Thioester common

While the primary antioxidant serves a critical role, it cannot stop all polymer peroxy radicals from propagating. This is where a second class of antioxidants, called peroxide decomposers, comes in. These molecules catalyze the decomposition of the peroxides to nonradical species, thus breaking the repetitive cycle of radical formation. Phosphites and thioesters commonly serve as secondary antioxidants. Phosphites are commonly used in HIPS resins, but care must be taken to use hydrolysis-resistant molecules to avoid the degradation of these species into black specks that render the final product unacceptable. Phosphites are usually found at levels between 500 and 2000 ppm. 

Thioesters, common in living systems, are more reactive than esters. Explain. 

Chemists and biochemists And it convenient to divide the principal organic substances present m cells into four mam groups carbohydrates proteins nucleic acids and lipids Structural differences separate carbo hydrates from proteins and both of these are structurally distinct from nucleic acids Lipids on the other hand are characterized by a physical property their solubility m nonpolar solvents rather than by their structure In this chapter we have examined lipid molecules that share a common biosynthetic origin m that all their carbons are derived from acetic acid (acetate) The form m which acetate occurs m many of these processes is a thioester called acetyl coenzyme A... 

A.ntioxidants. PhenoHc antioxidants, added at about 0.1—0.5 phr, are usually chosen from among butylated hydroxytoluene [128-37-0] (BHT), and Nnonylphenol [104-40-5] for Hquid stabilizer formulations and bisphenol A [80-05-7] (2,2-bis-(/)-hydroxyphenyl)propane) for the soHd systems. Low melting thioesters, dilauryl thiodipropionate [123-28-4] (DLTDP) or distearyl thiodipropionate [693-36-7] (DSTDP) are commonly added along with the phenoHcs to enhance their antioxidant performance. Usually a 3 1 ratio of thiodipropionate to phenoHc antioxidant provides the desired protection. Most mixed metal stabilizer products contain the antioxidant iagredient. 

Fluorinated heterodienophiles and heterodienes Diels-Alder reactions in which the dienophiles have perfluoroalkyl-substituted multiple bonds between carbon and a heteroatom are quite common Reported earlier were reactions of perfluoroketones, thiones, ketimines, thioesters, nitroso compounds, and nitriles [9] Examples of a-fluoroimines [107], co-hydroperfluorothioaldehydes [108], perfluorosulfines [109, IIO], and selenocarbonyidifluoride [III] (equations 89-92) have been reported recently... 

A variety of cellular and viral proteins contain fatty acids covalently bound via ester linkages to the side chains of cysteine and sometimes to serine or threonine residues within a polypeptide chain (Figure 9.18). This type of fatty acyl chain linkage has a broader fatty acid specificity than A myristoylation. Myristate, palmitate, stearate, and oleate can all be esterified in this way, with the Cjg and Cjg chain lengths being most commonly found. Proteins anchored to membranes via fatty acyl thioesters include G-protein-coupled receptors, the surface glycoproteins of several viruses, and the transferrin receptor protein. 

All three elimination reactions--E2, El, and ElcB—occur in biological pathways, but the ElcB mechanism is particularly common. The substrate is usually an alcohol, and the H atom removed is usually adjacent to a carbonyl group, just as in laboratory reactions. Thus, 3-hydroxy carbonyl compounds are frequently converted to unsaturated carbonyl compounds by elimination reactions. A typical example occurs during the biosynthesis of fats when a 3-hydroxybutyryl thioester is dehydrated to the corresponding unsaturated (crotonyl) thioester. The base in this reaction is a histidine amino acid in the enzyme, and loss of the OH group is assisted by simultaneous protonation. 

Closely related to the carboxylic acids and nitriles discussed in the previous chapter are the carboxylic acid derivatives, compounds in which an acyl group is bonded to an electronegative atom or substituent that can net as a leaving group in a substitution reaction. Many kinds of acid derivatives are known, but we ll be concerned primarily with four of the more common ones acid halides, acid anhydrides, esters, and amides. Esters and amides are common in both laboratory and biological chemistry, while acid halides and acid anhydrides are used only in the laboratory. Thioesters and acyl phosphates are encountered primarily in biological chemistry. Note the structural similarity between acid anhydrides and acy) phosphates. 

Thioesters are named like the corresponding esters. If the related ester has a common name, the prefix thio- is added to the name of the carboxylate acetate becomes thioacetate, for instance. If the related ester has a systematic name, the -oate or -carboxylate ending is replaced by -thioate or -carbothioate butanoate becomes butanelhioate and cyclohexanecarboxylate becomes cyclohexane-carbothioate, for instance. 

Acyl CoA s, such as acetyl CoA, are the most common thioesters in nature. Coenzyme A, abbreviated CoA, is a thiol formed by a phosphoric anhydride linkage (0 = P—O—P=0) between phosphopantetheine and adenosine 3, 5 -bisphosphate. (The prefix "bis" means "two" and indicates that adenosine 3, 5 -bisphosphate has two phosphate groups, one on C3 and one on C5. ) Reaction of coenzyme A with an acyl phosphate or acyl adenylate... 

Carboxylic acids can be transformed into a variety of carboxylic acid derivatives in which the carboxyl -OH group has been replaced by another substituent. Acid halides, acid anhydrides, esters, and amides arc the most common such derivatives in the laboratory thioesters and acyl phosphates are common in biological molecules. 

