A compound having the formula (I): wherein R1 is H, HO, acyl-O, TOM-O, ACE-O, alkyl-O, orthoester, TBDMS-O, HSe, diselenide, or alkyl-Se, R2 is H, HO, acyl-O, TOM-O, ACE-O, CH3-Se, alkyl-O, TBDMS-O, TMS-O, HSe, diselenide, alkyl-Se, phosphoramidite, phosphoroselenoamidite, phosphate, phosphoroselenoate, a 5'linked nucleotide, a 5'linked seleno-nucleotide, a 5'linked oligonucleotide, a 5'linked seleno-oligonucleotide, a 5'linked nucleic acid chain, or a 5'linked seleno-nucleic acid chain, R3 is H, HO, acyl-O, alkyI-O)3Si-O, BzH-O, DOD-O, alkyl-O, TBDMS-O, HSe, diselenide, alkyl-Se, DMTr- O, phosphate, phosphoroselenoate, diphosphate, diphosphoroselenoate, triphosphate, triphosphoroselenoate, a 3'linked nucleotide, a 3'linked oligonucleotide, a 3'linked nucleotide, a 3'linked oligonucleotide, or a nucleic acid chain, R4 is H or CH3, and X is an oxygen atom or selenium atom.