Metal-substituted chlorophylls

Commercially produced metal-substituted chlorophylls such as copper chlorophylls and copper chlorophyllins that can be obtained by chemical modification of natural chlorophylls have better stability, solubility, and tinctorial strength, but they cannot be considered natural food colorants and will be discussed later. 

Photosynthesis can be affected in many ways. Metals can influence biosynthesis of biomembranes and photosynthetic pigments, especially chlorophyll. They may inactivate enzymes by oxidising SH-groups necessary for catalytic activity or by substitution for other divalent cations in metalloenzymes. They finally can also interact with the photosynthetic electron transport and with the related photophosphorylation. 

Many synthetic dyes such as porphyrins, acridine, thlonlne and flavin dyes have been used in photosensltlzatlon of electron transfer reactions. In the past few years several promising organometalllc compounds have been prepared as substitutes for the natural labile chlorophyll. These organometalllcs Include a variety of metals, chelated to bipyridine or porphyrin ligands ( ). The photophysical properties of these sensitizers and their potential use in artificial photosynthetic devices have been extensively reviewed ( 7, ). In particular, sensitizers such as... 

Alzawa et al. showed that chlorophyll-liquid crystal electrodes in acidic buffer solutions gave cathodic photocurrents accompanied by the evolution of hydrogen gas (52). Substitution of the central metal of native chlorophyll (Mg-pheophytin) resulted in a drastic change of photoelectrochemlcal behavior. A Mn-pheophytin/ liquid crystal/Pt electrode generated anodic photocurrent upon illumination. In contrast, Ru-pheophytin/liquid crystal/Pt electrode gave a cathodic photocurrent with a quantum efficiency of approximately 0.5% (54). Fong s work on photoelectrochemlcal cell has proved that chlorophyll hydrates are photoactive (49). 

The chemical answer to this is to identify the sites on the porph)rrin molecule where reactivity occurs and to bl(x k them. These sites are the mcso-carbons and the metal centre. Nature, of course, solved this problem eons ago by embedding the sensitive chlorophyll molecule in the protein-lipid matrix of the chloroplast membrane. Synthetically, this problem is solved by substituting water-solubilising groups in the wicso-positions so that a molecular chain covers that position and the metal centre like an umbrella. 

Metal complexes of various porphyrins 47, phthalocyanines 48 and naphthalocyanines 49, with intense absorptions in the visible region of light, have also been covalently bound to macromolecular carriers. Early work in this field was reviewed in 1977 and 1983 [124,125], These reviews, for example, reported the fixation of metal-free and metal-containing protoporphyrin-IX, chlorophyll, mesoporphyrin-IX and substituted 5,10,15,20-tetraphenyl-porphyrins. The binding of substituted phthalocyanines to substituted polymers has also been reviewed [125,126] and covers mainly the binding of carboxylic or sulfonic acid-substituted phthalocyanines to polystyrene. 

Adenosylcobalamin, cyanocobalamin (both active forms of vitamin B12), heme, and chlorophyll a aU enjoy the structural similarity of possessing a metal (cobalt in B12, iron in heme, and magnesium in chlorophyll a) centrally held in a core of four cojoined pyrrole (or pyrrole derived) rings. That these tetrapyrroUc systems are individually adorned with other substitution elements allows them and a large number of related materials, which are also composed of the same four pyrrole (or pyrrole-derived) rings, their respective functions. 

From the examples shown it became clear that most studies have been motivated so far from the structural point of view. As a typical example, lightharvesting chlorophyll dye assemblies motivated researchers to design por-phyrinoid model systems. For synthetic reasons chlorin dyes were substituted by porphyrin dyes and the magnesium ion was substituted for other metals like cobalt (e.g. Hunters cychc array 17). As a consequence even though the metal-assembled structures look quite related to the natural counterparts, the properties are so different that photophysical studies on these assembhes have not even been initiated (because cobalt totally quenches the fluorescence of the porphyrins). This example clearly points towards our future goal... 

---