Scheme 1.9: Different types of interactions of o-quinone with metal ions
1.7: Electron transfer in quinones and their derivatives
Biological photosynthesis process is characterized by a series of electron transfer events in the reaction center which produces long-lived charge-separated states.159 The initial step in this series of electron transfer processes is transfer of an electron from one photoexcited chlorophyll to an adjacent quinone.160 A large numbers of porphyrin- quinone systems are developed for understanding photo-induced charge separation in photosynthetic reaction centers. Most of the model compounds consist of porphyrin molecules covalently linked to electron acceptor quinones.161-163 Spectroscopic study on these materials have revealed the electron-transfer rates depends on donor-acceptor coupling, solvent and temperature.
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Electron transfer in non-covalently bound quinone system has been explored in zinc porphyrin complexes. For example, a calix[4]arene substituted zinc (II) metalloporphyrin (1.81) binds benzoquinone through hydrogen bonds of two phenolic O-H groups serving as a tweezer with carbonyl of quinone was studied.164 In this case spectroscopic evidences on through space electron transfer were observed.
N
N N
N Zn O
O
MeO
MeO NO2
O O
Electron transfer hv
H H
Figure 1.32: Calix[4]arene substituted zinc (II) metalloporphyrin bound to benzoquinone receptor (1.81).
The electron transfer in the quinone-linked zinc porphyrin with a flexible peptide spacer (1.82) was studied by 1H-NMR and fluorescence spectroscopy.165 Such study indicated that the flexibility of peptide group contributes to the electron transfer process.
N
N N
N
MeO2C C-(NHCH2C=O)nNH-CH2
O
O
OMe OMe Zn
O
n = 0-2
Figure 1.33: Quinone-linked zinc-porphyrin with flexible peptide spacer (1.82).
Covalently linked quinone to porphyrin exhibited enhanced fluorescence on interaction with hydroquinone (1.83).166 This occurs due to the unfavorable condition for an electron transfer process between the singlets excited porphyrin and quinone-hydroquinone entity.
Addition of 1,4-hydroquinone to a solution of 1.83 showed a drastic enhancement of the fluorescence intensity at 652 nm and 720 nm.
N NH N
HN O
O O
O H
H
Rn = Alkyl groups Rn
Figure 1.34: Adduct of porphyrin linked quinone receptor with hydroquinone (1.83).
1.7.1: Quinone and their derivatives in photosynthesis
Quinone derivatives play important role in biological systems such as respiration and photosynthesis.167-170 The functional and structural protein units participating in photosynthesis is called photosystems. Their function is to absorb light and transfer energy and electron. The reaction centers in the photosystem are enzymes that use light to reduce molecules. Two families of reaction centers are present in the photosystems namely, photosystem I (PS I) and photosystem II (PS II). There are differentiated by the response to light by which they are reactive namely; photosystem I (PS I) 700 nm and photosystem II (PS II) 680 nm in chloroplasts. Photosynthesis is initiated by a series of photochemical reactions in which light energy absorbed by chlorophylls is converted to redox energy that is used to power a series of metabolic reactions.171-178 The reactions by light absorption in photosynthetic organisms can be divided into two separate processes.
In the first step, light is absorbed by antennae, which are proteins that bind several chlorophyll molecules. In many cases, a large numbers of such proteins are available to trap light energy. Chlorophylls absorb light and pass on this energy to other adjacent pigments in a process called exciton transfer or resonance energy transfer. Structural and organization of antennae proteins reveal that these proteins are ligated to chlorophylls in highly organized manner. The photosynthetic reaction centers of photosystem I and II of oxygenic organisms such as cyanobacteria, red and green algae, higher plants and in photosynthetic bacteria, contain a single photosystem. There are multi-subunit membrane
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protein complexes that function as photochemical devices. Photosystem II contains two quinones, one function as one-electron acceptor and other functions as two-electron and two-proton accumulator. In photosystem II, plastoquinone-9 (PQ-9) serves the role of mobile quinone, which shuttles electrons to Photosystem I via the cytochrome complex.
Scheme 1.8: Role of quinone systems in photosynthesis process.
The reaction centers of photosystem I of cyanobacteria and green plants, contain two bound phylloquinone (1.7). Thus, phylloquinone (1.7) is a focus of structure function relationships due to its central role in electron transfer.179-188 In this context, the light- induced189-193 reduction of quinoidal units in photosystem II is shown in Scheme 1.8. The core of photosystem II has water and plastoquinone oxidoreductase. The photosystem II catalyzes the light-induced electron transfer from water to plastoquinone. These are accompanied by net transport of protons from the cytoplasm to the lumen and accompany the production of molecular oxygen and also release of the plastoquinol into the membrane phase.
1.8.2: Quinone derivatives in environment
Quinones interact with biological systems to promote inflammatory, anticancer actions and also induce toxicities.194-195 Naphthoquinone derivatives are of a particular interest for environmental science because of their prevalence as natural products and their presence in the atmosphere as by-products of fuel and tobacco combustion.196-197 For example, 1,2- and 1,4-naphthoquinones (1.3 and 1.4) are toxic metabolites of naphthalene and these are also major polynuclear aromatic hydrocarbons present in ambient air.
Quinones are formed by direct and indirect ozone-based processes in which polynuclear aromatic hydrocarbons are adsorbed on particle surfaces. They are oxidized by ozone or
by ozone-based oxidized alkenes derivative.198-201 There are differences in reactivities between particle bound and vapour phase quinones each of which could also cause toxicity in different ways. Quinones in vapour phase have ready access to the lung components, whereas particle exposure varies with size and content. Studies with radiolabeled benzo[a]pyrene-7-8-dione (1.76) have shown that when it is in the particle bound state to diesel exhaust particles, this poly aromatic hydrocarbon (PAH) persists for protracted periods relative to free vapour phase PAH.202-204 Quinone and their derivatives can easily get distributed to cellular components by dissociation from the particles in accordance with their partition characteristics,
O O
Figure 1.35: Benzo[a]pyrene-7-8-dione (1.84).