Thiamin Vitamin BI

Toxopyrimidine (pyramin lOlO) may be obtained from thiamine (vitamin Bi 1011) by acidic hydrolysis or by treatment with the thiaminase of Bacillus aneurinolyticus. Toxopyrimidine produces convulsions and death in rodents as do analogues, e.g. 2,4-dimethylpyrimidin-5-ylmethanol (1012) (65JMC750), but the effect is minimized or even... 

This factor is particularly significant in OFBD since biological samples or isolates are used. In addition to background interference, fluorescence quenching has been demonstrated in a variety of biomolecules such as thiamine (vitamin Bi),(27) nicotinamide/28 nucleosides/nucleotides,(29) and pyruvate/30 To circumvent the obvious limitations associated with the use of UV or visible fluorophores in OFD, the potential... 

Thiamine (vitamin Bi), in the form of thiamine diphosphate (TPP), is a coenzyme of some considerable importance in carbohydrate metabolism. Dietary deficiency leads to the condition beriberi, characterized by neurological disorders, loss of appetite, fatigue, and muscular weakness. We shall study a number of... 

Thiamin (vitamin Bi, 22) (Fig. 14) - an important cofactor of decarboxylases, transketolases, carboxy-lyases, and some other enzymes - was successfully glycosylated by enzymatic transglycosylation using p-galactosidase [59] and p-A-acetyl-hexosaminidase [60] from A. oryzae. 

Thiamine (vitamin Bi) is phosphorylated by ATP to thiamine pyrophosphate. This is a coenzyme for, among others, alpha-ketoglutarate dehydrogenase, transketolase and pyruvate dehydrogenase. Thiamine pyrophosphate is involved in fatty acid... 

Thiamin, vitamin Bi, is the next most labile vitamin. 

Nearly all the water-soluble vitamins are heterocyclic compounds. Among the first to be isolated was thiamine (vitamin Bi) (62), deficiency of which causes degenerative changes in the nervous system, including the multiple peripheral neuritis characteristic of beriberi. Thiamine deficiency can arise from decomposition of the vitamin by bacteria in the gut. In mammalian metabolism the hydroxy group of thiamine is esterified to give cocarboxylase (thiamine pyrophosphate) which catalyzes the decarboxylation of a-keto acids to aldehydes, acyloins or acids, and their transformation into acyl phosphates. 

THIAMINE (Vitamin Bi). Some earlier designations for this substance included aneurin, antmeuntic factor, antibenben factor, and oryzamin. Thiamine is metabolically active as thiamine pyrophosphate (TPP). the formula of which is ... 

When, in 1832, Wohler and Liebig first discovered the cyanide-catalyzed coupling of benzaldehyde that became known as the benzoin condensation , they laid the foundations for a wide field of growing organic chemistry [1]. In 1903, Lapworth proposed a mechanistical model with an intermediate carbanion formed in a hydrogen cyanide addition to the benzaldehyde substrate and subsequent deprotonation [2]. In the intermediate active aldehyde , the former carbonyl carbon atom exhibits an inverted, nucleophilic reactivity, which exemplifies the Umpo-lung concept of Seebach [3]. In 1943, Ukai et al. reported that thiazolium salts also surprisingly catalyze the benzoin condensation [4], an observation which attracted even more attention when Mizuhara et al. found, in 1954, that the thiazolium unit of the coenzyme thiamine (vitamin Bi) (1, Fig. 9.1) is essential for its activity in enzyme biocatalysis [5]. Subsequently, the biochemistry of thiamine-dependent enzymes has been extensively studied, and this has resulted in widespread applications of the enzymes as synthetic tools [6]. 

In this transformation, l-G1u can be replaced by L-Asp, i-Om, or by other amino acids the reaction requires the presence of pyridoxal-5-phosphate (vitamin B ), or, thiamine (vitamin Bi) [39, 40], as coenzyme. 

Referring to a mechanistic classification of organocatalysts (Seayad and List 2005), currently the two most prominent classes are Brpnsted acid catalysts and Lewis base catalysts. Within the latter class chiral secondary amines (enamine, iminium, dienamine activation for a short review please refer to List 2006) play an important role and can be considered as—by now—already widely extended mimetics of type I aldolases, whereas acylation catalysts, for example, refer to hydrolases or peptidases (Spivey and McDaid 2007). Thiamine-dependent enzymes, a versatile class of C-C bond forming and destructing biocatalysts (Pohl et al. 2002) with their common catalytically active coenzyme thiamine (vitamin Bi), are understood to be the biomimetic roots ofcar-bene catalysis, a further class of nucleophilic, Lewis base catalysis with increasing importance in the last 5 years. 

Studies on thiamine (vitamin Bi) catalyzed formation of acyloins from aliphatic aldehydes and on thiamine or thiamine diphosphate catalyzed decarboxylation of pyruvate have established the mechanism for the catalytic activity of 1,3-thiazolium salts in carbonyl condensation reactions. In the presence of bases, quaternary thiazolium salts are transformed into the ylide structure (2), the ylide being able to exert a cat ytic effect resembling that of the cyanide ion in the benzoin condensation (Scheme 2). Like cyanide, the zwitterion (2), formed by the reaction of thiazolium salts with base, is nucleophilic and reacts at the carbonyl group of aldehy s. The resultant intermediate can undergo base-catalyzed proton... 

Group transfer reactions often involve vitamins, which humans need to obtain in their diet, since we are incapable of realising their synthesis. These include nicotinamide (derived from the vitamin nicotinic acid), and riboflavin (vitamin B2) derivatives, required for electron transfer reactions, biotin for the transfer of CO2, pantothenate for acyl group transfer, thiamine (vitamin Bi, as thiamine pyrophosphate) for transfer of aldehyde groups, folic acid (as tetrahydrofolate) for exchange of one-carbon fragments. Lipoic acid (not a vitamin) is both an acyl and an electron carrier. In addition, vitamins like pyridoxine (vitamin Bs, as pyridoxal phosphate), vitamin B12, and vitamin C (ascorbic acid) participate as cofactors in an important number of metabolic reactions. 

Structures of thiamine (vitamin Bi) and its phosphorylated metabolites. They consist of a six-membered pyrimidine ring and a five-membered thiazole ring, linked through a methylene group. 

Thiazole gives a transient radical in aqueous solution upon pulse radiolysis. The radical formed under neutral conditions is the 3-hydrothiazolyl radical (201) for which electronic absorption spectra, acid-base characteristics, and second-order decay kinetics were measured. A derivative of this, the radical from thiamine (vitamin Bi) (202) was also characterized. A role is implied for related radicals (e.g., 203) during the electrochemical dimerization reported for benzothiazolium salts. ... 

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