SEC-RI/UV has also been used to analyse some 26 thioorganotin compounds, organotin carboxylates and chlorides, essentially PVC stabilisers, and some of their main by-products and related compounds (thioesters and dithioesters, n-alkanes) [803]. Not all organotin chlorides were stable in the adopted analysis conditions. N, Ai -ethylene-bis-stearamide and -oleamide in common plastics (ABS, SAN, PUR, LDPE, PA6.6) can be analysed by SEC after derivatisation with trifluo-roacetic anhydride. SEC analysis of fatty alcohol ethoxy-lates (FAE), used as nonionic surfactants, has also been described [759]. 

Simple alkyl or aryl thioesters are commonly assayed as substrates of hydrolases, witness the hydrolysis of phenyl thioesters by horse serum carbox-ylesterase [150], For most substrates investigated, e.g., phenyl thioacetate, phenyl thiopropionate, and phenyl thiobutyrate (7.66, R = Me, Et, and Bu, respectively), kcat values were found, which were a few times larger than those of corresponding nitrophenyl esters, whereas the affinities were lower by approximately one order of magnitude. Methyl and phenyl esters of various linear thioacids were also found to be good substrates of mammalian liver carboxylesterases and serum cholinesterases [151]. 

Proteins can undergo different rounds of palmitoylation and depalmitoylation, either constitutively or as a response to signals." " Here the Ras proteins are the most commonly discussed examples. As described above, all Ras proteins are expressed with the CAAX-box and are subject to post-translational modifications. First, they get farnesylated and after proteolysis and methylation of the C-terminus, H-/N-Ras as well as K-Ras 4A get further palmitoylated at additional cysteines present in their C-terminus. Palmitoylation occurs in the Golgi apparatus and via vesicular transport the farnesylated and palmitoylated proteins are directed to the plasma membrane (PM). The palmitoyl thioester is hydrolyzed at multiple cellular sites and the protein is transported back to the Golgi via a nonvesicular pathway (Scheme 3)." ... 

Perhaps the most important example of the reverse Claisen reaction in biochemistry is that involved in the P-oxidation of fatty acids, used to optimize energy release from storage fats, or fats ingested as food (see Section 15.4). In common with most biochemical sequences, thioesters rather than oxygen esters are utilized (see Box 10.8). 

Acetyl-CoA (see Box 10.8) is a thioester of acetic acid with coenzyme A. It is a remarkably common intermediate in many metabolic degradative and synthetic pathways, for which the reactivity of the thioester fnnction plays a critical role. There are two major sonrces of the acetyl-CoA entering the Krebs cycle glycolysis via the oxidative... 

Many such activated acyl derivatives have been developed, and the field has been reviewed [7-9]. The most commonly used irreversible acyl donors are various types of vinyl esters. During the acylation of the enzyme, vinyl alcohols are liberated, which rapidly tautomerize to non-nucleophilic carbonyl compounds (Scheme 4.5). The acyl-enzyme then reacts with the racemic nucleophile (e.g., an alcohol or amine). Many vinyl esters and isopropenyl acetate are commercially available, and others can be made from vinyl and isopropenyl acetate by Lewis acid- or palladium-catalyzed reactions with acids [10-12] or from transition metal-catalyzed additions to acetylenes [13-15]. If ethoxyacetylene is used in such reactions, R1 in the resulting acyl donor will be OEt (Scheme 4.5), and hence the end product from the acyl donor leaving group will be the innocuous ethyl acetate [16]. Other frequently used acylation agents that act as more or less irreversible acyl donors are the easily prepared 2,2,2-trifluoro- and 2,2,2-trichloro-ethyl esters [17-23]. Less frequently used are oxime esters and cyanomethyl ester [7]. S-ethyl thioesters such as the thiooctanoate has also been used, and here the ethanethiol formed is allowed to evaporate to displace the equilibrium [24, 25]. Some anhydrides can also serve as irreversible acyl donors. 

The fourth and last step of the /3-oxidation cycle is catalyzed by acyl-CoA acetyltransferase, more commonly called thiolase, which promotes reaction of /3-ketoacyl-CoA with a molecule of free coenzyme A to split off the carboxyl-terminal two-carbon fragment of the original fatty acid as acetyl-CoA The other product is the coenzyme A thioester of the fatty acid, now shortened by two carbon atoms (Fig. 17-8a). This reaction is called thiolysis, by analogy with the process of hydrolysis, because the /3-ketoacyl-CoA is cleaved by reaction with the thiol group of coenzyme A... 

Formation of the internal thioester bond can be also accomplished by mixed anhydride assisted coupling of the N-protected peptide with a mercaptoacetylated C-terminal peptide fragment. 6 Sulfanylacids used for the preparation of peptide thioesters can be obtained from the common amino acids by the method of Pfister et al. 41 further described by Yan-keelov et al. 2 ... 

In the synthesis of a-amino acids [290] through addition of the carbanion of MMTS to nitriles the overall process involves three other steps frequently encountered in sulfur-mediated chemistry a Pummerer-type rearrangement, with a less common migration of a methylthio group, and a Raney nickel desulfurization following transesterification of the thioester function. 

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