| Abstract |
In recent years, scientific community’s attention has been drawn to the development and design of new synthetic systems which catalyze reactions using solar energy as a sole source of energy. These novel technologies are catalytically active for reactions like hydrogen oxidation, dioxygen reduction or water splitting to molecular Ο2 and Η2. Various studies have demonstrated that the combination of photosensitizers and enzyme can support reactions like O2 reduction or H2 oxidation.
Photosynthesis is a complex procedure taking place in plants where the sunlight is the source of energy triggering the decomposition of water into molecular Ο2 and Η2. The equation is shown below
2Η2Ο → 2Η2 + Ο2.
By mimicking the natural photosynthesis, solar energy can be used to drive catalysis and convert light to stored chemical energy. Our target is to develop robust, stable systems powered by sunlight associating an electron donor to a unit capable to react with dioxygen. Photosensitizers are excellent candidates as electron shuttle since they are capable of harvesting solar energy. On the other hand, metalloenzymes like laccase act as “oxygen catalytic units”.
In the present study, we report two hybrid photocatalytic systems designed to function in visible-light driven dioxygen reduction. The first system – the so called grafted system- contains a photosensitizer covalently bound on the surface of an enzyme (laccase). In the second system β-cyclodextrin is grafted on laccase and a suitable photosensitizer was used to interact deeply into the cavity of cyclodextrin, affording that way a supramolecular system.
Porphyrins (Zn-P and COOCH3_NH_Pyrene in particular) were selected as photosensitizers since they demonstrate high stability, high absorption in the visible spectrum and display long life time in their excited states. Laccases, on the other side, are robust oxidoreductases which catalyze the oxidation of various organic and inorganic substrates with the concomitant reduction of dioxygen into water as by-product (abundant, not dangerous, non toxic, inexpensive). At first, activity tests were performed to determine the percentage of activity laccase carries after the coupling we made in both systems. It was revealed that in covalent attachment of Zn-porphyrin_aldehyde on enzyme’s surface, 80% of the initial activity of the enzyme was kept whereas in covalent attachment of cyclodextrin_aldehyde we have only 40%. Fluorescence titrations showed a 1 : 1 COOCH3_NH_Pyrene – β-cyclodextrin inclusion complex. Last but not least, some dioxygen consumption experiments were conducted in order to investigate these two systems for their ability to function in visible light driven dioxygen reduction. Grafted system showed the best performance whereas the effectiveness of supramolecular system was almost absent, possibly due to the powerless and weak intermolecular forces developed among the components.
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A grafted versus supramolecular moiety of Porphyrin , Laccase and β-cyclodextrin assembly for O2 reduction
