Aminoacids configuration

The protein polymers are highly sterospecific, the aminoacid residues always adopting the L-configuration, i.e. the same configuration as the reference substance L-malic acid. 

The expression homochiral in its original meaning concerns isometric molecules of various kinds with the same configuration, for example, the (i,)-aminoacids. There were propositions to use this word in a narrow sense, as the equivalent of enantiopure. For a discussion, see Ref. 6, p. 215. 

Figure 3.8 The two enantiomers of a-aminoacids. Here we follow the classic nomenclature of l- and D-aminoacids for indicating the two chiral forms. In terms of the S, R nomenclature, L-aminoacids correspond to the S absolute configuration -except for cystein, which is R.

Chiral bicyclic guanidinium receptors (e.g. 12-14, S,S-configuration shown) have been developed in our group from aminoacid precursors [16]. Improved synthetic procedures for such compounds were later reported by Schmidtchen [17] (who in 1980 presented a first type of achiral bicycloguanidinium hosts [18] that form complexes with several oxoanions [19]). Most derivatives are highly... 

All aminoacids have L-configuration except where indicated. 

Copper has been employed commonly in relation with carbohydrates and aminoacids but other analytes can also be determined. Microalignment of the electrode with the channel outlet [89], deposition of a thin layer [81,108] or incorporation of a wire [21] are examples of in-channel formats. End-channel configurations such as those obtained by using a guide tube [11] or attaching a Cu wire perpendicularly to the outlet [32] after chip bonding were tested. Carbon nanotube (CNT)/copper composite electrodes have proved to be more sensitive, compared to the Cu and CNT alone, in the determination of carbohydrates [109]. 

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. 

In the synthesis of peptides such as bradykinin the use of the configurationally correct aminoacid is obligatory and semi-synthesis and total synthesis here become indistinguishable. By contrast the unnatural dipeptide sweetening agent, aspartame (ref. 106) illustrates the scope for semi-synthesis. 

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