METHOD OF OLIGONUCLEOTIDE SYNTHESIS

1 . A method for the synthesis of a plurality of immobilised nucleic acids of differing sequence, comprising:

a. taking a system with a solid support having a plurality of 5′-end immobilised nucleic acids which are 3′-ONH2 protected and a nitrite deprotection solution that is inactive at the basal pH of the system;

b. lowering the pH at a site localized to one or more selected immobilised nucleic acids, thereby activating the deprotection solution to deprotect the 3′-ends of a subset of the immobilised nucleic acids;

c. extending the deprotected 3′-ends of the immobilized nucleic acids using nucleotides with 3′-ONH2 protection and an optionally modified terminal transferase enzyme (TdT);

d. lowering the pH at a site localized to one or more selected immobilised nucleic acids, thereby activating the deprotection solution to deprotect the 3′-ends of a subset of the immobilised nucleic acids, wherein the localized sites are different to those of step b;

e. extending the deprotected 3′-ends of the immobilized nucleic acids using nucleotides with 3′-ONH2 protection and an optionally modified terminal transferase enzyme (TdT), thereby synthesizing a plurality of immobilised nucleic acids of differing sequence.

2 . The method of claim 1 comprising the steps of

a. taking a system with a solid support having a plurality of 5′-end immobilised nucleic acids which are 3′-ONH2 protected;

b. adding a nitrite deprotection solution that is inactive at the basal pH of the system;

c. lowering the pH at a site localized to one or more selected immobilised nucleic acids, thereby activating the deprotection solution to deprotect the 3′-ends of a subset of the immobilised nucleic acids;

d. removing the nitrite deprotection solution;

e. extending the deprotected 3′-ends of the immobilized nucleic acids using nucleotides with 3′ -ONH2 protection and an optionally modified terminal transferase enzyme (TdT);

f. adding a nitrite deprotection solution that is inactive at the basal pH of the system;

g. lowering the pH at a site localized to one or more selected immobilised nucleic acids, thereby activating the deprotection solution to deprotect the 3′-ends of a subset of the immobilised nucleic acids, wherein the localized sites are different to those of step c;

h. extending the deprotected 3′-ends of the immobilized nucleic acids using nucleotides with 3′ -ONH 2 protection and an optionally modified terminal transferase enzyme (TdT), thereby synthesizing a plurality of immobilised nucleic acids of differing sequence.

3 . The method of claim 1 wherein a different nucleotide solution is added compared to the previous cycle of extension, and the solutions are repeated in cycles to grow differing sequences in differing areas of the solid support.

4 . The method of claim 1 , wherein the immobilised nucleic acids are single stranded DNA species or double stranded DNA species, with a 3′ overhang, or a mixture thereof.

5 . The method of claim 1 , wherein the pH change is the result of an electrochemically generated acid (EGA).

6 . The method of claim 5 , wherein the method used to generate the EGA is selected from: the electrolysis of water or the modulation of a hydroquinone/benzoquinone system.

7 . The method of claim 1 , wherein the pH change is the result of a photogenerated acid.

8 . The method of claim 1 , wherein the modified TdT is active at the basal pH of the system and inactive at the altered pH required for deprotection of the 3′-ends of the immobilised nucleic acids.

9 . The method of claim 1 , wherein the altered pH required for deprotection of the 3′-ends of the immobilised nucleic acids is pH 5.5 or lower.

10 . The method of claim 9 , wherein basal pH of the system is 7.5 or higher.

11 . The method of claim 1 , wherein the nitrite solution is buffered.

12 . The method of claim 11 , wherein the buffer is selected from MES, citrate, phosphate, acetate or a combination thereof.

13 . The method according to claim 11 wherein the concentration of buffer is between 500 mM and 2500 mM.

14 . The method of claim 1 wherein the nitrite is present at a concentration of between 500-1000 mM.

15 . The method of claim 1 wherein the nitrite is sodium nitrite.

16 . The method of claim 1 , wherein the system comprises alternating anodic and cathodic electrodes.

17 . The method of claim 1 , wherein each of the plurality of immobilized nucleic acids is extended by at least 25 bases.

18 . The method of claim 1 , wherein the oligonucleotide sequences are released from being immobilized.

19 . A method for the selective deprotection of immobilised nucleic acids, comprising:

a. taking a system comprising:

i. a solid support wherein the solid support has a plurality of immobilised nucleic acids which are 3′-ONH 2 protected;

ii. a nitrite deprotection solution that is inactive at the basal pH of the system; and

b. temporarily lowering the pH at a site localized to one or more selected immobilised nucleic acids, thereby activating the deprotection solution to deprotect the 3′-ends of a subset of the immobilised nucleic acids.

20 . A kit for preparing a plurality of immobilised nucleic acids of differing sequence, comprising:

a. a solid support having a plurality of 5′-end immobilised nucleic acids which are 3′-ONH 2 protected;

b. a buffered nitrite deprotection solution that is inactive at the basal pH of the system;

c. nucleotides with 3′-ONH 2 protection; and

d. an optionally modified terminal transferase enzyme (TdT).