Ribo-oligonucleotides

Felix et al. (1960) and Sulkowski and Laskowski (1962) used the solvents ethanol M ammonium acetate (75 30, v/v) and saturated ammonium sulphate isopropanol water (80 2 18, v/v/v) in two dimensions. This provides high resolution of small ribo- and deoxy-ribo-oligonucleotides. The method distinguishes between Ap and Cp residues. 

Trichloroethanol has been used as a protecting group in the triester approach for the synthesis of deoxyribo- [29] as well as ribo-oligonucleotides [30]. It can be removed selectively by reduction with Zn or Zn/Cu, as previously discussed. The triesters may be synthesized either by stepwise addition of the two properly protected nucleosides to, , -trichloroethyl phos-phorodichloridate (Fig. 6.11a) or by condensation of a/3, 3, 3-tri-chloroethyl ester of a nucleotide with a nucleoside using an arylsulphonyl chloride as a condensing agent (Fig. 6.11b). 

The jS-cyanoethyl group has been employed for the protection of phosphate diesters in the synthesis of deoxyribo- [34] and ribo-oligonucleotides [35]. It is introduced using a procedure similar to Fig. 6.11b, and is selectively removed on treatment wdth sodium hydroxide. In the synthesis of ribo-oligonucleotides protection by the phenyl group, also alkali-labile, has been described [36] (Fig. 6.14). It is introduced as in Fig. 6.11a. 

Spleen exonuclease is active on the 5 -OH oligonucleotides of both the ribo and the deoxyribo series. These are sequentially split from the 5 -OH end with formation of 3 -mononucleotides. It has been suggested that an enzyme-product intermediate may exist in the form of nucleoside-3 -phosphoryl-enzyme complex (3) since transfer of nucleoside-3 -phosphate to available 5 -hydroxyl functions (or other alcoholic functions) occurs at high substrate (or acceptor) concentrations 15, 18). 

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