Hydrolysis of Thymidine Boranomonophosphate and Stepwise Deuterium Substitution of the Borane Hydrogens. 31P and 11B NMR Studies

The α- P -boranophosphate nucleosides comprise a new class of modified nucleotides that may find use as therapeutic and DNA diagnostic agents. Hydrolysis of thymidine 5‘-boranomonophosphate, d(p B T), has been studied in H 2 O and D 2 O using 1 H, 31 P, and 11 B NMR spectroscopies. Although d(p B T) is quite stable at 25 °C, it hydrolyzes slowly at higher temperatures. At 50 or 60 °C, d(p B T) hydrolyzes first into thymidine (dT) and boranophosphate (O 3 P−BH 3 3 - ), followed by subsequent hydrolysis of the O 3 P−BH 3 3 - to produce phosphonate and boric acid. A three-step deuterium substitution of the borane hydrogens in O 3 P−BH 3 3 - was detected in D 2 O by the presence of a 31 P isotope shift. The 31 P resonances shifted downfield by 0.14 ppm upon substitution of each of three 1 H atoms by 2 H. Exchange of the borane hydrogens with D 2 O occurs as sequential processes superimposed upon hydrolysis of O 3 P−BH 3 3 - . The hydrolysis and deuteration steps were characterized in terms of pseudo-first-order rate constants. Hydrolysis of O 3 P−BH 3 3 - is an order of magnitude slower than deuterium substitution.