Phosphorothioates (PTOs) increase cell uptake, nuclease resistance, and elicit RNAse H activity. PTO consists in the substitution of a non-bridging oxygen in the phosphate of an oligo’s backbone with a sulfur atom. This type of bond can be introduced at the last 5 nucleotides at the 3’ and 5’-end, or internally. PTO bonds are not recognized nor cleaved by nucleases. Therefore, full-PTO oligonucleotides are protected against both, endonucleases and exonucleases degradation. However, this might also result in cytotoxicity. It is important to remember that PTOs destabilize duplexes by reducing the Tm of 1-3° C per PTO.
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H.P. Vosberg, and F. Eckstein. Effect of deoxynucleoside phosphorothioates incorporated in DNA on cleavage by restriction enzymes. J. Biol. Ch. 257.11 (1982): 6595-6599.