Polynucleotide phosphorodithioate

An oligonucleotide of at least 2 nucleotides in length and having a 5'-terminal nucleotide of the formula:

• ##STR9## and a 3'-terminal nucleotide of the formula:
• ##STR10## and wherein the 5'- and 3'-terminal nucleotides of oligonucleotides containing at least three nucleotides are separated by a series of nucleotides of the formula:
• ##STR11## wherein each individual R is hydrogen or a blocking group;
• wherein each individual A is hydrogen, hydroxyl, halogen, SH, NH.sub.2, azide or OR.sub.4 wherein R.sub.4 is a blocking group;
• wherein each individual B is a purine or pyrimidine nucleotide base;
• and wherein each nucleotide in the oligonucleotide is connected to the preceding and following nucleotide by an internucleotide linkage;
• and wherein the oligonucleotide has at least one phosphorodithioate internucleotide linkage having the structure:
• ##STR12##