Quinone/semiquinone

C/B and D/A, which correspond to the quinone/semiquinone/catecholate sequence of [Ir(triphos)(Phencat)]+. 

Os(bpy)2LL]2+/1+/0 (L-L are quinone, semiquinone, or catecholato ligands derived from catechol, 3,5-di-ter<-butylcatechol, or tetrachlo-rocatechol) have been characterized by UV/Vis/NIR and EPR spectroscopies. These spectroscopic properties and the crystal structure of [0s(bpy)2(dbcat)]C104 confirm an Os( III (-catecholate ground state for the +1 ion. This contrasts with the ground state of the +1 ions of Ru analogs, which are best described as Ru(II)-semiquinone complexes... 

Roginsky VA, Pisarenko LM, Bors W, Michel C (1999) The kinetics and thermodynamics of quinone-semiquinone-hydroquinone systems under physiological conditions. J Chem Soc Perkin Trans 2 871-876... 

Quinones, Semiquinones, and Catechols. All molecules with unsaturated bonds (olefins, acetylenes, aromatics, carbonyls, quinones, etc.) have a degree of electrophilicity and electron affinity. Within a class, the extent of conjugation... 

Dopaminergic neurons contain neuromelanin, a pigment composed of lipofus-cin along with a complex mixture of polymers of the various catecholamines, metal ions, cysteine, and possibly other substances (A4). This waste pigment is presumed to be derived from the oxidation of dopamine and other catecholamines to produce quinones, semiquinones, and quinhydrones, some of which undergo... 

Superoxide (02 ) and the peroxyl radical (H02) have been intensively studied, and a good account of their thermochemistry is presented in Standard Potentials (pp. 60-63). They are related by the pKa of H02, which is 4.8 0.1 (51). The reduction potential of the 02/02 couple has been determined by a variety of methods, including, for example, the equilibria with various quinone-semiquinone systems. The value cited, — 0.33 V, is taken with respect to a standard state of 1 atm 02 pressure. When expressed relative to the 1 Mstandard state of 02, the potential is —0.16 V. Standard NBS data permit calculation of AfG° = 4.4 kJ/mol and 31.8 kJ/mol for H02 and 02, respectively. Some related potentials include 0.12 V for (H+, 02)/H02,1.44 V for (H+, H02)/H202, and 0.75 V for H02/H02 . 

The quinone, semiquinone, and catecholate species particularly of iron are important in electron-transfer processes in photosynthesis and respiration. 

A study of several quinone/semiquinone reactions [46] used similar conditions (low-exchange limit) and an in situ photolysis-ESR flow instrument. The chemistry involved in the generation of semiquinones is shown in Eq. 38... 

In photosystem I, the primary electron acceptor is a monomeric chlorophyll called A. The secondary electron acceptor is a bound phylloquinone (abbreviated as OQ in this book), whose in situ quinone/ semiquinone redox potential has been estimated to be < -0.81 V. Phylloquinone has the same phytol side... 

Scheme 2 Redox active ligand participation in catalytic water oxidation by a binuclear Ru catalyst containing ortho-quinone/semiquinone ligands...

Quinone / semiquinone anion / catecholate dianion Effective magnetic moment (in Bohr magnetons)... 

Qumones are capable of reacting with a large number of radiation-produced primary species like eaq, H and OH [14-16]. While eaq and H may reduce quinones by one-electron reduction process to the semiquinone, OH may either add to the ring or some suitable substituent position and give rise to one-electron oxidation to some form of transient. Formation of semiquinones by y-radiolysis has been discussed in detail m earlier monographs [7,9,17-19] and will not be included here. Pulse radiolysis kinetic spectrophotometry technique [20,21] has opened up new scope for detailed studies [8-12,15,16,22-25] on semiquinones. On pulse radiolytic one-electron reduction of quinones, semiquinones may be formed as follows -... 

It was conclusively established that such exceptional stabihty was due to the nearness of the E (one-electron reduction potential), E (second one-electron reduction potential) and E (two-electron reduction potential) values of the quinone and the characteristic changes of E, E and E as a fimction of pH as a consequence of the pK values of the quinones, semiquinones and hydroquinones. These are discussed later in the section dealing with redox potentials. 

In a biological environment, the redox properties of the quinone/semiquinone system will be profoundly affected by solvation as well as by complexation with other surroimding molecules, and also by availability of large number of free and bound protons. Semiquinones are capable of complexing with quinones, other semiquinones, as well as hydroquinones. They can also complex with metal ions [91,92]. Even in the simplest case of Cu /benzosemiquinone system, the latter substitute for a water molecule in the co-ordination sphere of the metal ion. Although the reduction potential of benzoquinone is +99 mV and that of Cu " is +153 mV, no rapid inner-sphere electron transfer is seen. 

Quinone Semiquinone radical formation/ oxidative stress/thiol trapping... 

Coenzyme Q is capable of accepting either one or two electrons to form either a semiquinone or hydroquinone form. Figure 11-4 shows the quinone, semiquinone, and hydroquinone forms of the coenzyme. Coenzyme Q is not bound to a protein instead it is a mobile electron carrier and can float within the inner membrane, where it can transfer electrons from Complex I and Complex II to Complex 111. 

Distinguish among the quinone, semiquinone, and ubiquinol forms of coenzyme Q. Explain how reduction of a quinone can consume two protons. 

Transfer of the second electron results in formation of the hydroquinone. Because this reaction is readily reversible, the equilibrium between the quinone, semiquinone, and hydroquinone will be dependent on the reduction potential of the system of interest. Few studies of quinone-containing pollutants in natural reducing systems exist. Studies of quinones have focused primarily on elucidating their role as electron mediators in the reduction of other pollutants of interest (see Section... 

Peroxidase (quinones, semiquinones free radicals) Chlorophyll (in light)... 

Redox active dioxolene derivatives exist as different isomers of benzo-quinone, semiquinonate and catecholate by two-step one-electron transfer (Scheme 55). A linear chain structure with direct a Rh Rh bond was reported with 3,6-di-tert-butyl-l,2- benzosemiquinonate (3,6-DBSQ) radical anion in [Rh (C0)2(3,6-DBSQ)]. It was originally synthesized by the reaction of T1(3,6-DBSQ)... 

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