Amino acids secondary, chiral

Only certain a-amino acids are present in any protein structure, and they have the general formula R-CH(NH2)-COOH, with the exception of proline and hydroxyproline that have secondary amino groups. With few exceptions, amino acids have chiral molecules with L (S) configuration at the chiral a carbon ... 

Amino acids are chiral substances. Usually only one of the enantiomers is found to be biologically active. Protein structure is determined by the sequence of amino acids in the chain (its primary structure), the coiling or stretching of the chain (its secondary structure), and the overall shape of the complete molecule (its tertiary structure). Two important secondary structures are the a-helix and the p sheet. The process by which a protein assumes its biologically active tertiary structure is called folding. Sometimes several proteins aggregate together to form a quaternary structure. 

Circular dicliroism has been a useful servant to tire biophysical chemist since it allows tire non-invasive detennination of secondary stmcture (a-helices and P-sheets) in dissolved biopolymers. Due to tire dissymmetry of tliese stmctures (containing chiral centres) tliey are biaxial and show circular birefringence. Circular dicliroism is tlie Kramers-Kronig transfonnation of tlie resulting optical rotatory dispersion. The spectral window useful for distinguishing between a-helices and so on lies in tlie region 200-250 nm and hence is masked by certain salts. The metliod as usually applied is only semi-quantitative, since tlie measured optical rotations also depend on tlie exact amino acid sequence. 

Organic-Base Catalyzed. Asymmetric direct aldol reactions have received considerable attention recently (Eq. 8.98).251 Direct asymmetric catalytic aldol reactions have been successfully performed using aldehydes and unmodified ketones together with chiral cyclic secondary amines as catalysts.252 L-proline and 5,5-dimethylthiazolidinium-4-carboxylate (DMTC) were found to be the most powerful amino acid catalysts for the reaction of both acyclic and cyclic ketones as aldol donors with aromatic and aliphatic aldehydes to afford the corresponding... 

Noncovalent interactions play a key role in biodisciplines. A celebrated example is the secondary structure of proteins. The 20 natural amino acids are each characterized by different structures with more or less acidic or basic, hydrophilic or hydrophobic functionalities and thus capable of different intermolecular interactions. Due to the formation of hydrogen bonds between nearby C=0 and N-H groups, protein polypeptide backbones can be twisted into a-helixes, even in the gas phase in the absence of any solvent." A protein function is determined more directly by its three-dimensional structure and dynamics than by its sequence of amino acids. Three-dimensional structures are strongly influenced by weak non-covalent interactions between side functionalities, but the central importance of these weak interactions is by no means limited to structural effects. Life relies on biological specificity, which arises from the fact that individual biomolecules communicate through non-covalent interactions." " Molecular and chiral recognition rely on... 

The relevant AGB values, calculated by using Te = 970 are listed in Table 8. According to the reported values, the heterochiral Trp/Pro and Phe/Ala complexes are more stable than the homochiral ones. The reverse is true for the Phe/ Pro and PheA al complexes. By the same token, the chiral discrimination factor, AR measured by ESl-MS for 19 amino acids was found to vary between 0.3 and 3. ° The stereochemistry associated to the CID of diastereomeric peptides has been investigated using a similar approach. The results suggest that the secondary structure of protonated peptides may play an important role in their gas-phase behavior. ... 

Various polymer-bound (polystyrene-bound) oxazaboroHdine catalysts for the reduction of secondary alcohols were reported [128]. These can simply be prepared by condensation of the resin-bound boronic acid with chiral 1,2-amino alcohols. The best results as far as enatioselectivity is concerned were obtained with oxaza-borohdine (59) (Scheme 4.36). 

Chirality derived from the readily accessible a-amino acids has been incorporated into the side chains of aza and diaza macrocyclic polyethers. A number of procedures suitable for peptide synthesis have proved (178) to be unsuitable for acylating the relatively unreactive secondary amine groups of aza crown ethers. Eventually, it was discovered that mixed anhydrides of diphenylphos-phinic acid and alkoxycarbonyl-L-alanine derivatives do yield amides, which can be reduced to the corresponding amines, e.g., l-172. By contrast, the corresponding bisamides of diaza-15-crown-S derivatives could not be reduced and so an alternative approach, involving the use of chiral A-chloroacetamido alcohols derived from a-amino acids, has been employed (178) in the synthesis of chiral receptors, such as ll-173 to ll-175, based on this constitution. 

These intermediates were used to synthesize optically pure ephedrines and amphetamines without recourse to resolution, since the chirality of the amino acid educt was entirely conserved throughout the process. The reduction of (S)-2-(ethoxycarbo-nyl)amino-propiophenone (74) first produced a mixture of alcohols (75a, b) Lithium aluminium hydride reduction then produced the desired secondary amino alcohols (76a, b). Table 1 illustrates the reduction scheme and the diastereomer ratios obtained "K... 

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