Stepwise Degradation

Decisive progress in stepwise degradation was brought about by Pehr Edman (Plate 15) in 1950 [11], who found that phenylisothiocyanate, the sulfur analog of Bergmann s reagent, reacts with the N-terminal amino group at pH 9 even at room temperature, and that the thioureido compound hereby formed is readily split by weak acids, e.g. trifluoroacetic acid, to yield a 2-anilinothiazolin-5-one. 

Stepwise degradation of a tripeptide via phenylhydantion Fig. 4. Peptide degradation via phenylhydantoines [10] 

The peptide, shortened by the corresponding amino acid, is then subjected to a new cycle of degradation. The thiazolinones are treated with stronger acids or heated and thus converted to the more stable 3-phenyl-2-thiohydatoins (PTH-s) which can be identified by chromatographic methods or by mass spectrometry (Fig. 5). 

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Barbier-Wieland degradation A method for the stepwise degradation of aliphatic acids... 

Reactions and Uses. The common reactions that a-hydroxy acids undergo such as self- or bimolecular esterification to oligomers or cycHc esters, hydrogenation, oxidation, etc, have been discussed in connection with lactic and hydroxyacetic acid. A reaction that is of value for the synthesis of higher aldehydes is decarbonylation under boiling sulfuric acid with loss of water. Since one carbon atom is lost in the process, the series of reactions may be used for stepwise degradation of a carbon chain. 

Application of NMR spectroscopy to heterocyclic chemistry has developed very rapidly during the past 15 years, and the technique is now used almost as routinely as H NMR spectroscopy. There are four main areas of application of interest to the heterocyclic chemist (i) elucidation of structure, where the method can be particularly valuable for complex natural products such as alkaloids and carbohydrate antibiotics (ii) stereochemical studies, especially conformational analysis of saturated heterocyclic systems (iii) the correlation of various theoretical aspects of structure and electronic distribution with chemical shifts, coupling constants and other NMR derived parameters and (iv) the unravelling of biosynthetic pathways to natural products, where, in contrast to related studies with " C-labelled precursors, stepwise degradation of the secondary metabolite is usually unnecessary. 

Amylose is thus extremely susceptible to degradation in the presence of hydroxyl ion. Two mechanisms may be involved (1) a stepwise degradation caused by the reducing end-group s undergoing enolization followed by degradation to acidic products (this must occur in the presence or absence of oxygen), and (2) a random, catalytic, hydrolytic scission of 4—>l-a-D... 

As to the homogeneity of microgels, their composition and their structure has to be considered. In an aqueous alkaline solution a stepwise degradation of microgels by hydrolysis is possible [133], by which especially the unreacted terminal EUP-units are removed [115]. The degradation rate increases with the EUP-fraction incorporated in the microgel. 

Peptides containing serine or threonine may undergo an N—>0 acyl shift upon exposure to strong acids (Scheme 40). 592,594 This reaction has been exploited in the structure elucidation of cyclosporin A, since the acid-catalyzed acyl shift with formation of an ester allowed its selective hydrolysis to the linear peptide for further stepwise degradation. 593 ... 

The present article will be devoted in its first chapters to the chemical basis of the stepwise degradation procedure and to a comparative description of design and function of the two instruments, and in its later chapters to the results so far achieved. Finally, present limitations of the automated method as well as novel approaches to the sequence analysis of peptides will be discussed. 

Doolittle, L.R., G.A. Mross, L.A. FothergUl, and R.E Doolittle, A simple solid-phase amino acid sequencer employing a thioacety-lation stepwise degradation procedure. Anal Biochem, 1977.78(2) 491-505. 

This may be explained by considering the nitration of hexamine with nitric acid as a stepwise degradation by nitrolysis, i.e. the nitration of amine involving the stepwise cleavage of the bond between the nitrogen and carbon atoms. According to Lamberton [39] this idea was first advanced by Linstead. The main work on this subject has been done by British [39] and Canadian [40] authors. 

Often, a polysaccharide gives a polymeric residue after a Smith degradation. Methylation analyses of this material and of the original polysaccharide may nonetheless furnish valuable structural information. It is also possible to subject the material to successive Smith degradations, a notable example being the stepwise degradation of the ovarian-cyst, blood-group H substance (42) by Lloyd and Kabat.100... 

The structures of oligosaccharides and polysaccharides are usually determined by a combination of methods specific enzymatic hydrolysis to determine stereochemistry and produce smaller fragments for further analysis methylation analysis to locate glycosidic bonds and stepwise degradation to determine sequence and configuration of anomeric carbons. 

Kim TI, Seo HJ, Choi JS et al (2005) Synthesis of biodegradable cross-linked poly(beta-amino ester) for gene delivery and its modification, inducing enhanced transfection efficiency and stepwise degradation. Bioconjug Chem 16 1140-1148... 

Stepwise degradation of sugars under catalytic conditions leads to shorter-chain sugars and such by-products as CO, C02, and H20 depending on the conditions (see also next section).348 Sucrose or other carbohydrates have been used for the preparation of new solid acidic catalysts by partial carbonization followed by sulfonation. This leads to sheets of amorphous carbon bearing hydroxyl, carboxyl, and sulfonic groups. These have been used for the production of biodiesel fuel.349... 

A second degradation can ensue following the splitting of acetamide or jV-methylacetamide (McLafferty rearrangement) to a followed by stepwise degradation to c. ... 

At The American University, Isbell s major interest in research turned to the study of the oxidation of saccharides with hydrogen peroxide. In collaboration with Dr. Frush, he published some forty papers on the subject. A number of major discoveries were made, including that of a stepwise degradative peroxidation, which is catalyzed by base or by such metals as iron(II). It starts at the anomeric carbon of an aldose, either in the acyclic or the cyclic form, and affords the lower aldose and formic acid (see Fig. 8). Two mechanisms were recognized an ionic one prevalent in strong alkali, and a free-radical process catalyzed by Fe(II) (see Fig. 9). 

FIG. 8.—The stepwise degradative peroxidation of aldoses starts at the anomeric carbon and forms a lower aldose and formic acid. 

The first insight into the nature of the nonaromatic portion was obtained by H. Leuchs and his school, whose exemplary skill and thoroughness led to the first stepwise degradation of these alkaloids. 

The mass spectra of sulfonamides derived from heterocyclic bases, e.g., sulfathiazole (306) (Fig. 10), show mainly fragments corresponding to a stepwise degradation of the sulfanilamide moiety, (306)—>[307]—>[308]—>[309]—>[49].178... 

When such strains as E. coli 83-24, which are blocked after shikimic acid, were grown on minimal medium plus aromatic supplement, they accumulated 400-800 mg. of shikimic acid per liter, together with variable amounts of shikimate 5-phosphate. Since no mutants that are blocked between shikimic acid and its phosphorylated form were found, it was considered that the phosphate ester is not on the main path of biosynthesis. As will be pointed out later, enzymic studies showed that shikimate 5-phosphate is actually an intermediate between shikimate and the aromatic compounds. It would appear, therefore, that the block in such strains as E. coli 83-24 is probably immediately after shikimate 5-phosphate. With filtrates from this organism, methods were developed for the isolation of pure shikimate and for its stepwise degradation. ... 

ENZYMATIC LIQUEFACTION OF THE RAW MATERIAL. Mixtures of pectinolytic and cellulolytic enzymes are used. Together they bring about a stepwise degradation, with initial loosening and separation of the cell aggregates, then breakdown of the cell walls and finally conversion of cellulose to sugar. After liquefaction of the mash, the remaining solids are usually separated off in a decanter. 

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