Porphine ring

The various porphyrinogens are colorless, whereas the various porphyrins are all colored. In the smdy of porphyrins or porphyrin derivatives, the characteristic absorption spectrum that each exhibits—in both the visible and the ultraviolet regions of the spectrum—is of great value. An example is the absorption curve for a solution of porphyrin in 5% hydrochloric acid (Figure 32-10). Note particularly the sharp absorption band near 400 nm. This is a distinguishing feamre of the porphin ring and is characteristic of all porphyrins regardless of the... 

Appropriately placed groups on the porphyrin periphery can aid considerably the incorporation of metal ions into the ring. The porphyrin shown, Hjtmpp, 13 reacts rapidly with Co(II), Ni(II), Cu(II) in dmf/H20. The rate data indicate that an initial adduct is formed which allows the metal to more easily transfer into the porphine ring. See also Ref. 129 for a similar behavior with an azamacrocycle. 

A macrocyclic ligand called the corrin ring, which bears various substituents. It resembles the porphine ring, but differs in various ways, notably in the absence of one methine, =CH, bridge between a pair of pyrrole rings. 

Figure 13.4 The anion of porphine. Porphyrins are porphine rings with peripheral substituents. The porphine ring is flat, and metal ions can be bound within the square planar arrangement of nitrogen atoms.

This behaviour is due to the formation of an extended network of electrostatic interactions (i.e. those between the oppositely charged peripheral groups and the 7t—n between the porphine rings) [40]. 

V-methylpyridyl groups (permanently cationic) in the me so position of the porphine ring with four pyridyl groups which are ionisable (Fig. 14 pKa 4). and can cycle between neutral (deprotonated) and cationic (protonated) states. In order to avoid nitrogen protonation of the inner core of H2TPPS (pAfa 5) we have used CuTPPS instead of the naked anionic parent (Fig. 1). L-Phe and D-Phe are the chiral templates. 

Left detail of Mg coordination notice that the metal is slightly out of the plane of the porphin ring. Right plan view of Mg within the porphin ligand. A histidine residue from an associated protein forms the fifth coordination point to the Mg atom. 

Substituting some or all of the hydrogen atom(s) of the porphin ring leads to the formation of so-called porprin derivatives, porphyrin (For) (Figure 4.22). 

To date, few porphyrin derivatives has been revealed to exist in natural plant and animals despite that a large number of porphyrin derivatives having been artificially synthesized. Usually, artificial porphyrin derivatives are prepared by substituting the hydrogen atoms at the 5,10,15, and 20 positions of porphin ring, but the natural porphyrin derivatives are the product substituted at the 2, 3, 7, 8, 12, 13, 17, and 18 positions [38]. Some of the most representive porphyrin derivatives are shown in Figure 4.23. 

Quadridentate ligands can be obtained by condensing suitably substituted aldehydes and ketones with bidentate primary amines, and the resulting Schiff bases closely resemble the porphin ring system. Generally, one of the structural units—N=C—C=N—,—N=C—NHC=N—, N=C—NH—N=C—, —C=N—CH=CH—N=C—, or —C=N— CH2—CH2—N=C—is incorporated into the structure for maximum stability. For example, if or /io-aminobenzaldehyde is condensed with ortho-... 

FIGURE 8.22 Structures of (a) porphine, (b) heme, and (c) bacteriochlorophyll. The porphine ring common to the three structures is highlighted in blue. 

Stacking by 2,3,7,8 dioxin and PCBs. Note that the 2.2, 4,4, 6,6 HCB does not effectively interact with the Porphine ring system. 

From the total kinetic energy of the 24 porphin ring atoms, we calculated the instantaneous kinetic temperature... 

The effect of Cl" binding on the location of the lowest (71,71 ) state is minimal, since this state is essentially localized on the free-base porphine ring. However, the negative ion coordination facilities loss of an electron by the MgP moiety, resulting in a CT state of lower energy than in the uncoordinated complex. Thus, the present studies have shown that environmental effects, such as stabilization by a polarizable micro-environment and/or negative ion coordination, are essential in order to produce a low-lying excited CT state in the MgP-P dimer system. 

Proton transfer exchange in a porphin ring. The intramolecular transfer of protons within a porphin ring reported by Limbach (T) is of interest, both because it shows the special types of information accessible by n.m.r. methods (3l) and because the protons migrate between nitrogen atoms at a fixed distance (equation 6), so that this is an instance of a bound" proton... 

Determine the symmetry elements present in the molecule, and its point group. Does the point group change if an Fe ion is substituted for the two hydrogen atoms in the center of the porphine ring ... 

Porphine has symmetry as a moiecuie substituting metai ions for the two H atoms inside the porphine ring, and the symmetry becomes... 

The type and sequence of substituents on the porphine ring is determined during its biosynthesis (see Section 1). 

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