The invention features a mononucleotide comprising a nucleobase bonded to a sugar having a 3′-carbon and a 5′-carbon, where the 5′-carbon is bonded to a phosphorus (V) atom of a phosphate group through an oxygen atom, the phosphorus (V) atom being bonded to (i) a disulfide bioreversible group through an oxygen atom; and (ii) (a) optionally substituted amino, optionally substituted alkoxy, optionally substituted aryloxy, or optionally substituted heteroaryloxy; or (b) the 3′-carbon through an oxygen atom. The invention also features methods of delivering the mononucleotide to a cell and methods of treating a subject having Hepatitis C.