Adenosine acid-base chemistry

Biological chemistry takes place in the aqueous environment of the body of an animal or plant. Therefore, in spite of the fact that the molecules involved are largely organic in nature, the chemistry is essential acid-base chemistry and therefore a good candidate for the application of the bond valence model, as the example of adenosine triphosphate discussed in the previous section (Section 13.5.4) shows. This section illustrates a number of other ways in which the model has been used in biological systems. 

Adenosine is similar to cytosine in its acid-base chemistry N-l, adjacent to the -NH2 group, is the principal site of protonation. A tautomer of the cation protonated at N-3 is formed in smaller amounts. Guanosine is electronically more complex, being protonated mainly at N-7 and to a lesser extent at N-329. This can be understood in terms of electronic interaction with the adjacent oxygen as indicated in the resonance structure to the right in the following diagram ... 

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. 

The imidazole nucleus appears in a number of naturally occurring products, among which the most important are the amino acid histidine (203) and the purines, which comprise many of the important bases in nucleic acids. These purines, of which adenosine (204) is an example, are fused imidazopyrimidines. Their chemistry is discussed in Chapter 4.09. 

Compounds other than nucleic acids can also serve as templates for polymerization to an extent. Tjivikua et al. [33] showed that an adenosine derivative will couple with a pentafluorophenyl ester derivative to form an amide bond. The product is self-complementary hydrogen bonds form between two products in a manner analogous to Watson-Crick base pairing, though the products are not nucleotides. The formation of one product enhances the formation of future products therefore the system is autocatalytic, and may be a model for self-repUcating systems using alternate chemistry (Fig. 9.7). 

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