Phosphorothioate group

The terminal base linkage at the 5 and 3 ends of these sequences was modified to a phosphorothioate group. 

The Sp and Rp stereoisomers of the nonbridging phosphorothioate group also provide the wherewithal for different modes of metal binding (Figure 12a,b). The positions of the phosphorothioate groups (a, /3, or y) (Figure 12c) affect both the stabihty of the formed M-S bonds (e g. there is a... 

Figure 9.34. Labeling with Phosphorothioates. Phosphorothioates, groups in which one of the nonbridging oxygen atoms is replaced with a sulfur atom, can be used to label specific sites in the DNA backbone to determine the overall stereochemical course of a displacement reaction. Here, a phosphorothioate is placed at sites that can be cleaved by i coRV endonuclease.

Another approach being used to enhance the ionization process is the chemical modification of the nucleic acid molecule in order to improve ionic volatility. For example, the replacement of phosphate protons from native DNA backbones by alkyl groups,25 or the replacement of phosphate groups by phosphorothioate groups followed by alkylation26 have been reported. 

Random incorporation of modified residues in in vitro transcribed RNA is of particular interest in analogue interference experiments (Section 4.6.3) where randomly modified RNA molecules with altered properties are selected and compared to the original pool of RNA molecules. A crucial prerequisite for the interference analysis is the ability to identify the modified residues. A convenient and efficient method is the use of nucleotides which contain both the modification of interest (e.g. 2 -0-methyl NTP, dNTP, inosinetri-phosphates) and an a-phosphorothioate group (one of the nonbridging oxygens is replaced with a sulphur).14-16 The concurrent incorporation of the phosphorothioate renders the modified nucleotides sensitive to a selective cleavage by iodine. 

Of 30 restriction enzymes tested we found eight (Ncil, Aval, Hindu, Avail, Pvul, Banll, PstI and FspI) that will not hydrolyze the phosphorothioate group (18,19). When such enzymes are confronted with dsDNA which has been modified so that one strand contains normal phosphate internucleotidic linkages, whilst the other strand contains phosphorothioate groups at the position of cleavage, nicked DNA is formed as the final product. This nicked... 

Synthesis and characterization of an octanucleotide containing the EcoRI recognition sequence with a phosphorothioate group at the cleavage site. Biochemistry 23, 3443-3453... 

Cosstick, R. and Eckstein, F. (1985) Synthesis of d(GC), and d(CG) octamers containing alternating phosphorothioate linkages effect of the phosphorothioate group on the B-Z transition Biochemistry 24, 3630-3638... 

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