Porphins side chains

Effect op Porphin Side Chains on Activity op Hemoclobin and Peroxidase and on Growth of Strtgomonas/oscicttiota... 

Porphyrins are formally derived from the porphin (1) nucleus by substitution of some or all peripheral positions with various side chains. In the classical system of nomenclature T, introduced by H. Fischer,Sc the peripheral /5-pyrrolic positions are numbered front 1 to 8 and the methine positions (also named meso positions) between the pyrrole rings are designated a, //, y, and 5. The rings are lettered clockwise A, B, C, and D. The classical nomenclature was in the past more and more displaced by a nomenclature which numbers all the carbon... 

Natural Porphyrins Have Substituent Side Chains on the Porphin Nucleus... 

The porphyrins found in nature are compounds in which various side chains are substituted for the eight hydrogen atoms numbered in the porphin nucleus shown in Figure 32-1. As a simple means of showing these substitutions, Fischer proposed a shorthand formula in which the methenyl bridges are omitted and each pyrrole ring is shown as indicated with the eight... 

Attempts to co-crystallize amphiphilic porphyrins with micellar fibres failed completely. The van der Waals interactions between porphyrin planes ( stacking ) are so strong that unsubstituted porphin and (3-octamethylporphyrin are insoluble in practically all solvents. Porphyrins with larger substituents such as ethyl or acetic acid side chains allow face-to-face dimerization only, an action which occurs in all solvents and also probably in micelles down to a concentra-... 

N4 mol wt 310.34. C 77.407 , H 4.55%, N 18.06%. Parent substance of the porphyrins, a group of compounds found in all living matter which are the basis of respiratory pigments in animals and plants. In porphyrins, side chains are substi -luted for the hydrogens in the porphine pyrrole rings. Chfo-rins are dihydroporphyrins. See also chlorophyll, hemoglobin, vitamin Bl . hematin. Prepn of porphine Fischer,... 

The H93G mutant and modified Mb proteins serve as excellent model systems and have been used in theory and experiment to probe the fundamental principles of energy transfer and heme cooling. Similar Mb models were studied in subsequent experiments. Champion and coworkers studied the spectral dynamics of the photo-excited horse heart Mb, both the native and H93G mutant [63]. It was found that the transient relaxation of each compound is completed within 10 ps the optical transient of H93G was observed to be quite similar to that of native deoxyMb. When the protoheme in Mb was replaced by an iron porphine, in which all heme side chains were replaced by hydrogens, the relaxation is also completed within 10 ps but is obviously slower than the native Mb, consistent with simulation results [62],... 

FIGURE 9.6 Depiction of the computed porphine V4 and V7 modes. The ligand imidazole and porphyrin side chains are not shown. 

To clarify the role of the imidazole ligand in the VER process, the VER properties of the V4 and v-, modes in iron porphine (FeP), a porphyrin model withont side chains or axial ligands, were examined [88], Relaxation of the excited V4 and V7 modes in FeP is predicted to be slower than relaxation in FeP-lm, consistent with the fact that there are fewer bath modes in FeP relative to FeP-Im. However, we find the identified energy transfer pathways for FeP to be similar to those for FeP-Im for the V4 and V7 modes. 

For each bath mode, the contribution to the norm from each atom can be calculated from the normalized eigenvectors. Modes with a total contribution larger than 0.5 from the side chain atoms were identified as side chain dominant modes those with a contribution larger than 0.8 from the side chain atoms were identified as side chain localized modes. Similar definitions can be made for porphine core atoms or the two isopropionate groups. Table 9.6 summarizes the results for the dominant modes or the localized modes for those modes involved in the important energy transfer pathways in V4 and V7 modes relaxation. 

For the excited V4 mode, both porphine core motions and side chain motions are predicted to be involved in the VER pathways. We draw similar conclusions for the excited mode. In spite of the distortion of the porphine core, comparable to that observed in Ni-heme, the v-, mode is not predicted to be involved in the energy transfer pathways from the excited V4 mode in NiOEP. The overtone is strongly coupled with the mode in NiOEP, but the third order Fermi resonance parameter is smaller than observed for other pathways. This result is similar to the predictions for the nonplanar Ni-heme (see Table 9.5). 

Porphine itself does not occur in nature, but analogous compounds with various side chains on the pyrrole rings are some of the most important life-sustaining compounds of nature. One example is heme, the iron-porphyrin complex responsible for the red color of arterial blood. 

In the protoporphyrins, the eight carbon atoms of porphin carry two vinyl groups, four methyl groups and two propionic acid groups. One of the fifteen possible isomers oif this structiure is protoporph3rrin IX or porphin-l 3 5 8-tetramethyl-2 4-divinyl-6 7-dipropionic acid, commonly called protoporphyrin. This is the porphyrin most widely distributed in nature, it is a constituent of haemoglobin, catalase, peroxidase, of cytochrome-6, etc. Diuing the course of putrefaction protoporphyrin loses its unsaturated side chains and is transformed into deuteroporphyrin III. On the other hand, it can be derived by a series of operations from copro-porphyrin III, another proof of its chemical kinship with the porphyrins of Series III. 

The type structure of the porphyrins porphin. The side chains of representative porphyrins. Metal porphyrin derivatives. Azaporphin. 

Porphin by itself has no catalytic function except when iron is inserted into the ring. The specific side chains at the positions of the pyrrole rings undoubtedly are also related to the functioning of this pigment. Two approaches have been used to determine their function. One has been to vary the side chains and note how this influences the function of the modified heme protein. Another approach has been to supply modified porphins to organisms which require iron protoporphin for a growth factor and note whether these modified porphins could replace iron protoporphin in the catalytic functions essential for growth. 

The effect of side chains on the activity of peroxidase has been studied recently by Theorell, Bergstrom, and Akeson (135). The normal prosthetic group, iron protoporphin IX, was replaced by other iron porphins on the... 

Concerning the function of the other side chains around the porphin ring, namely, the methyl groups, no direct evidence is available. From the chemical point of view a reasonable postulate is that they function as a protection for the reactive pyrrole rings, preventing them from entering into undesirable side reactions which might occur in the cells. 

Oxidation of hemes, bile pigments, and most dipyrrylmethenes with hydrogen peroxide in alkaline solution ves rise to propentdyopents with different side chains. Monopyrroles, dip3rriyl ketones, porphins, and stercobilin do not give rise to propentdyopents (42). 

Porphin, the parent substance, containing no side chains, is of the phyllo type (Fig. 3, curve). As side chains are added, the third band gradually increases in height as in 1,4-dipropionic acid porphin, is almost of the etio type in l,5-dimethyl-2,6-diethylporphin (124a) (Fig. 3, curve 2), and is of the etio type (or stepladder absorption spectrum) in the naturally occurring porphins (Fig. 4) in which all the pyrroles have side... 

In the fully substituted porphins, changing the relative positions of the side chains (with the exception of the C=0 groups) to form different isomers has no effect on the absorption spectrum. Substitution of one... 

The effects of the differences in solubility due to the side chains, and the differences in pK at which the protons are added at the pyrrolenine nitrogens, may be combined to bring about the separation of closely related porphins. A method based on a combination of these properties was first systematically applied and used with great skill by Willstatter (158). The Willstatter method consists of the solution of the porphins in ether and extraction with aqueous hydrochloric acid of different concentrations. The HCl numbers are characteristic physical constants of the porphins. The HCl number is defined as that per cent of aqueous hydrochloric acid which will extract two-thirds of the porphin from w ater-saturated ether into an equal volume of the aqueous layer. The ether solution is made up to contain 20 milligrams per cent porphin or a saturated solution containing less. 

---