Th and U in the form of the bisporphyrin complexes, An(P)2 (An = Th, U P = octaethylporphyrin, tetra-j -tolylporphyrin). These sandwich-type complexes are useM for studying the electronic structure of porphyrins. For example, Th is electrochemically inactive, which allows the porphyrin-based electrochemistry to be studied exclusively for its role in photosynthesis. 

Labeling of 6-Type Haemoprotein Using Reconstitution Effect of Labeling on the Electronic Structure of Porphyrin... 

MCD spectroscopy [68, 131,132, 182] is widely used in inorganic chemistry—often focusing on elucidating the electronic structure of porphyrins and phthalocya-nines [183, 184]—and in biological chemistry [185—188] for probing the electronic structure of metalloenzyme active sites [185]. 

The indicated formal potential E° n of the corresponding monomer (-1.17V) in solution is very near that of the surface film (-1.13V vs. SSCE). That formal potentials of surface films on chemically modified electrodes are near those of their corresponding dissolved monomers (13,18) is actually a common, and quite useful, observation. In the present case, it demonstrates that the electronic structures of the porphyrin rings embedded in the polymer film are not seriously perturbed from that of the monomer. 

For a review of absorption and emission spectroscopy of porphyrins and metalloporphyrins, see Gouterman, M. Optical spectra and electronic structure of porphyrins and related rings. In The Porphyrins Dolphin, D., Ed., Academic Press New York 1978, Vol. Ill, pp 1-165. 

To get a deeper insight into the factors which affect the electronic structure of hemopro-teins, systematic studies on heme model compounds have been undertaken by Scholes and coworkers79 247 255. The model systems used by these authors are protohemin, deuterohemin and ferric tetraphenylporphyrin with a series of axial ligands. All the published ENDOR data on high-spin heme model compounds were obtained in frozen solutions with B0 along gi = 2 which is oriented normal to the porphyrin plane. A 1 1... 

Consequently, there has been much interest in understanding the electronic structure of these molecules. Since the partial occupancy of the 3electronic states within a narrow energy range, it has long been debated as to what is the ground state for unligated, four-coordinate Fe(II) and Co(II) porphyrins (FeP and CoP, respectively). 

ESR studies also show the similarity between oxycoboglobin and oxygenated cobalt porphyrin, and suggest that the protein does not measurably influence the electronic structure of the heme group, although the protein does control the extent to which dioxygen is bound. 

The prevalence for epoxidation seems to be independent of the electronic structure of the porphyrin or the type of oxidant. For example, the catalytic oxidation of cyclohexene la with molecular oxygen and isobutyraldehyde in the presence of Fe3+(TPP)C1 (TPP = tetraphenylporphyrin) in 1,2-dichloroethane yielded exclusively the epoxide 4a and only trace amounts of the allylic alcohol 2a or ketone 3a, as demonstrated by Nam et al. [111]. 

In the case of porphyrins different substitution patterns induce relevant electronic variations the energy level of the two HOMOs of the porphyrin n system (a/u and a2u) are in fact reversed in going from a meso-substituted porphyrin, such as tetraphenylporphyrin, to a -substituted one, such as octaethylporphyrin [44]. Electronic variations may also derive from conformational changes Smith and coworkers have reported hybrid compounds substituted both at meso and P positions, the Zn2+ complexes of 2,3,7,8,12,13,17,18-octaethyl- and 2,3,7,8,12,13,17,18-octamethyl-5,10,15,20-tetraphenylporphyrin [46] (see Sect. 3.3). These complexes have a nonplanar conformation caused by the steric interactions of the substituents. Theoretical calculations indicate that such conformational changes cause a differential shift in the energies of the HOMO and LUMO and so modulate the electronic structure of the macrocycles. 

The H NMR and X-ray crystallography results discussed in Section III have clearly shown that the environments of the heme pockets of the a and P chains of Hb as manifested by the conformations of proximal histidine (F8), distal histidine (E7), and distal valine (Ell), as well as the conformation of several porphyrin protons and the electronic structure of the heme group in both deoxy and ligated states, are not equivalent. It has not been clear what this difference in structure in the a and p heme pockets means in terms of differences in function, because there are discrepancies in the published results regarding the relative ligand affinities of the a and P chains of Hb A. 

Their absorption spectra were typical for macrocycles of this type (Fig. 1). They all displayed an intense split Q band and an intense single peak in the Soret region. The peripheral functionalization affected the electronic structure of the tetraaza-porphyrin ir-system and resulted in perturbations of the Q bands. Their maximum extinction coefficients were much higher than those of ordinary diarylethenes. 

The stereochemistry and functions of all iron porphyrin-containing proteins can be attributed to the varied electronic structure of iron for the oxidation and spin states that are stable in physiological environments. Theoretical descriptions of the electronic structure of iron should be, in principle, applicable to the understanding of structure-function relationships in hemeproteins. 

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