2′-Substituted 2-amino-3-methylpyridine ribonucleosides in triplex-forming oligonucleotides: triplex stability is determined by chemical environment

A new synthetic route to the phosphoramidite monomer of 2-amino-3-methyl-5-(2′-O-methyl-β-D-ribofuranosyl)pyridine (Me-MAP) and its 2′-O-methoxyethyl analogue (MOE-MAP) has been established using D-ribose and 2-amino-3-methyl-5-bromopyridine as precursors. Ultraviolet melting and DNase I footprinting studies indicate that the triplex stabilizing properties of 2′-modified MAPs are determined by the conformation of the entire oligonucleotide backbone. Me-MAP confers a higher triplex stability than 2′-deoxycytidine whereas triplex stabilization by MOE-MAP is similar to that of dC. Incorporation of Me-MAP or MOE-MAP into oligonucleotides renders them dramatically more resistant to degradation by serum nucleases than incorporation of 2-amino-3-methyl-5-(2′-deoxy-β-D-ribofuranosyl)pyridine (dMAP) or dC.