Helices homochiral

Fig. 5.21. Top Self-assembly drives the formation of helical, homochiral dimeric titanium tris-catecholate complexes. Dimerization is only mediated by LC, while Na and K" " do not lead to comparable products. Bottom Crystal structures of the dimers formed from the aldehyde (left, R = H), the ethyl ketone (centre, R = C2H5), and the methyl ester (right, R = OCH3). Shown is a side view in space-filling representation and a ball-and-stick model with a view along the Ti—Ti...

The presence of a "handedness" in the molecules that make up living systems is a key feature of life on Earth. The insistence of nature on just one chiral form in the molecules of life is called homochirality. How did the dominance of the L- amino acids arise Why are the naturally occurring helices of protein and DNA, which we will discuss in Section 25.11, all right-turn helices Homochirality could have arisen by chance in the course of evolution or because it was "seeded" in some way at the beginnings of life. One theory is that chirality was introduced early in Earth s evolutionary history, through seeding by chiral amino acids that fell on the planet from outer space. 

Enantiomeric enrichment of homochiral peptides was achieved during partial hydrolysis of polypeptides composed from polymers that contain blocks of both random and a-helix [177] or random and /1-sheet sequences [178]. Hydrolysis of the atactic part of the polypeptides was found to occur much faster than within the isotactic blocks of a-helix and /3-sheets. Based on these observations, Bonner suggested that environmental dry/wet cycles on the primitive Earth might have caused repeated polymerization/hydrolysis cycles that permitted the eventual evolution of optical purity from a small abiotic ee value for the amino acids [177,179]. 

RNA but of both handednesses. Upon appending a single homochiral residue such as L-lysine at the carboxy terminus of the helix, the peptide nucleic acid polymers predominantly fold into duplexes of a single handedness. This phenomenal positive cooperativity can be considered a form of molecular amplification. Additional examples on spontaneous and induced formations of helical morphologies were reviewed [114,182]. 

Surprisingly little work has been carried out on the resolution of homochiral helicates into the two enantiomers. Self-resolution upon crystallization has been observed for two homonuclear triple helicates [37,38], but there seem to be only two well-authenticated cases of enantiomeric resolution, both using antimonyl tartrate the complex [ 02(9)3] " , a dinuclear triple helix [39], and a trinuclear double helical complex of iron(ll) [Fe3(19)2] " with a tm-terpyridyl ligand [40]. The circular dichroism spectrum of [ 02(9)3] " is shown in Figure 13. 

Figure 10.82 (a) Electron microscope images of materials formed by PLP + ULU (right-handed helix), (b) PDP + UDU (left-handed) and (c) PMP -I- UMU (nonhelical), (d) Schematic representation of the model for the triple-helical organisation of three self-assembled homochiral polymer strands (PTP, UTU)n (T = L or d not m). PU hydrogen-bonded pairs on the same strand are represented by circles of the same shading. Alkyl chains stick ont of the side of the cylinder. (Reproduced with permission from J.-M. Lehn, Supramolecular Chemistry, VCH Weinheim, 1995.)... 

R. Janoschek, Theories on the origins of biomolecular homochirality, in R. Janoschek (Ed.), Chirality From weak bosons to the a-helix, Springer, Berlin, 1991, Ch. 2, pp. 18-33. 

DNA binding with octahedral metal complexes has been the focus of many studies of different groups however. Barton and coworkers were the first to report on homochiral association (A, A) between A-[Ru(phen)3] with B-DNA (A-helix) on the basis of equihbrium dialysis Although the enantiospecific interaction of the A-... 

Fig. 9 Synthesis of a racemic octaaryl double helix utilizing tetra[2,3-thienylene] as a core. Supramolecular self-assembly of the racemic mixture yields homochiral supramolecular polymers (dimer repeat shown).

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