Carboxyl component side chain

There is only a small selection of nonprotein amino acids that contain carbonyl groups in the form of ketone, aldehyde, and carboxylic acid moieties, as part of the side chain. The examples given in Table 6 are components of nonribosomal peptides isolated from bacteria or fungi and siderophores from bacteria. The biosynthesis of these amino acids is not clear however, some of the amino acids with carboxylic acid side chains may be traced back to the L-a-amino acids aspartic acid and glutamic acid. 

When side chains of two or more different kinds are attached to a cyclic component, only the senior side chain is named by the conjunctive method. The remaining side chains are named as prefixes. Likewise, when there is a choice of cyclic component, the senior is chosen. Benzene derivatives may be named by the conjunctive method only when two or more identical side chains are present. Trivial names for oxo carboxylic acids may be used for the acyclic component. If the cyclic and acyclic components are joined by a double bond, the locants of this bond are placed as superscripts to a Greek capital delta that is inserted between the two names. The locant for the cyclic component precedes that for the acyclic component, e.g., indene-A - -acetic acid. 

Wool has a complex chemical stmcture, composed mainly of a large number of different proteins (87). It is amphoteric in character because of the presence of basic amino and acidic carboxyl groups in the side chains of some of the component amino acids. In an aqueous acidic dyebath, protonation of the amino and carboxyl groups results in a net positive charge on the fiber. 

Biocytin is e-N-biotinyl-L-lysine, a derivative of D-biotin containing a lysine group coupled at its e-amino side chain to the valeric acid carboxylate. It is a naturally occurring complex of biotin that is typically found in serum and urine, and probably represents breakdown products of recycling biotinylated proteins. The enzyme biotinidase specifically cleaves the lysine residue and releases the biotin component from biocytin (Ebrahim and Dakshinamurti, 1986, 1987). 

Fig. 12.3. Principle of the two-component pathway. The fignre shows the principal steps of the two-component pathway in bacterial systems. An extraceUnlar signal (change in osmolarity, N availability, etc.) is registered by a receptor. An interaction takes place with the first component, the sensor kinase", which undergoes autophosphorylation at a His residue (H). The phosphate residue is transferred to the carboxyl side chain of an Asp residue (D) of the reaction regulator. Phosphorylation of the second component activates this for further signal conduction. The sen-sor kinase" may also be localized in the cytoplasmic domain of the receptor.

However, they have not yet found many applications in asymmetric Ugi reactions [41-43], and this is probably due to the fact that diastereomeric excesses are often only moderate and strongly influenced by the structure of the side chain of the a-amino acid. A thorough study was carried out by Yamada et al. [42], who observed that the configuration of the newly generated stereocenter of the major diastereoisomer is always opposite to that of the amino ester. Representative examples are shown in Scheme 1.15. Although Yamada often also used chiral protected aminoacids as the carboxylic component, they were proved to have a negligible influence on the stereoselectivity. 

One of the technically and commercially most important cation-exchange membranes developed in recent years is based on perfluorocarbon polymers. Membranes of this type have extreme chemical and thermal stability and they are the key component in the chlorine-alkaline electrolysis as well as in most of today s fuel cells. They are prepared by copolymerization of tetrafluoroethylene with perfluorovinylether having a carboxylic or sulfonic acid group at the end of a side chain. There are several variations of a general basic structure commercially available today [11]. The various preparation techniques are described in detail in the patent literature. 

The following aspects of this structural component are important in relation to control and recognition mechanisms first, a hydroxy-amino acid is located on the carboxyl side of the asparagine residue that forms the site of attachment second, there is a spacer residue (B) in the polypeptide chain and third, the polarity of residue B appears to control the type of oligosaccharide side-chain that becomes attached to asparagine. If residue B is polar, a complex type of poly-... 

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