Peptides heterochiral

In recent work, Balaram and coworkers have demonstrated that /9-amino acids can be tolerated at specific positions in the strands and turn segments of a-pep-tide hairpins [197, 198]. Induction of turn structure in tetrapeptides by incorporation of a / -amino acid has also been described [115]. Similarly, studies by Gell-man have shown that the dinipecotic acid heterochiral turn segment can be used to nucleate a-peptide hairpin structures [199]. 

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. ... 

We have published a theoretical study of peptide model dimers in what homo-vs. heterochiral recognition is concerned for For-Gly-NH2, For-Ala-NH2, and four of their fluoro substituted derivatives using DFT methods (B3LYP/6-31+G and B3LYP/6-311+G ) [27]. This is related to the work of Kojo and Tanaka [129], and Kojo et al. [130] that experimentally studies the cocrystallization of asparagine with other aminoacid. 

I. Alkorta, J. Elguero, Theoretical study of peptide model dimers. Homo versus heterochiral complexes. Theochem 680, 191-198 (2004)... 

Fig. 22 A schematic representation of chiroselective replication cycles. Homochiral peptides Tll and Tdd are produced autocatalytically while the heterochiral peptides TDL and Tdl result from uncatalyzed background reactions. Template-directed ligation reactions proceed through the intermediary of stereospecific noncovalent complexes and pass on stereochemical information from the homochiral products to the substrates, thus resulting in the amplification of homochiral products. Light and dark backgrounds denote regions of the sequence composed of L- and D-amino acids, respectively (Reproduced from [218], Copyright 2001, Nature)...

Nagarajan, V., Pattabhi, V., Johnson, A., Bobde, V., and Durani, S. (1997) Crystal structures of heterochiral peptides. Part II. tert-Boc-valyl-D-alanyl-leucyl-alanyl methoxide, J. Peptide Res. 49, 74-79. 

Furthermore, during the chain elongation process, the reaction may proceed along either pathway to yield atactic polymers. Indeed, the polymerization experiments performed on the racemic crystals suspended both in hexane and in water containing the n-butylamine initiator yielded a complex library that was dominated by the long heterochiral peptides (Figure 8.16a). 

Moreover, it was also resistant towards small errors in the template chain substitution of one monomeric unit by its enantiomer created a heterochiral template that nevertheless catalysed the formation of the homochiral peptide. The peptide s homochirality would be preserved this way even if racemization of some of its building blocks occiured. 

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