Oligonucleotides comprising modified nucleosides

1. A method for identifying one or more aptamers capable of binding to a target molecule comprising:

(a) contacting a library of aptamers with the target molecule to form a mixture, and allowing for the formation of an aptamer-target complex, wherein the aptamer-target complex forms when an aptamer has affinity for the target molecule;

(b) partitioning the aptamer-target complex from the remainder of the mixture (or enriching for the aptamer-target complex);

(c) dissociating the aptamer-target complex; and

(d) identifying the one or more aptamers capable of binding to the target molecule;

wherein the library of aptamers comprises a plurality of polynucleotides, wherein each polynucleotide comprises at least one first 5-position modified pyrimidine and at least one second 5-position modified pyrimidine, wherein the first 5-position modified pyrimidine and the second 5-position modified pyrimidine are different 5-position modified pyrimidines;

wherein the first 5-position modified pyrimidine is a 5-position modified uridine and wherein the second 5-position modified pyrimidine is a 5-position modified cytidine; or

wherein the first 5-position modified pyrimidine is a 5-position modified cytidine and wherein the second 5-position modified pyrimidine is a 5-position modified uridine;

wherein the 5-position modified uridine comprises a moiety at the 5-position selected from a naphthyl moiety, a benzyl moiety, a tyrosyl moiety, an indole moiety and a morpholino moiety; and wherein the 5-position modified cytidine comprises a moiety at the 5-position selected from a naphthyl moiety, a benzyl moiety, a tyrosyl moiety, and a morpholino moiety.

2. The method of claim 1 , wherein each polynucleotide comprises a fixed region at the 5′ end of the polynucleotide.

3. The method of claim 2 , wherein the fixed region at the 5′ end of each polynucleotide is at least 10, at least 15, at least 20, at least 25 or at least 30 nucleotides in length, or 5 to 30, 10 to 30, 15 to 30, 5 to 20, or 10 to 20 nucleotides in length.

4. The method of claim 1 , wherein each polynucleotide comprises a fixed region at the 3′ end of the polynucleotide.

5. The method of claim 4 , wherein the fixed region at the 3′ end of the polynucleotide is at least 10, at least 15, at least 20, at least 25 or at least 30 nucleotides in length, or 5 to 30, 10 to 30, 15 to 30, 5 to 20, or 10 to 20 nucleotides in length.

6. The method of claim 1 , wherein the moiety of the 5-position modified uridine is covalently linked via a linker comprising a group selected from an amide linker, a carbonyl linker, a propynyl linker, an alkyne linker, an ester linker, a urea linker, a carbamate linker, a guanidine linker, an amidine linker, a sulfoxide linker, and a sulfone linker.

7. The method of claim 1 , wherein the moiety of the 5-position modified cytidine is covalently linked via a linker comprising a group selected from an amide linker, a carbonyl linker, a propynyl linker, an alkyne linker, an ester linker, a urea linker, a carbamate linker, a guanidine linker, an amidine linker, a sulfoxide linker, and a sulfone linker.

8. The method of claim 1 , wherein the 5-position modified cytidine is selected from NapdC, 2NapdC, TyrdC, and PPdC.

9. The method of claim 1 , wherein the 5-position modified uridine is selected from NapdU, PPdU, MOEdU, TyrdU, TrpdU, and ThrdU.

10. The method of claim 1 , wherein the 5-position modified cytidine is a NapdC and the 5-position modified uridine is selected from NapdU, 2NapdU, PPdU, MOEdU, TrydU, TrpdU and ThrdU.

11. The method of claim 1 , wherein the 5-position modified cytidine is a PPdC and the 5-position modified uridine is selected from NapdU, 2NapdU, PPdU, MOEdU, TyrdU, TrpdU, and ThrdU.

12. The method of claim 1 , wherein each polynucleotide comprises a random region.

13. The method of claim 12 , wherein the random region is 20 to 100, or 20 to 90, or 20 to 80, or 20 to 70, or 20 to 60, or 20 to 50, or 20 to 40, or 30 to 100, or 30 to 90, or 30 to 70, or 30 to 60, or 30 to 50, or 30 to 40 nucleotides in length.

14. The method of claim 1 , wherein each polynucleotide is 20 to 100, or 20 to 90, or 20 to 80, or 20 to 70, or 20 to 60, or 20 to 50, or 30 to 100, or 30 to 90, or 30 to 80, or 30 to 70, or 30 to 60, or 30 to 50, or 40 to 100, or 40 to 90, or 40 to 80, or 40 to 70, or 40 to 60, or 40 to 50 nucleotides in length.

15. The method of claim 1 , wherein each polynucleotide is an aptamer that binds a target, and wherein the library comprises at least 1000 aptamers, wherein each aptamer comprises a different nucleotide sequence.

16. The method of claim 1 , wherein steps (a), (b) and/or (c) are repeated at least one time, two times, three times, four times, five times, six times, seven times, eight times, nine times or ten times.

17. The method of claim 1 , wherein the one or more aptamers capable of binding to the target molecule are amplified.

18. The method of claim 1 , wherein the mixture comprises a polyanionic competitor molecule.

19. The method of claim 18 , wherein the polyanionic competitor is selected from an oligonucleotide, polydextran, DNA, heparin and dNTPs.

20. The method of claim 19 , wherein polydextran is dextran sulfate; and DNA is herring sperm DNA or salmon sperm DNA.

21. The method of claim 1 , wherein the target molecule is selected from a protein, a peptide, a carbohydrate, a small molecule, a cell and a tissue.