Sulphur Reagent II (3-[(Dimethylaminomethylene)amino]-3H-1,2,4-dithiazole-5-thione, also known as DDTT)⁽¹⁾ is a bright-yellow crystalline compound. It does not contain an oxygen atom and thus affords a very low percentage of oxidation in the process of oligonucleotide synthesis (< 2% for 20-mer oligodeoxynucleotide phosphorothioates). DDTT forms stable solutions that do not display any loss of functional activity or precipitation of the reagent for a period of over 6 months.
​The Use of DDTT in DNA Synthesis
Regardless of the sulfurization agent used, the presence of water in the reaction mixtures increases the degree of oxidation and leads to a higher PO/PS ratio. The use of anhydrous solvents for the preparation of the solutions of DDTT is strongly recommended. For the sulfurization step in the synthesis of DNA phosphorothioate oligos on small scale, using 5 to 6 molar excess DDTT (a contact time of 30 s, 1, 2, and 2.5 min for 0.1 M, 0.05 M, 0.03 M, and 0.02 M solutions respectively) is recommended.
The Use of DDTT in RNA Synthesis
The use of anhydrous solvents for the preparation of the solutions of DDTT is strongly recommended. To synthesize full-length RNA phosphorothioates, 0.05 M solution of DDTT, with a 4 min contact time is recommended. Under these conditions, >90% yield of full-PS 20-mer RNA may be expected. RNA sequences containing stretches of purine nucleoside residues are more difficult to sulfurize irrespective of the reagent used. To obtain the high degree of sulfurization with those oligonucleotides, 0.1 M solution of DDTT and/or an extended contact time may be required.

DDTT is a proprietary product developed by AM Chemicals, LLC. It is protected under US Patents 7,723,528 and 8,552,175.

Ref:

1. Andrei P. Guzaev, Tetrahedron Letters 52 (2011) 434–437.