Hydroporphyrins, structure

We examine here possible structural effects that may result from or accompany the generation of the primary photoproducts, and speculate about the consequences of concomitant changes in distances,conformations, relative orientations and charges on the electronic profiles of and interactions between the BChls, BPheos and their radicals. Because the primary events in green plant photosynthesis also involve a series of chlorophyll donors and acceptors ( ), similar trends should therefore prevail for chlorophyll radicals as well. Furthermore, radicals of porphyrins and hydroporphyrins (saturated porphyrins such as chlorins and isobacteriochlorins) have been... 

The examples cited above represent part of an increasing body of structural information on chlorophylls, chlorins, bacteriochlorins and isobacteriochlorins (10-14 and references therein) that points to the remarkable flexibility of these molecules This ability of the macrocycle to adjust is not limited to hydroporphyrins but is also observed in porphyrins 5,10,15,20-tetra-n-propylporphinato lead (II) assumes a "roof" shape by folding along an axis defined by two opposite methine carbons with the two planes of the "roof" inclined at 22 to one another (15) In contrast, triclinic 5,10,15, 20-tetraphenylporphinato cobalt (II) is distinctly saddle shaped with the 3 carbons of adjacent pyrrole rings lying 40 66 and -0 66A above and below the plane of the four nitrogens (16) ... 

Porphyrin can add up to six hydrogen atoms on the pyrrolic j8-carbons without loss of aromaticity. Except for 2,3,12,13-tetrahydro derivatives (Bchl) (27), the porphyrin normal is no more than a Cs axis and the four nitrogens are nonequivalent in the other hydroporphyrins (Chi (25), iBchl (26) and hexahydroporphyrin). The X-ray structures of some metallochlorins [M(Chl)] show the asymmetry of the macrocycles (Figure 9 and Table 8),21,162,163... 

Several macrocyclic ligands are shown in Figure 2. The porphyrin and corrin ring systems are well known, the latter for the cobalt-containing vitamin Bi2 coenzymes. Of more recent interest are the hydroporphyrins. Siroheme (an isobacteriochlorin) is the prosthetic group of the sulfite and nitrite reductases which catalyze the six-electron reductions of sulfite and nitrite to H2S and NH3 respectively. The demetallated form of siroheme, sirohydrochlorin, is an intermediate in the biosynthesis of vitamin Bi2, and so links the porphyrin and corrin macrocycles. Factor 430 is a tetrahydroporphyrin, and as its nickel complex is the prosthetic group of methyl coenzyme M reductase. F430 shows structural similarities to both siroheme and corrin. 

The structure of iron(II) octaethylchlorin shows the iron and four nitrogen atoms to be rigorously planar, but the rest of the chlorin macrocycle to be significantly S4 ruffled.732 Hydroporphyrins intrinsically have larger cores than porphyrins, but the distortion from planarity leads to a reduction in core size and shorter metal-nitrogen distances. The enhanced ability of hydroporphyrins to undergo distortion so as to adjust their core size in response to the size of the metal may be responsible for the differences between iron porphyrins and hydroporphyrins.733... 

This nomenclature procedure parallels that normally used for porphyrins, which is based on the compound with the highest level of unsaturation known thus the porphyrin is the parent compound and the related reduced structures are named hydroporphyrins. 

The synthetic conditions are unsatisfactory with respect to the intended role of the reaction as a potentially biomimetic model, but demonstrate that it is possible to achieve interconversion between the structures of hydroporphyrin, secocorrin and corrin. 

Hydroporphyrins may also answer theoretical questions. Porphyrin(I) is generally regarded as a very stable aromatic system. A convincing explanation of its aromaticity has been given (86PAC67). Several publications deal with the question of the preferred path of delocalization, which is related to structure, reactivity, and tautomerism of porphyrins. The connection of this problem to the properties of hydroporphyrins is obvious. As a consequence, some aspects of porphyrin chemistry are included in this article. 

Nomenclature follows IUPAC recommendations (79PAC2251). The basic macrocyclic system I is named porphyrin, which implies that the pyrrolic nitrogen atoms are placed in positions-21 and -23, regardless of the structure of the actual tautomer. Hydro prefixes are then considered in ascending numerical order. I attempt here to maintain the arrangements of the formulas of hydroporphyrins resulting from these rules. 

Norporphyrins 1-4 and hydroporphyrins II-V all represent types of 18 n-electron conjugation pathways in porphyrins (75CRV85). The properties of these compounds may therefore reflect the importance of a particular path of delocalization in a discussion of structure or reactivity. Even more important are differences of particular hydroporphyrins in view of many biochemical processes (81JA5890). 

X-Ray structure determinations of porphyrins have been reviewed (75MI3 78MI9). Selected data of porphyrins and hydroporphyrins are given in Table I. 

Given that all structurally characterized Ni(II) and Fe(II) hydroporphyrins are distorted while both planar and distorted porphyrins are known, two points have been made (1) hydroporphyrins distort more easily than porphyrins (2) the energy barrier for contraction of the macrocyclic core (with resulting nonplanarity) upon complexation to small metals is low (85JA4207). 

The greater structure compliance of hydroporphyrins over porphyrins may be a factor of chemical and biological significance (83JA4108). 

Structure 22 of the hydroporphyrin ligand system of factor F-430 has been elucidated mainly by a combination of incorporation and nC-NMR experiments (82HCA828 84HCA334). Isomerization of 22, which may occur during work-up was mentioned in Section III, D(85HCA1338). Factor F-430 (22) contains a chromophore system not previously encountered among natural products. 

The properties of hydroporphyrins described here and numerous investigations of porphyrins render possible some generalizations concerning the structure of these chromophores. 

The Ni(I) forms of these hydroporphyrin model compounds have not been isolated, and thus have been studied in solution by EXAFS (see below). However, several Ni(I) complexes with tetraazamacrocyclic ligands have been isolated and structurally characterized by X-ray crystallography [66 - 70]. These studies showed that depending on the macrocyclic ligand s flexibility, reduction of Ni(II) to Ni(I) was associated with either expansion of the Ni-N distances, a distortion in the NiN4 core (i. e., two sets of significantly different Ni(I)-N bond lengths) or both expansion and distortion. 

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