Exergonic process

ENDERGONIC PROCESSES PROCEED BY COUPLING TO EXERGONIC PROCESSES... 

The cyclodimers are liberated from the respective elimination products 8a and 10a via successive substitution processes with incoming butadiene, that regenerates the active catalyst la in an overall exergonic process. For the rate determining reductive elimination step of the C8-channel free-energy activation barriers of 20.1-24.1 kcalmol-1 are predicted for catalysts TTV, that are in excellent agreement with experimental estimates.43 Thus, moderate reaction conditions are required for the catalytic cyclodimerization of 1,3-butadiene.6... 

An exergonic process is one for which the free energy change is negative. 

Acyl residues are usually activated by transfer to coenzyme A (2). In coenzyme A (see p. 12), pantetheine is linked to 3 -phos-pho-ADP by a phosphoric acid anhydride bond. Pantetheine consists of three components connected by amide bonds—pantoic acid, alanine, and cysteamine. The latter two components are biogenic amines formed by the decarboxylation of aspartate and cysteine, respectively. The compound formed from pantoic acid and p-alanine (pantothenic acid) has vitamin-like characteristics for humans (see p. 368). Reactions between the thiol group of the cysteamine residue and carboxylic acids give rise to thioesters, such as acetyl CoA. This reaction is strongly endergonic, and it is therefore coupled to exergonic processes. Thioesters represent the activated form of carboxylic adds, because acyl residues of this type have a high chemical potential and are easily transferred to other molecules. This property is often exploited in metabolism. 

The oxidation of glyceraldehyde 3-phosphate is considerably more complex. The oxidation of an aldehyde to a carboxylic acid is a strongly exergonic process and the oxidation of glyceraldehyde 3-phos-phate by cells is almost always coupled to the synthesis... 

Reoxidation of the reduced carriers NADH and FADH2 actually involves a sequence of electron carriers, the electron transport chain, whose function is indicated below the circle near the center of Fig. 10-1. The oxidation of reduced NADH by 02 (Eq. 10-7) is a highly exergonic process and is accompanied by the... 

The oxidation of an aldehyde to a carboxylic acid, a highly exergonic process, often proceeds through a thioester intermediate whose cleavage can then be coupled to synthesis of ATP. This sequence is of central importance to the energy metabolism of cells (Chapters 10 and 17) and is shown in Fig. 15-6. 

These resonance stabilization energies of free radicals can be quite large, e.g. 50 kj mol 1 for benzyl-, and 70kjmol 1 for methyl-Np. These must be included in the overall energy balance of the reaction, and can make all the difference between a fast, highly exergonic process, and an endergonic process which in practice does no take place at all. 

Highly exergonic process—carbon oxide oxidation—conjugated with this reaction, proceeds in the presence of palladium (II) complexes ... 

However, H202 injection to the system simultaneously with ethylbenzene sharply increases the dehydrogenation rate, whereas thermolysis of H202 is a highly exergonic process ... 

The inverted regime (—AG > X) for strongly exergonic processes leads to a decrease of the ET rate. 

Upon electronic excitation the redox properties of either the electron donor (D) or the acceptor (A) are enhanced. The feasibility of an electron transfer can be estimated from a simple free reaction energy consideration as customary in the frame of the Rehm-Weller approach (Eq. (1)) [11], where Efy2 (P) and 4) represent the oxidation and reduction potential of the donor or the acceptor, respectively. AEexcit stands for the electronic excitation energy, whereas Aiscoui indicates the coulombic interaction energy of the products formed (most commonly radical ions). This simplified approach allows a first approximation on the feasibility of a PET process without considering the more complex kinetics as controlled by the Marcus theory [6c]. For exergonic processes (AG<0) a PET process becomes thermodynamically favorable. 

Figure 19. Combination of enderdonic process 1 and exergonic process 2. ...

The addition of an alkaline earth metal enolate A to a carbonyl compound is always an exergonic process irrespective of whether the enolate is derived from a ketone, an ester, or an amide and whether the carbonyl compound is an aldehyde or a ketone (Figure 10.39, top). One of the reasons for this exergonidty lies in the fact that the alkaline earth metal ion is part of a chelate in the alkoxide B of the aldol addition product. The driving forces for the additions of alkaline earth metal enolates of esters and amides to carbonyl compounds are further increased because the aldol adducts B are resonance-stabilized, whereas the enolates are not. 

The conversion of glucose to two molecules of pyruvate is an exergonic process (Chap. 10). C6H1206-+ 2C3H4O3 AG° = -147 kJ mol 1... 

The overall consumption of one molecule of acetyl-CoA in the citric acid cycle is an exergonic process AG° = —60 kJ mol-1. All but two of the individual reactions are exergonic. Step 2 (citrate— isocitrate) and step 8 (malate —>oxaloacetate) are endergonic (Fig. 12-3). 

In the first control point, citrate synthase catalyzes the condensation of acetyl-CoA with oxaloacetate to produce citrate (AG° = -32.2 kJ mol ). Although the reaction is reversible, the equilibrium lies very much in favor of citrate formation because of the hydrolysis of a bond in the intermediate compound, citroyl-CoA (Fig. 12-4). Citroyl-CoA is bound to citrate synthase, and the hydrolysis of the thioester bond, to produce citrate and coenzyme A, is an exergonic process. Citrate synthase is inhibited by its substrates (acetyl-CoA and oxaloacetate), and its activity is affected by... 

The back electron-transfer in a triplet geminate pair seems strictly related to the triplet energy of one of the reactants. In fact, if one of them has a triplet energy (Et) lower than the energy stored in the pair [85], the exergonic process [Eq. (4)] will be significant [86] ... 

Carbohydrate is then oxidized back to C02 and H20 by plant cells and by plant-consuming eukaryotes (animals and fungi) and prokaryotes (bacteria), this exergonic process being mechanistically coupled to the endergonic formation of ATP, the so-called energy currency of living cells. 

This exergonic process is coupled to the endergonic process of ATP synthesis catalysed by the ATP synthase (or Fq-F complex) of the mitochondrial inner membrane ... 

The E° for the Au+/Au redox couple in MeCN is -1-1.58 V versus NHE compared to +0.9 V for the HOVHO couple hence electron transfer is an exergonic process. Electrochemical oxidation of HO at a gold electrode in... 

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