Substitution of one non-bridging oxygen in a natural phosphodiester internucleotide linkage with a borano (-BH3) group results in a chiral phosphorus center in boranophosphate. UV thermal melting profiles were recorded for DNA duplexes formed between a DNA 9-mer with either an Sp or a Rp boranophosphate linkage in the middle and the complementary DNA 9-mer, as well as for their unmodified parent duplex. The thermal stability of the DNA duplexes was in the order of normal > Sp borano > Rp borano. CD spectra of all three duplexes exhibited typical B-DNA profile, which closely resembled each other.
In order to introduce pyrimidine C5-propynyl modification into boranophosphate oligodeoxyribonucleotides (BP- ODNs), 5-(1-propynyl)-2'-deoxyuridine 5'-(alpha-P-borano) triphosphate (d5PUTPalphaB) was synthesized. The two diastereomers were separated by reverse-phase HPLC. Kinetic studies showed that the Rp isomer was a slightly better substrate for MMLV reverse transcriptase than thymidine triphosphate or Rp-thymidine 5'-(alpha-P-borano)triphosphate. Using the Rp isomers of d5PUTPalphaB and the other three 5'-(alpha-P-borano) triphosphates, a DNA primer could be extended to the full length of the template.
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