Phosphorothioate links

The stereochemical mechanism of 3 processing and strand transfer has been investigated using DNA substrates that incorporate phosphorothioate link-ages [8]. For both reactions, the introduced chiral centers are inverted in the products, implying that the reactions occur via a one-step in-line displacement mechanism rather than via a covalent intermediate. 

The oligonucleotide d(C)28 containing only phosphorothioate internucleotidic links (prepared chemically as a diastereoisomeric mixture) is a remarkably powerful linear competitive inhibitor of HIV reverse transcriptase, with a Ki value more than two orders of magnitude lower than that of all-phosphodiester d(C)28- Other d(C)28 analogues with cumulative runs of nine phosphorothioate links at the beginning, middle, and end of the sequence gave intermediate Ki values. Unfortunately, the all-phosphorothiate d(C)28 showed substantial inhibition also of DNA polymerases a and y, and may thus be cytotoxic. 

Starting with backbone modifications, we will consider first phosphorothioate links that result from the exchange of one non-bridging oxygen atom for a sulphur atom in a given... 

E. Chueng, and A.A. Levin. 2002. Development of an ultrasensitive noncompetitive hybridization-ligation enzyme-linked immunosorbent assay for the determination of phosphorothioate oligodeoxynucleotide in plasma. Anal. Biochem. 304 19-25. 

Fig. 1 Modified nucleic acids with enhanced nuclease stability, a DNA, b phosphorothioate, c Z -OjT-C-methylene-bridged (locked) nucleic acid (LNA), d 2f-0,A -C-ethylene-bridged nucleic acid (ENA), e Z -O-methyl nucleic acid, f methylphosphonate-linked nucleic acid, g morpholino-linked nucleic acid, h peptide nucleic acid (PNA)...

Oligonucleotides are only very poorly taken up into cells, and a variety of conjugates have been studied with the aim of improving cellular uptake. The cellular uptake of the ISIS ICAM-1-specific phosphorothioate ODN conjugated to cholesterol (ISIS-9388) has been studied in vivo for uptake into liver cells in rats." The presence of the 3 -cholesterol on the ODN resulted in a two-fold increase in accumulation of the ODN in various liver cell types, and the authors conclude that conjugation of ODNs with cholesterol is likely to be beneficial for antisense therapy of liver-associated diseases. ODNs which have 3 -linked cholesterol have been shown to interact with cell membranes in macrophages, and... 

The phosphorothiolate linkage was used by Sommer and Cramer [255] for the thymidine 5 -phosphorothioate attachment to chloromethylated PS resin (5% cross-linking) in the early days of phosphodiester chemistry (Figure 19.9). In DMF at 60 °C after 6h of reaction with a stoichiometric amount of the nucleoside, a substitution level of about 35% of the initial chlorine has been obtained. The phosphorothiolate linkage between the oligonucleotide and the resin was split by treatment with iodine solution ( 5 mg ml.1) in 75% aqueous pyridine at ambient temperature for 22 h. 

In the group of phosphorothioates, an increase in the number of sulfur atoms linked to the phosphorus atom increases the fungicidal effect of the aliphatic esters (Hacskaylo and Stewart, 1962). Thus, simple trialkyl phosphorodithioates are more efficient than trialkyl phosphorothioates, and the fungicidal activity of phosphorotrithioates and tetrathioates is even more potent (Mandelbaum et al.,... 

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