Effect of 5-bromo- and 5-methyldeoxycytosine on duplex stability and discrimination of the NotI octadeoxynucleotide. Quantitative measurements using thin-layer chromatography.

Octadeoxynucleotides based on the recognition sequence of the restriction endonuclease NotI were synthesized containing unmodified nucleotides and nucleotides with methyl and bromide additions at the C5 position of the pyrimidine ring of deoxycytosine. On annealing to single-stranded DNA bearing one NotI site, thin-layer chromatography (TLC) of the different oligonucleotides was used quantitatively to determine differences in dissociation temperature (Td) and binding equilibrium. Buffers used in filter hybridization experiments could be used in this TLC system. In addition, actual hybridizations were carried out to filter-bound DNA with and without a NotI site. The incorporation of 5-methyldeoxycytosine and 5-bromodeoxycytosine led to a significant increase in stability of homoduplex formation during hybridization, due to a shift in the binding equilibrium and an increase of the Td, thereby improving discrimination considerably. Some implications of the results for several techniques involving oligomer hybridization are discussed.