Carbohydrates pyrimidine nucleosides

The most important derivatives of pyrimidines and purines are nucleosides Nucleosides are N glycosides m which a pyrimidine or purine nitrogen is bonded to the anomeric carbon of a carbohydrate The nucleosides listed m Table 28 2 are the mam building blocks of nucleic acids In RNA the carbohydrate component is d ribofuranose m DNA It IS 2 deoxy d ribofuranose... 

Although there are a number of inefficient steps in most proposed prebiotic syntheses of ribotides, the major objection to RNA as the primogenitor of life has been the relatively small yield of ribose in the formose reaction, a simple condensation of glycoaldehyde. Muller et a/.,18 however, have discovered a variation of the formose reaction that produces a limited mix of pentose diphosphates in which the ribose forms predominate (52 14 23 11, ribose arabinose lyxose xylose). Although many critical chemical roadblocks remain (such as the extremely low yield of pyrimidine nucleosides following the condensation of ribose and free bases), this advance belies the previously held view that products of the formose reaction are necessarily so chemically diverse that they are the carbohydrate analog of petroleum. 19... 

In the study of 1352 hydrogen bonds of type NH -0=C [561, only 6 were regarded as four-centered, but this analysis used an arbitrary van der Waals cut-off criterion which may have biased the conclusions. In the carbohydrates, purines, pyrimidines, nucleosides, and nucleotides, the proportion was about or less than 1%. 

Hydrates of Small Biological Molecules Carbohydrates, Amino Acids, Peptides, Purines, Pyrimidines, Nucleosides and Nucleotides... 

Table 22.1. The distribution of water hydrogen-bond configurations observed in hydrates of the carbohydrates, amino adds and peptides, purines and pyrimidines, nucleosides and nucleotides (number of hydrates)...

Table 22.2. Sixteen configurations observed for water molecules in hydrates of organic molecules (carbohydrates, amino acids, peptides, purines, pyrimidines, nucleosides and nucleotides). First number refers to acceptor bonds, X—H -Ow. Second number refers to donor bonds OwH A. CD indicates chelated... 

Fig. 22.1. H 0 hydrogen-bond length distributions in carbohydrate, purine, pyrimidine, nucleoside, nucleotide and peptide hydrates. Note that OH - Ow

An analysis of the water molecule configurations in hydrates of the amino adds, peptides, carbohydrates, purines, pyrimidines, nucleosides and nucleotides is presented in Ihble 22.1. The key to the 12 different observed configurations is given in Ihble 22.2. The water molecules, which are involved only in two-center bonds constitute 63% of the total number of bonds, the others are donors or acceptors of three-center bonds. Similar proportions are observed in all three different classes of molecules. Thq only possibly significant difference is a trend toward more three-coordinated pyramidal configurations with the nucleosides and nucleotides. 

The term nucleoside was once restricted to pyrimidine and purine A-glycosides of D-ribofuranose and 2-deoxy-D-ribofuranose, because these are the substances present in nucleic acids. The term is used more liberally now with respect to the carbohydrate portion, but is still usually limited to pyrimidine and purine substituents at the anomeric carbon. Uridine is a representative pyrimidine nucleoside it bears a D-ribofuranose group at N-1. Adenosine is a representative purine nucleoside its carbohydrate unit is attached at N-9. 

Natural nucleoside bases can be one of five heterocycUc aromatic compounds and include adenine, guanine (the purines), cytosine, thymine and uracil (the pyrimidines). Nucleosides are composed of a base which is attached to a carbohydrate moiety through one of the aromatic ring nitrogen atoms. The specific carbohydrate distinguishes RNA (ribose) from DNA (deoxyribose). Nucleotides are phosphory-lated derivatives of nucleosides and contain one, two, three or (less commonly) four or more phosphate residues either at the 5 position or at the 3 position (Fig. 11.1.1). 

The anomeric carbon of the carbohydrate is attached to N-1 in pyrimidine nucleosides and to N-9 in purines. 

The N-glycosidic purine-carbohydrate bond in both types of nucleic acid is particularly susceptible to acids whereas the pyrimidine nucleosides and -nucleotides are quite resistant. Acid removes the phosphoric acid moiety much more easily from purine nucleotides than from pyrimidine nucleotides. 

Ammonium hydroxide Pyrimidine nucleosides from carbohydrates and cyanoacetylenes via carbohydrate 2-amino-Zl -oxazoline ring... 

Using doubly-labeled cytidine-C, it was demonstrated that the ribose and pyrimidine moieties were incorporated to the same extent into the pyrimidine and carbohydrate moieties of both RNA and DNA pyrimidine nucleosides. This established that the postulated intact incorporation of the labeled nucleosides had taken place, at least with cytidine (331). Other studies with several doubly-labeled nucleosides, cytidine, uridine, deoxycytidine, and thymine, also indicated a relatively direct conversion of a ribonucleoside to a deoxyribonucleoside. (333). The latter studies also... 

In order to potentially improve solubility and molecular recognition of anti-VZV bicyclic furo[2,3- f] pyrimidine nucleosides, Jin etal. [52] prepared new carbohydrate conjugated analogs 42 (Figure 10.10). Evaluation of these compounds has not been reported yet. 

Section 28 2 Nucleosides are carbohydrate derivatives of pyrimidine and purine bases The most important nucleosides are derived from d ribose and 2 deoxy D ribose... 

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