Synthesis and characteristics of a self-complementary tetramer d(CGCG) with methylphosphonate linkages of defined stereochemistry

Starting from protected deoxynucleosides CpG methylphosphonate dimers (p indicating methylphosphonate linkage) were prepared and separated into diastereomers by flash chromatography. The Rp isomer was converted into the beta-cyanoethyl phosphoramidite and coupled in solution with 5'-deprotected CpG to form a self-complementary tetramer d(CpGCpG) with two Rp configurated methylphosphonate moieties. This molecule is used in thermodynamic studies of the duplex to coil, and the B to Z transition and its structure is investigated by 2D NMR methods.