L-Configuration

Glycine is the simplest ammo acid and the only one m Table 27 1 that is achiral The a carbon atom is a chirality center m all the others Configurations m ammo acids are normally specified by the d l notational system All the chiral ammo acids obtained from proteins have the l configuration at their a carbon atom meaning that the amine group IS at the left when a Fischer projection is arranged so the carboxyl group is at the top... 

Except for glycine which is achiral all of the a ammo acids present m proteins are chiral and have the l configuration at the a carbon... 

Where R H the amino acids may incorporated in either a D- or L-configuration and so it is possible for configurational polymers to be produced. There do not, however, show the same mechanical properties as the configurational homopolymers, which are more regular in structure. 

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 amino acids found in proteins have the L-configuration, as illustrated for alanine, serine, and leucine. 

Although all the chiial amino acids obtained from proteins have the l configuration at then a carbon, that should not be taken to mean that D-amino acids are unknown. In fact, quite a number of D-anino acids occur naturally. D-Alanine, for example, is a constituent of bacterial cell walls and D-seiine occurs in brain tissue. The point is that D-fflnino acids are not constituents of proteins. 

A different situation arises when one considers a stereospecific catalyst which is endowed with optical activity and which favors therefore a specific configuration. Such a catalyst, if highly stereospecific, should form polymers, for example of all d configuration with an occasional inclusion here and there of l units. Of course if a racemic mixture of such a catalyst is used, then formation of a racemic mixture of polymers is expected, each polymeric molecule having an all d configuration incrusted with l units or an all l configuration incrusted with d units. 

An interesting and important fact is that almost all amino acids isolated from proteins L-configuration have the L-configuration at the a-carbon, although some amino acids isolated from microbiological sources are the mirror image isomers, ie in the D-configuration. We shall consider amino add stereochemistry in more detail in section 8.3. 

The D-configuration is assigned to (+)-glyceraldehyde and the L-configuration is assigned to (-)-glyceraldehyde. 

Conversion to penicillin N is achieved by an epimerase that converts the L configuration amino group into the D configuration. 

Figure 10.21 Aldolase-catalyzed asymmetric synthesis of uncommon L-configured sugars (a), and selected examples of carbohydrate-related product structures that are accessible by enzymatic aldolization (b).

Whereas SHMT in vivo has a biosynthetic function, threonine aldolase catalyzes the degradation of threonine both l- and D-spedfic ThrA enzymes are known [16,192]. Typically, ThrA enzymes show complete enantiopreference for their natural a-D- or a-t-amino configuration but, with few exceptions, have only low specificity for the relative threo/erythro-configuration (e.g. (122)/(123)) [193]. Likewise, SHMT is highly selective for the L-configuration, but has poor threo/erythro selectivity [194]. For biocatalytic applications, the knovm SHMT, d- and t-ThrA show broad substrate tolerance for various acceptor aldehydes, notably induding aromatic aldehydes [193-196] however, a,P-unsaturated aldehydes are not accepted [197]. For preparative reactions, excess of (120) must compensate for the unfavorable equilibrium constant [34] to achieve economical yields. 

Phenylserine derivative (132), precursor to the enantiomer of the antibiotic thiamphenicol, has been prepared with 92% de and >99% ee using a recombinant D-ThrA from Alcaligenes xylosoxidans, whereas the opposite L-configurated isomer was obtained by i-ThrA catalysis with only low diastereoselectivity [196]. 

Despite the fact that a-L-rhamnosylpyranoside usually occurs in the anthocya-nins as the dissacharide rutinose, a-rhamnose was found in acerola, linked at position 3 to cyanidin and pelargonidin and to delphinidin in Vaccinium padifolium blueberries,and at the 5 position in black raspberries, " red raspberries, " and purple mashua. The D- and L-configurations were not assigned in any of these studies. 

Conclusive evidence for the a-L configuration of the idosyluronic-to-hexosamine bond was provided by 1H-n.m.r. studies on disaccharide 7, obtained as the major product ( 75% yield) of the cleavage of heparin by heparinase.106 This configuration is consistent also with the 1H- and 13C-n.m.r. spectra of disaccharide 6 (the major product from nitrous acid cleavage of heparin103-106) and of unmodified heparin.82-84... 

The building blocks of proteins are the alpha-amino acids, and exclusively those with the L-configuration. There are 20 that occur in na- ture. They too all consist of the four elements C, H, N, and 0 two amino acids additionally contain sulfur (cysteine and methionine). In certain, but vital, enzymes (the peroxidases), sulfur is replaced by selenium. 

The importance of the selenium-analog of cysteine, selenocysteine (Se-cysteine), HSeCH2CHNH2COOH, and its incorporation into protein via a ribosomal mechanism has earned it the label of the 21st amino acid.112 115 Assuming L configuration at the a carbon, Se-cysteine is represented by 49, R=H (Scheme 17). 

They confirmed that the leucine was the d-form while the tryptophane and alanine had the L-configuration. The... 

Another oxidizing enzyme with very interesting synthetic potential is galactose oxidase [14]. This copper protein oxidizes primary hydroxy functions in polyols enantioselectively to the corresponding aldehydes. Thus, sugar alcohols may be transformed into the interesting non-natural L-configurated... 

---