Pairing, oxidative

Formally, in redox reactions there is transfer of electrons from a donor (the reductant) to the acceptor (the oxidant), forming a redox couple or pair. Oxidations in biological systems are often reactions in which hydrogen is removed from a compound or in which oxygen is added to a compound. An example is the oxidation of ethanol to acetaldehyde and then to acetic acid where the oxidant is NAD. catalyzed by alcohol dehydrogenase and acetaldehyde dehydrogenase, respectively. 

Redox pairs Oxidation (loss of electrons) of one compound is always accompanied by reduction (gain of electrons) of a second substance. For example, Figure 6.11 shows the oxidation of NADH to NAD+ accompanied by the reduction of FAD to FADH2. Such oxidation-reduction reactions can be written as the sum of two halfreactions an isolated oxidation reaction and a separate reduction reaction (see Figure 6.11). NAD+ and NADH form a redox pair, as do FAD and FADH2. Redox pairs differ in their tendency to lose electrons. This tendency is a characteristic of a particular redox pair, and can be quantitatively specified by a constant, E (the standard reduction potential), with units in volts. 

Recently, Do and Yeh [113] have considered the degradation of formaldehyde by paired oxidation with electrogenerated H202 and hypo-... 

Addition of an electron pair (reduction) or ligand can result in M-M bond breakage, whereas removal of an electron pair (oxidation) or ligand can result in M-M bond formation. The most common form of this reactivity pattern is the elimination of CO induced either thermally or photochemi-cally. In the case of E[ML ]3 complexes, intermediates with only one or two M-M bonds are often not observed, and the resultant complexes are the doxo-tetrahedral complexes as exemplified in Eqs. (141)65,139 and... 

Introducing excessive amounts of standards into the samples, which disturbs the equilibria in the material as a result of physicochemical modification of the nature of the matrix Introducing an internal standard that interacts with the investigated compounds (via complexing, formation of ion pairs, oxidation-reduction reaction, addition or hydrolysis reactions) Conducting extraction in uncontrolled pH conditions, or under elevated pressure and temperature, triggering processes of hydrolysis or alkylation... 

Oxidation (s. a. Ammoxidation, Autoxidation, Pairing, oxidative, Ring closure, —, Solvolysis, —) ON OS OC —, promoted by steric strain 19 923... 

Ion association effects are summarized as follows. Saveant s work indicates that the activation energy barrier for electron hopping between fully associated ions is large due to the fact that electron transfer between oxidized and reduced components requires dissociation of an ion-paired linkage and formation of a new one. Consequently predissociation mechanisms in which the fully ion-paired oxidized half of the redox couple dissociates prior to electron transfer, which is then a simple outer-sphere process, are energetically more favorable. What about the... 

Eq values have been determined for many couples. When used in pairs, oxidation-reduction potentials give the same sort of information as other measures of free energy changes. This information can be expressed as the extent to which a reaction will proceed. This function is useful for determining the answer to such practical questions as in which direction will a reaction actually proceed can a given system pull another is a given reaction likely to be reversible etc. 

Hydrogen Atom Acceptor Pairs Oxidant Base E°(V) pKa BDFE ... 

According to reference [13], at low temperatures the R.>ta6>tam fluorescence kinetics change with the observation wavelength in case of both open and closed (special pair oxidized) RCs. With closed RCs most fluorescence kinetics at 77 K could be well approximated by the sum of two exponents with 15-5 ps and 170-20 ps decay constants. Based on our model, the former of these constants allow us to estimate the average time for excitation stay in the lattice site, which is 5-8 ps, and the approximate value of the lattice constant, which is 32 8. 

---