Autocatalytic build

Illustration 9.5 indicates that one may have parallel paths leading from reactants to products and that in the case of an autocatalytic reaction, one path may be preferred over a second until the product level builds up to a point where the second becomes appreciable. In this example, the magnitudes of the rate constants are such that the vast majority of the reaction occurs by the autocatalytic path. In cases such as these it is desirable to use a CSTR or recycle reactor to enhance the reaction rate by virtue of the back-mixing of product species. 

In accordance with the autocatalytic process, matrices are again formed. It is surprising that the autocatalysis decreases when only 1 of the 15 building blocks of the peptide has the opposite handedness, e.g., when the N-peptide fragment contains one D-amino acid as well as the 14 L-amino acids. These experimental results show that such a system is able to form homochiral products via self-replication. It can be assumed that similar mechanisms influenced the origin of homochirality on Earth (Saghatelian et al., 2001 Siegel, 2001). 

Building on an early theoretical scheme by Frank [16] (see below) for the spontaneous autocatalytic symmetry breaking in which enantiomers act as catalysts for their own production, Kondepudi and Nelson [17-20] further suggested a generalized scheme by which the minor excess of one enantiomer, caused by PVED, might lead to a state of enantiomeric homogeneity. They then calculated that a period as short as 15 000 years could cause chiral domination. [19] In 1989 MacDermott and Tranter [21] maintained that this amplification time would be reduced from 104 years to just one year if the PVED were increased to 10-16 kT , i.e., by a factor of only ten. 

Let us recall the micellar aqueous system, as this procedure is actually the basic one. The chemistry is based on fatty acids, that build micelles in higher pH ranges and vesicles at pH c. 8.0-8.5 (Hargreaves and Deamer, 1978a). The interest in fatty acids lies also in the fact that they are considered possible candidates for the first prebiotic membranes, as will be seen later on. The experimental apparatus is particularly simple, also a reminder of a possible prebiotic situation the water-insoluble ethyl caprylate is overlaid on an aqueous alkaline solution, so that at the macroscopic interphase there is an hydrolysis reaction that produces caprylate ions. The reaction is very slow, as shown in Figure 7.15, but eventually the critical micelle concentration (cmc) is reached in solution, and thus the first caprylate micelles are formed. Aqueous micelles can actually be seen as lipophylic spherical surfaces, to which the lipophylic ethyl caprylate (EC) avidly binds. The efficient molecular dispersion of EC on the micellar surface speeds up its hydrolysis, (a kind of physical micellar catalysis) and caprylate ions are rapidly formed. This results in the formation of more micelles. However, more micelles determine more binding of the water-insoluble EC, with the formation of more and more micelles a typical autocatalytic behavior. The increase in micelle population was directly monitored by fluorescence quenching techniques, as already used in the case of the... 

The oxidation is initiated (a) by Fe to yield the benzoyl radical (99) which adds on a molecule of oxygen to form the perbenzoate radical (1(X)), this reacts with benzaldehyde (97) to yield perbenzoic acid (101) and another benzoyl radical (99)—these two steps constituting the chain reaction (b). The actual end-product is not perbenzoic acid (101), however, as this undergoes a rapid acid-catalysed, non-radical reaction (c) with more benzaldehyde (97) to yield benzoic acid (98). This latter reaction (c), being acid-catalysed, speeds up as the concentration of product benzoic acid (98) builds up, i.e. it is autocatalytic. That benzoyl radicals (99) are involved is borne out by the observation that carrying out the reaction at higher temperatures ( 100°), and at low oxygen concentrations, results in the formation of CO, i.e. by PhCO-> Ph- + CO. 

The question is therefore, what are the principal requirements of an autotrophic carbon-fixation mechanism An organic molecule serves as a C02 acceptor molecule, which becomes carboxylated by a carboxylase enzyme. This C02 acceptor molecule needs to be regenerated in a reductive autocatalytic cycle. The product that can be drained off from such a metabolic cycle should be a central cellular metabolite, from which all cellular building blocks for polymers can be derived examples of such central metabolites are acetyl-CoA, pyruvate, oxaloacetate, 2-oxoghitarate, phosphoe-nolpyruvate, and 3-phosphoglycerate. Importantly, the intermediates should not be toxic to the cell. The irreversible steps of the pathway are driven by ATP hydrolysis, while the reduction steps are driven by low-potential reduced coenzymes. 

For most straight chains, termination balances initiation, and the radical concentrations remain in the steady state. However, there are some straight chains where the termination becomes negligible and the radical concentration can build up to large values well past the steady state region. The reaction becomes autocatalytic as the reaction rate increases dramatically with the build-up in radical concentration, and if the increase is very rapid an explosion can occur. 

The facts are readily explained on the theory that decomposition of the ammonium amalgam occurs very slowly as an homogeneous reaction at temperatures below zero and that a rapid, heterogeneous reaction takes place at the surface of little droplets of liquid ammonia which are dispersed throughout the mercury. The surface tension of the mercury is sufficient to cause liquefaction of the first ammonia which is liberated. The autocatalytic effect occurs when the reaction has had time to build up these droplets of liquid ammonia. 

Mathys, S., Evans, T. C., Chute, I. C., et al. (1999) Characterization of a self-sphcing mini-intein and its conversion into autocatalytic N- and C-terminal cleavage elements facile production of protein building blocks for protein ligation. Gene 231, 1—13. 

The agent responsible for autocatalytic behavior need not be the product of the reaction, it can be an intermediate. Low-temperature oxidation of methane provides an example [59,85], The key free radical turns out to be CH3-, produced from methane by initiation and giving rise to other free radicals. The propagation mechanism with six interlocking steps and chain branching is such that build-up of CH3- accelerates the rate (see Figure 9.4, previous page). 

Silicones were commercially introduced in the United States in 1943. Initial commercialization in any new field of science is a seminal event, because it sets in motion an autocatalytic cycle of science, invention, and innovation (Figure 1). Responsible product development and production demand more precise information, which in turn calls for a deeper and a more critical scientific understanding of materials and processes. This fundamental understanding builds a knowledge base, which allows more invention and greater commercial innovation. This cycle accelerates the demand for more and better science. 

According to a general rule of organic chemistry, reactions involving the smallest molecules are catalytically the most restrictive. This rule holds notably for the build-up of carbon skeletons with the arithmetic Cl -F Cl = C2 (e.g., C2 = glycine or acetyl thioester). Therefore, it may not come as a surprise that in the course of metabolic evolution, these most simple carbon fixation reactions may fall by the wayside. Under these conditions, an autotrophic carbon fixation metabolism can only be maintained by a metabolic cycle, which multiplies the C2 unit autocatalytically in the absence of its de novo synthesis. A prominent example is the reductive citric acid cycle (C2 -F Cl... 

Molecular oxygen is the major cause of irreversible deterioration of hydrocarbon substrates, leading to the loss of useful properties and to the ultimate failure of the substrate. The oxidation process of hydrocarbons is autocatalytic oxidation starts slowly, sometimes with a short induction period, followed by a gradual increase in the rate, concomitant with the build up of hydroperoxides, which eventually subside, giving rise to a sigmoidal oxidation curve. When initiators such as peroxides are present, the length of the induction period is absent, or very short, but it can be prolonged by antioxidants, as shown in Fig. 1. The basic autoxidation theory of hydrocarbons involves a complex set of elementary reaction steps in a free radical-initiated chain reaction mechanism the basic tenets of this theory apply equally to polymer oxidation. 

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