Activated thioester

However, if the expressed protein is treated on the affinity support using thiophenol, this also will release the protein and result in a phenylthioester at its C-terminal, which is the reactive intermediate imminendy suitable for native chemical ligation. Treatment of this activated thioester protein with a N-terminal cysteine peptide induces the native chemical ligation reaction and couples the peptide to the expressed protein through an amide bond (Severinov and Muir, 1998) (Figure 17.27). 

Scheme 26 Segment Coupling via Active Thioesters Obtained by Solid-Phase Synthesis...

Pyridine disulfide reacts with triphenylphosphine and carboxylic acids to yield active thioesters, which can be amidated with secondary amines. 

Due to their obnoxious smell and catalyst-poisoning properties, active thioesters have only encountered limited use. 

Unlike two previous theories of life origin, only a few pieces of experimental evidence exist at present to prove the theoretical speculations. However, we have to notice the verification of the basic mechanism of molecular hydrogen generation as a reducing power, furthermore, the amide bond synthesis has been also demonstrated, both at temperatures within the range of hydrothermal vents (100 °C). In addition, the evidence for at least sulfide-based amino acid synthesis and polymerization from simple precursors has been shown. The formation of acetic acid and an activated thioester from carbon monoxide, methanethiol and various combinations of ferrous and nickel sulfides has been experimentally proved as well. However, further verification is necessary for the modes and rates of organic synthesis. 

Gerlach has also prepared pyrenophorin (9) and its meso diastereomer, as shown in Scheme 4.3,The activated thioester 13 was condensed with Grignard reagent 14, affording a 65% yield of ketone 15. After protection of the ketone carbonyl, introduction of the double bond via a selenylation-oxidation sequence produced 16 (36%). Seco acid 17 was obtained in 79% yield by desilylation followed by basic hydrolysis of the ester. Cyclization to the dimer, again under Mitsunobu conditions, afforded a 24% yield of a 1 1 mixture of the protected precursor of 9 and the corresponding meso diastereomer, which were then converted to 9 and 18 by hydrolysis. 

With the main exception of some nonenzymatic peroxidation reactions on oils or membrane lipids (in which total lipid oxidation is involved), most areas of acyl lipid oxidation in plants are concerned with the fatty acid components, either as free fatty acids or as activated thioester forms. Lipid peroxidation reactions are outside the scope of this chapter but are covered by Galliard and Chan (this volume, Chapter 5). The following discussion will relate mainly to fatty acid oxidation. 

A related mechanism was proposed by Wachtershauser [174] for the formation of the activated thioester HjC-CjOj-SCHj from CO and CH3SH on coprecipitated... 

Racemization is an unavoidable problem when hydrolysis of thioesters is carried out under basic conditions. PS-SOsH-catalyzed hydrolysis of an optically active thioester proceeded with only a slight loss in enantiomeric excess imder water-reflux conditions (Scheme 3.34). This is one of the advantages of the acid-catalyzed hydrolysis of thioesters. 

---