Amino acid moieties

In principle, aspartame is produced through the coupling of two amino acid moieties. One moiety consists of T.-phenylalanine methyl ester hydrochloride (2) made by treating the amino acid ia methanol and hydrochloric acid the other is aspartic acid anhydride hydrochloride or formic acid salt. The coupling reaction generates two positional isomers, a and p. 

Small chiral molecules. These CSPs were introduced by Pirkle about two decades ago [31, 32]. The original brush -phases included selectors that contained a chiral amino acid moiety carrying aromatic 7t-electron acceptor or tt-electron donor functionality attached to porous silica beads. In addition to the amino acids, a large variety of other chiral scaffolds such as 1,2-disubstituted cyclohexanes [33] and cinchona alkaloids [34] have also been used for the preparation of various brush CSPs. 

The first example of a grafted dendrimer carrying non-racemic amino acid moieties at the surface has been reported in 1991 byNewkome et al. [20]. A four-directional core molecule, which they prepared from pentaerythrol [21], has been elongated with the branching units fra[carboxyethoxymethyl]amino-... 

Figure 7.16 Enzymatic synthesis of the a-hydroxy-/3-amino acid moiety of (—)-hestatin... 

Fig. 4.23 Preparation of mesoporous silica materials with chirally twisted rod-like structures by using surfactant with a chiral amino acid moiety as a structure-directing reagent.

Barboiu, M., Hovnanian, N., Luca, C., Popescu, G. and Cot, L. (1998) Functionalized derivatives of benzocrown-ethers, III, New macrocydic derivatives containing chiral and linear lateral amino-acid moieties. European Journal of Organic Chemistry, 1705—1708. 

Table 6.3 displays the solution pKa values of the promoters capable of facilitating the electron transfer of Cu, Zn-SOD listed in Table 6.1. The —COOH-termi-nated SAMs are mostly negatively charged in phosphate buffer (pH 7.0). Although the bovine erythrocyte Cu, Zn-SOD has a net negative charge at pH 7.0 (p/ = 4.9), an electrostatic interaction is still expected to occur between the SAMs and the positively charged amino acid moieties (typically —NI13). Besides, the hydrogen bonding between —COOH groups and the amino acid residues is believed to comprehensively...

We have also studied the effect that moving the double bond closer to the amino acid moiety has upon the reactivity of unsaturated a-amino acids [43]. To this end, the cross-metathesis reactions of similarly protected homoallyl-, allyl-and vinylglycine with oct-l-ene were investigated under identical conditions (Eq. 25) (Table 3). 

Most coenzymes have aromatic heterocycles as major constituents. While enzymes possess purely protein structures, coenzymes incorporate non-amino acid moieties, most of them aromatic nitrogen het-erocycles. Coenzymes are essential for the redox biochemical transformations, e.g., nicotinamide adenine dinucleotide (NAD, 13) and flavin adenine dinucleotide (FAD, 14) (Scheme 5). Both are hydrogen transporters through their tautomeric forms that allow hydrogen uptake at the termini of the quinon-oid chain. Thiamine pyrophosphate (15) is a coenzyme that assists the decarboxylation of pyruvic acid, a very important biologic reaction (Scheme 6). 

Medium pH. The pH influenees the eharge on the surface of the mucus and the polymer [38]. Charge density on the surfaee of mueus varies with pH due to the differences in dissociation of the functional groups on the earbohydrate and amino acid moieties. 

For example, on reaction with (S)-jV-(methylsulfonyl)phenylalanyl chloride, meso-2-cy-clopentene-l,4-diol yields, besides the diester, the two diastereomeric monoesters which are separated chromatographically95. Since the absolute configuration of the amino acid moiety is known, it is (in principle) sufficient to determine the relative configuration at the three stereogenic centers in the monoesters, e.g., by an X-ray analysis (see p 473 for assignment by chemical correlation). 

Bitterness of 0-Aminoacyl Sugars Containing Peptides Composed of Valine or Phenylalanine. Since hydrophobicity of the amino acid moiety seemed to be an important key for bitterness of 0-aminoacyl sugars, we then prepared O-... 

Table IV. Relation between Tastes of O-Aminoacyl Sugars and Side Chain Length of Amino Acid Moiety...

In Section 3 we turn to reactions which require at least equimolar amounts of chiral information for the induction of asymmetry in the products. The newly formed asymmetric center can be induced by either intramolecular or by intermolecular interactions. Having served its stereochemical purpose, the amino acid moiety may be destroyed so that it does not exist as a discrete entity in the product, although sections of it may survive. 

Kinetics and stereochemistry of deuterium exchange of the a-hydrogen of an amino acid moiety in metal complexes of amino acid Schiff bases with or/Jio-hydroxy-acetophenone have been studied by Belokon et al.249). 

From an industrial chemist s point of view the use of proline, phenylalanine, valine, and other commercially available amino acids, is fine. To date, however, tert.-(S)-leucine is still an exotic compound. It should also be noted that the recycling of the chiral amino acid moiety is of importance for possible technical processes. On the other hand, the recovery of the chiral auxiliary sometimes does not make sense, especially in syntheses which the require the use of stochiometric amounts of expensive reagents, e.g. LDA. 

The strategy of designing saquinavir was based on the transition-state mimetic concept, an approach that has been used successfully in the design of potent inhibitors of renin and other aspartic proteases [10]. From the variety of nonscissile transition-state analogs of a dipeptide, the hydroxyethylamine mimetic was selected because it most readily accommodates the amino acid moiety characteristic of the Phe-Pro and Tyr-Pro cleavage sequence of the... 

Maillard browning will yield the constituent amino acid upon 6N HCI hydrolysis and analysis, but the Amadori amino acid moiety may be unavailable to the rat (9) since intestinal hydrolysis is less harsh than 6N HCI hydroTysis. Thus, we have a clear distinction between potential nutritional value and actual availability. 

Compound Amino acid moiety Protecting groups ... 

A tandem palladium-catalyzed reaction can effect a similar transformation to produce 2-vinyl-substituted heterocyclic systems as in Eq. 8E.11. By varying the amino acid moiety of the ligand, 83% ee could be obtained from the use of the glycine-derived ligand 129 [161]. A maximum enantioselectivity of 65% ee has been recorded for this type of reaction in an earlier study with BINAP as ligand [ 162]. Because both ( )- and (Z)-isomers gave the same enantioselectivity, attack on the rapidly interconverting 7t-allyl intermediates seems to determine the selectivity. Modest enantioselectivities have been reported for the related asymmetric preparation of 2-vinylpiperazine and 1,4-benzodioxane derivatives [163,164],... 

In meats, of course, there are components which arise from the protein which cannot be present in the products from pure fat. Table III shows some of the sulfur compounds and aromatic compounds which are also found in irradiated meats. Many of these can be postulated as arising from direct bond cleavage of amino acid moieties. Benzene and toluene may come from phenylalanine and phenol and p-cresol from tyrosine. Recent studies have been directed to considering the origin of some of the compounds from proteinaceous substances. Some of the sulfides, disulfides, and mercaptans can derive directly from cysteine or methionine, but those containing more than two carbon atoms in a chain require more than a superficial explanation. In order to evaluate the contribution of the volatiles from the protein as well as the lipid constituents of meat, volatile components produced in various protein substances have also been analyzed. 

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