Carbon atoms amino acids

Now that the general considerations concerning the 13C-n.m.r.-spec-tral data for O-glycosylated model compounds have been discussed, attention may be focused on some specific cases. Table IV gives the 13C-n.m.r.-spectral data for L-serine that has been glycosylated with various carbohydrates, such as a- and / -d-G1c, a- and /9-D-Gal, a- and / -d-Xyl, a-D-Man, and a-D-GalNAc. As mentioned earlier, the specific resonance-assignments to amino acid carbon atoms were based on chemical... 

Chemical-shift Differences of the Amino Acid Carbon Atoms of a-Glycosylated and Nonglycosylated Di- and Tri-peptides... 

Various carboxylic acids are found as the necic acid parts of the pyrrolizidine alkaloids, e.g. senecionine 6.77), monocrotaline 6.78) and heliosupine 6.79). Available evidence [61, 62] points to the derivation of these acids from the branched-chain amino acids isoleucine and valine (see dotted lines). [In the case of monocrotaline C-4, C-5, and C-8 apparently derive from propionic acid. Carbon atoms 5 and 6 of the echimidinic acid fragment in 6.79) apparently derive from acetate.] The basic fragment in these alkaloids, retronecine 6.80), is formed [63] from two molecules of... 

The same nomenclature has been adopted for amino-acids, the configurational family to which the a-carbon atom belongs being denoted by the prefixes d- and L-. 

It is important to notice that the united-atom simplification cannot be applied to functional hydrogens which are involved in the formation of a hydrogen hond or a salt bridge. This would destroy interactions important for the structural integrity of the protein. Removing the hydrogen at the u-carbon of the peptide backbone is also dangerous, because it prevents racemization of the amino acid. 

Periodic acid has a selective oxidising action upon compounds having two hydroxyl groups or a hydroxyl and an amino group attached to adjacent carbon atoms and is characterised by the cleavage of the carbon - carbon bond (Malaprade reaction) ... 

The most general methods for the syntheses of 1,2-difunctional molecules are based on the oxidation of carbon-carbon multiple bonds (p. 117) and the opening of oxiranes by hetero atoms (p. 123fl.). There exist, however, also a few useful reactions in which an a - and a d -synthon or two r -synthons are combined. The classical polar reaction is the addition of cyanide anion to carbonyl groups, which leads to a-hydroxynitriles (cyanohydrins). It is used, for example, in Strecker s synthesis of amino acids and in the homologization of monosaccharides. The ff-hydroxy group of a nitrile can be easily substituted by various nucleophiles, the nitrile can be solvolyzed or reduced. Therefore a large variety of terminal difunctional molecules with one additional carbon atom can be made. Equally versatile are a-methylsulfinyl ketones (H.G. Hauthal, 1971 T. Durst, 1979 O. DeLucchi, 1991), which are available from acid chlorides or esters and the dimsyl anion. Carbanions of these compounds can also be used for the synthesis of 1,4-dicarbonyl compounds (p. 65f.). 

There also exists an acidregioselective condensation of the aldol type, namely the Mannich reaction (B. Reichert, 1959 H. Hellmann, 1960 see also p. 291f.). The condensation of secondary amines with aldehydes yields Immonium salts, which react with ketones to give 3-amino ketones (=Mannich bases). Ketones with two enolizable CHj-groupings may form 1,5-diamino-3-pentanones, but monosubstitution products can always be obtained in high yield. Unsymmetrical ketones react preferentially at the most highly substituted carbon atom. Sterical hindrance can reverse this regioselectivity. Thermal elimination of amines leads to the a,)3-unsaturated ketone. Another efficient pathway to vinyl ketones starts with the addition of terminal alkynes to immonium salts. On mercury(ll) catalyzed hydration the product is converted to the Mannich base (H. Smith, 1964). 

L Ammo acid (Section 27 2) A descnption of the stereochem istry at the a carbon atom of a chiral ammo acid The Fis cher projection of an a amino acid has the ammo group on the left when the carbon chain is vertical with the carboxyl group at the top... 

Critical micelle concentration (Section 19 5) Concentration above which substances such as salts of fatty acids aggre gate to form micelles in aqueous solution Crown ether (Section 16 4) A cyclic polyether that via lon-dipole attractive forces forms stable complexes with metal 10ns Such complexes along with their accompany mg anion are soluble in nonpolar solvents C terminus (Section 27 7) The amino acid at the end of a pep tide or protein chain that has its carboxyl group intact—that IS in which the carboxyl group is not part of a peptide bond Cumulated diene (Section 10 5) Diene of the type C=C=C in which a single carbon atom participates in double bonds with two others... 

Nucleophilic Ring Opening. Opening of the ethyleneimine ring with acid catalysis can generally be accompHshed by the formation of an iatermediate ayiridinium salt, with subsequent nucleophilic substitution on the carbon atom which loses the amino group. In the foUowiag, R represents a Lewis acid, usually A = the nucleophile. 

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