Self-replicating system

9 Kassianidis, E. and Philp, D., Design and implementation of a highly selective minimal self-replicating system , Angew. Chem., Int. Ed. 2006, 45, 6344-6348. 

capable of interacting with two new molecules of 12.48 (site A) and 12.49 (site B). In effect, the product, trans-12.51, is capable of forming a ternary complex 12.52 as seen in the previous catalytic and autocatalytic systems, which brings about the organisation of the two precursors to itself Reaction of the bound 12.48 and 12.49 generates a further molecule of trans-12.51, resulting in self-replication. Unfortunately, the resulting (trans-12.51)2 dimer is self-complementary with an association constant 

The ability of nucleic acids to act as templates for self-replication is a fundamental process in Nature s chemistry. The Rebek group have employed both templat-ing and recognition effects for the production of assembled systems that promote replication of the templating molecule. An important feature of this work is the fact that the presence of the usual weak intermolecular forces allowed the corresponding host-guest complexes to form and dissipate rapidly. The resulting dynamic behaviour provides an environment for an efficient autocatalytic replication process to occur. 

The initial self-replication system investigated involved recognition between adenine and an imide of Kemp s triacid via hydrogen bond formation. The 

A longer spacer group has also been inserted in the templating molecule by exchanging the 2,6-disubstituted naphthalene spacer of 38 for a 4,4 -biphenyldiyl 

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There was a sense however that the sequence that later came to be called RNA World was the most reasonable one RNA World denotes an early period in the development of self replicating systems... 

Winter, A. E. (1996), Autocatalysis and the generation of self-replicating systems , Acta Chem. Scand., 50, 469-485... 

Fig. 6.5 Scheme necessary for a minimal self-replicating system with an autocatalytic reaction cycle (von Kiedrowski, 1999)... 

Fig. 6.6 Schematic representation of the first minimal self-replicating system. The hexameric product obtained from linking the two trideoxynucleotides can itself function as a matrix (von Kiedrowski et al., 1992)...

The initial rate at which the matrix C is formed in these matrix-dependent experiments is related to the initial concentration c by a square-root dependence. This square root law of autocatalysis is found in most self-replicating systems ... 

M. Eigen describes this process roughly as follows let us consider a self-replicating system which is characterized by a quantity of information equal to N bits. The probability that a bit is incorrectly copied is w, and the selection reacts to errors by means of a selection factor S. In other words an error-free system has a selection advantage S over a system with an error. The survival criterion is then ... 

The theory that life began in an RNA World suggests that the first self-replicating system was a set of RNA molecules. The catalytic and informationtransferring properties of RNA indicate a possible scenario ... 

The similarities in products and pathways between interstellar molecules and terrestrial laboratory experiments imply a unity of physical and chemical laws in the universe. Given certain conditions and appropriate energy sources, the same chemical pathways will be followed to create certain products from the elements. That is not to say that life, even in primitive form, could be supported in interstellar space. The significant precursor molecules found in interstellar space are at extremely low concentrations, but if they were transported to planetary atmospheres, perhaps by comets, they might then react in the proper environment and evolve into self-replicating systems. 

Hydrogen-Bonding Templates in Self-Replicating Systems. 125... 

In the previous example the template is unrelated to the starting materials and to the final product. However, if the product formed by templation is the actual template, a self-replicating system emerges (see Scheme 42). 

Scheme 42 Schematic representation of a self-replicating system...

In 1990 Rebek reported an interesting self-replicating systems based on the Kemp s triacid derivative 82 (see Scheme 43) [112-114]. By a combination of hydrogen bonding and tt-tt stacking interactions, diamine 83 forms a binary complex with 82. In such a complex, the amine is ideally positioned to react with the activated ester in 82 yielding the ds-amide complex 84. 

Scheme 43 Rebek s self-replicating system based on Kemp s triacid derivative 82...

Scheme 44 Self-replicating system based on the condensation of aldehyde 86 and amine 87... 

These examples have demonstrated that it is possible to use the templating properties of a compound to accelerate its own formation. This is a potentially very attractive approach for the production of large quantities of a specific product with high selectivity. Furthermore, synthetic self-replicating systems also provide interesting models for their biological counterparts, which in turn could provide important clues to understand chemical evolution and indeed the origin of life itself. 

In spite of their attractive features, one of the problems usually encountered in self-replicating systems is the formation of very stable dimmers between the template and the complementary product formed. This obviously imposes important limitations to the use of self-replicating processes for the formation of large quantities of a specific product. 

Julius Rebek and his group were also active at about the same time with enzyme-free self-replication of chemical structures. Unhke von Kiedrowski group, he did not use nucleotides, but a rephcator consisting of an adenosine derivative and a derivative of Kemp s acid (Rotello etal., 1991 Rebek, 1994). See also Figure 7.7 for a self-replicating system not based on nucleic-acid chemistry. There are several variations of this scheme, which are not illustrated here - for reviews see Sievers etal, 1994 Orgel, 1995. 

Figure 7.6 Minimal representation of a cross-catalytic self-replicating system. (Adapted from Burmeister, 1998.)...

What does your intuition say did macromolecular self-replication systems come first in the origin of life or should they be seen as the product of a mature cellular or proto-cellular metabolism ... 

Achilles, T. and von Kiedrowski, G. (1993). A self-replicating system from three starting materials Angew. Chem., 32, 1198-201. 

Morowitz, H. J. (1967). Biological self-replicating systems. Prog. Theor. Biol, 1,... 

Minimal self-replicating systems. Curr. Opin. Chem. Biol, 8, 634-9. 

Takahashi, Y. and Mihara, H. (2004). Construction of a chemically and conformationally self-replicating system of amyloid-like fibrils. Bioorg. Med. Chem., 12, 693-9. 

A synthetic self-replicating system by Kiediowsky el at Figure 6.16. Examples of host-catalyzed reactions. 

A self-replicating system based on the catalytic action of reversed micelles has been presented in Chapter 1. Other cases of micellar catalysis have been discussed in Ref. 119. The use of semi crystalline fibers to immobilize catalysts and substrates was also proposed [120]. Another approach to the enhancement... 

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