Chemical synthesis of a very long oligoribonucleotide with 2-cyanoethoxymethyl (CEM) as the 2′-O-protecting group: structural identification and biological activity of a synthetic 110mer precursor-microRNA candidate

A long RNA oligomer, a 110mer with the sequence of a precursor-microRNA candidate, has been chemically synthesized in a single synthesizer run by means of standard automated phosphoramidite chemistry. The synthetic method involved the use of 2-cyanoethoxymethyl (CEM), a 2′-hydroxyl protecting group recently developed in our laboratory. We improved the methodology, introducing better coupling and capping conditions. The overall isolated yield of highly pure 110mer was 5.5%. Such a yield on a 1-μmol scale corresponds to 1 mg of product and emphasizes the practicality of the CEM method for synthesizing oligomers of more than 100 nt in sufficient quantity for biological research. We confirmed the identity of the 110mer by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, as well as HPLC, electrophoretic methods, and RNase-digestion experiments. The 110mer also showed sense-selective specific gene-silencing activity. As far as we know, this is the longest chemically synthesized RNA oligomer reported to date. Furthermore, the identity of the 110mer was confirmed by both physicochemical and biological methods.