Porphyrins Metal-binding properties

The metals are generally found either bound directly to proteins or in cofactors such as porphyrins or cobalamins, or in clusters that are in turn bound by the protein the ligands tire usually O, N, S, or C. Proteins with which transition metals and zinc are most commonly associated catalyze the intramolecular or intermolecular rearrangement of electrons. Although the redox properties of the metals are important in many of the reactions, in others the metal appears to contribute to the structure of the active state, e.g., zinc in the Cu-Zn dismutases and some of the iron in the photosynthetic reaction center. Sometimes equivalent reactions are catalyzed by proteins with different metal centers the metal binding sites and proteins have evolved separately for each type of metal center. 

The co-operative effect. That a metal can become much more chemically active after chelation is evident enough in the oxygen-binding properties of haemoglobin and the oxidizing powers of the heme enzymes. Inorganic salts of iron have catalase- and peroxidase-like properties, but these are enormously increased upon incorporation in the porphyrin nucleus attached to a specific protein. Similarly, cuprous ions catalyse the aerial oxidation of ascorbic acid, but this effect is immensely magnified in the enzyme ascorbic oxidase (Table 10.4). 

The cucurbit [n]uril family (CB[n]) of molecular containers possess remarkable binding affinities and selectivities (Ka values up to 1012M-1, Krei values up to 106) which renders them useful as a component of molecular machines, sensors, and biomimetic systems (123-125). Recently, Wagner and coworkers have reported (126) that CB[10] - with its spacious 870A3 cavity - is capable of acting as a host for free base and metalated tetra (Af-methylpyridinium)porphyrins 19a-d (Fig. 17). Despite the large ellipsoidal deformation of CB[10] upon complexation, the complexed porphyrins retain their fundamental UV/VIS, fluorescence, and electrochemical properties. The CB[ 10] porphyrin... 

---