Glyceraldehyde structure

The isolation of pyruvaldehyde enol ethers rather than glyceraldehyde structures during mild acidic hydrolysis may indicate that the latter structures are not stable in their free form but tend to lose water to produce pyruvaldehyde enol ether structures. Such structures may therefore be formed in lignin during technical processes carried out under mild acidic conditions such as the production of TMP. Pyruvaldehyde enol ether structures absorb UV-light above 300 nm and may thus contribute to the photo-yellowing process. Under conditions simulating the production of CTMP, on the other hand, it was previously shown that these types of structures are rapidly and completely eliminated presumably by sulfonation reactions (75). 

The ordered structures of proteins (alpha-helices, beta-sheexs) readily explain the fact that all protein molecules (on Earth at least) are comprised of L-a-amino acids (since an occasional D-amino add would disrupt the ordered structure of L-enantiomers). The configurations of L-amino acids are described by the structure below they are consistent with carbohydrate nomenclature as seen by comparison with the D-glyceraldehyde structure shown earlier in this chapter. In both classes. 

Fischer projection A method of representing three-dimensional structures in two-dimensional drawings in which the chiral atom(s) lies in the plane of the paper. The two enantiomeric forms of glyceraldehyde are represented as... 

Glyceraldehyde can be considered to be the simplest chiral carbohydrate It is an aldotriose and because it contains one chirality center exists in two stereoisomeric forms the D and l enantiomers Moving up the scale m complexity next come the aldotetroses Examining their structures illustrates the application of the Fischer system to compounds that contain more than one chirality center... 

Suggest a reasonable structure for the intermediate in the con ] version of dihydroxyacetone phosphate to o glyceraldehyde 3 phosphate J... 

In the Fischer convention, the ermfigurations of other molecules are described by the descriptors d and L, which are assigned comparison with the reference molecule glyceraldehyde. In ertqrloying the Fischer convention, it is convenient to use projection formulas. These are planar representations defined in such a w as to convey three-dimensional structural information. The molecule is oriented with the major carbon chain aligned vertically in such a marmer that the most oxidized terminal carbon is at the top. The vertical bonds at each carbon are directed back, away fiom the viewer, and the horizontal bonds are directed toward the viewer. The D and L forms of glyceraldehyde are shown below with the equivalent Fischer projection formulas. 

At the present time, use of the Fischer convention is almost entirely restricted to carbohydrates, amino acids, and biologically important molecules of closed related structural types. The problem with more general use is that there are no adequate rules for deciding whether a diiral atom is like D-glyceraldehyde or L-glyceraldehyde when the structures are not closely similar to the reference molecules. This relationship is clear for carbohydrates and amino acids. 

Absolute configurations of the amino acids are referenced to D- and L-glyceraldehyde on the basis of chemical transformations that can convert the molecule of interest to either of these reference isomeric structures. In such reactions, the stereochemical consequences for the asymmetric centers must be understood for each reaction step. Propose a sequence of reactions that would demonstrate that l( —)-serine is stereochemically related to l( —)-glyceraldehyde. 

FIGURE 6.34 Sheet structures formed from andparallel arrangements of /3-strands, (a) Streptomyces suh i x Xu inhibitor, (b) glutathione reductase domain 3, and (c) the second domain of glyceraldehyde-3-phosphate dehydrogenase represent minimal andparallel /S-sheet domain structures. In each of these cases, an andparallel /S-sheet is largely exposed to solvent on one face and covered by helices and random coils on the other face. (Jane Richardson)... 

FIGURE 7.1 Structure of a simple aldose (glyceraldehyde) aud a simple ketose (dihy-droxyacetoue). 

R)-Glyceraldehyde. Fischer projection of, 976 molecular model of, 976, 977 Glyceric acid, structure of. 753 Glycerol, catabolism of, 1132-1133 s/i-Glycerol 3-phosphate, naming of, 1132... 

I-Oialkoxy carbonyl compounds are a special class of chiral alkoxy carbonyl compounds because they combine the structural features, and, therefore, also the stereochemical behavior, of 7-alkoxy and /i-alkoxy carbonyl compounds. Prediction of the stereochemical outcome of nucleophilic additions to these substrates is very difficult and often impossible. As exemplified with isopropylidene glyceraldehyde (Table 15), one of the most widely investigated a,/J-di-alkoxy carbonyl compoundsI0S, the predominant formation of the syn-diastereomer 2 may be attributed to the formation of the a-chelate 1 A. The opposite stereochemistry can be rationalized by assuming the Felkin-Anh-type transition state IB. Formation of the /(-chelate 1C, which stabilizes the Felkin-Anh transition state, also leads to the predominant formation of the atm -diastereomeric reaction product. 

Aldehydes up to a chain length of four nonhydrogen atoms are tolerated as acceptors. 2-Hydroxyaldehydes are relatively good acceptors, and the D-isomers are preferred over the t-isomers [180]. Reactions that lead to thermodynamically unfavorable structures may proceed with low stereoselectivity at the reaction center [181]. Recently, a single-point mutant aldolase was found 2.5 times more effective than the wild type in accepting unphosphorylated glyceraldehyde [182,183]. 

I, 5-pyranose and 2, 3-0-isopropylidene-D-glyceraldehyde. Crystal structure of methyl 7-acetamido-7-deoxy-1,2 3,4-di-O-isopropylidene-L-t/ireoa-D-ga Zacto-octopyranuronate, Carbohydr. Res., 271 (1995) 79-99. 

The a-carbon of all amino acids, with the exception of glycine, has four different substituent groups and is therefore an asymmetric carbon atom. Such an atom can exist in two different spatial arrangements which are mirror images of each other. These structural forms of molecules are known as stereoisomers and the common notation of D and L forms is used, a nomenclature that refers to their absolute spatial configuration when compared with that of glyceraldehyde (Figure 10.4). 

Figure 10.4 The stereochemical relationship between amino acids and glycer-aldehyde. The designation of d or l to an amino acid refers to its absolute configuration relative to the structure of d- or L-glyceraldehyde respectively. The d and l forms of a particular compound are called enantiomers.

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