Previous methods for the preparation of phosphorothioate-containing oligodeoxyribonucleotides rely on the reaction of phosphite triesters with sulfurizing reagents such as tetraethylthiuram disulfide (TETD) and 3H-1,2-benzodithiol-3-one 1,1-dioxide (Beaucage reagent). However, these and other sulfurizing reagents suffer from several disadvantages, and there is great impetus for the development of improved methods for sulfur transfer that are fully compatible with standard automated DNA synthesis. The present report describes the use of 1,2,4-dithiazolidine-3,5-dione (DtsNH) and 3-ethoxy-1,2,4-dithiazoline-5-one (EDITH) as effective sulfurizing reagents that meet these needs. Both reagents are easily prepared, and are stable upon prolonged room temperature storage in acetonitrile solution. The reagents are used at low concentrations (0.05 M) and for short reaction times (30 s). The methodology has been proven for the automated synthesis on 0.2-1.0 μmol scales of oligodeoxyribonucleotides, of length 6-20 bases, containing the phosphorothioate substitution at either a single site or at all positions.
Bibliographical noteFunding Information:
We thank Dr James G. Coull (PerSeptive BioSystems) and anonymous referees for constructive suggestions of additional experiments that have been carried out and reported in the revised version of this paper. DtsNH and EDITH that were used in some of these studies were prepared over a 20-year period by the following students and technicians: Lin Chen, Steven J. Eastep, David A. Halsrud and Lydia Ong, in addition to two of the authors (G.B. and R.P.H.). Finally, we are grateful to NIH grants GM 28934, 42722 and 43552 to G.B.; GM 49928 to K.M.F.; and grants LEQSF(RF/19931996)-RD-A-42 and NSF CHE-9500992 to R.P.H. for support of this research.