porphyrin

A macrocyclic ring formed from the cyclisation of polypyrrole. Here's how to draw one:

1. Draw four Ns at the corners of a square.
2. Eight lines radiate out from these Ns. 2 up, 2 down, 2 left, 2 right.
3. The two lines connected to each N are joined into 4 separate pentagons.
4. Join the pentagons with methylene (1 carbon) bridges.

The structure can now be embellished with double bonds and other modifications. Two of the pentagons (diagonally opposite) are the same (N1=CC=CC1) while the other two are (N1C=CC=C1) and (N1CC=CC1). The double bonds in the bridges are such that the whole system has conjugated pi orbitals.

Porphrins(Pph) are found in many biological cofactors, with different metals in the center. Iron-Pph is haem, while magnesium-Pph is chlorophyll. More unusual metals (in biology) like nickel occur in some archaea (Ni-Pph is F450) while cobalt is found in vitamin B12.

Supramolecular chemists are also fond of making unlikely porphyrin structures, such as 'calix[4]arenoporphyrin'. This is the calixarene described in the node, but with porphyrin groups in place of the t-butyl groups. Weird.

Porphyrins have also been quite popular in inorganic chemistry as catalysts for the reduction of oxygen (potentially useful for fuel cells). Some porphyrins can polymerize, and some of these polymers can conduct electricity - possibly useful for molecular wires.

Phorphrins are a class of biologically significant pigments. They are composed of four pyrrole rings joined into flat macrocyclic compounds by one-carbon bridges in a conjugated system of 18 pi electrons. This lends such molecules exceptional stability. The parent molecule, porphine, has two hydrogens bonded to two of the central four nitrogen atoms, and is not found in nature.
Porphyrins are typically highly coloured, and form stable complexes with metallic ions which bind to each of the four central nitrogens and nestle in the exact middle of the molecule.

The most biologically important examples are haem, where an atom of Fe occupies the central position, and Chlorophyll, where the position is taken by an atom of magnesium. Haem is the oxygen-transport iron-porphyrin complex responsible for the red colour of arterial blood. It exists in red blood cells in a complex with the globin protein, as haemaglobin. The iron atom is complexed with the four porphyrin nitrogens but also has two additional coordination sites, one above and one below the plane of the porphyrin ring. One of these sites is occupied with an imadazole ring from a histidine unit in the globin proein, and the second site is available for binding with oxygen. Carbon monoxide can also bind to this site, with many times more affinity than oxygen, leading to the threat of suffocation in high CO environments.
The green colour of plants is due to chlorophyll, a magnesium complex of modified porphyrin. This pigment is found in large organelles called chloroplasts, one or more of which can be found in some types of cells, depending on the plant. Clorophyll-alpha is an ester of the long-chain alcohol phytol, which helps solubilize the pigment in the choloroplasts.

           
          H           H           H
           \          |          /
            C -- C == C -- C -- C
          //     |         ||    \\
      H - C      |         ||     C - H
           \     |         ||     /
            C -- NH         N -- C
           //                    \\
      H - C                       C - H
           \                     /
            C == N        HN -- C
           /     |         |     \\
      H - C      |         |      C - H
           \\    |         |     /
            C -- C == C -- C == C
           /          |          \
          H           H           H       
                      

                  Porphyrin

               
             CH2
           //
          C           H           CH3
           \          |          /
       H    C -- C == C -- C -- C     H    H
       |  //     |         ||    \\   |    | 
   H - C- C      |         ||     C - C == C - H
       |   \     |         ||     /
       H    C -- N-        N -- C
           //                    \\
      H - C           Fe2+        C - H
           \                     /
      H     C == N        -N -- C
      |    /     |         |     \\
  H - C - C      |         |      C - CH3
      |    \\    |         |     /
      H     C -- C == C -- C == C
           /          |          \
          C           H           C       
           \                     /
            C                   C
           /                     \
          C=0                  O=C
          |                      |
          OH                     OH

                  Haem