Hydrolysis of thymidine boranomonophosphate and stepwise deuterium substitution of the borane hydrogens. P-31 and B-11 NMR studies

The alpha-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 H2O and D2O using H-1, P-31, and B-11 NMR spectroscopies. Although d(p(B)T) is quite stable at 25 degrees C, it hydrolyzes slowly ac higher temperatures, At 50 or 60 degrees C, d(p(B)T) hydrolyzes first into thymidine (dT) and boranophosphate (O3P-BH33-), followed by subsequent hydrolysis of the O3P-BH33- to produce phosphonate and boric acid. A three-step deuterium substitution of the borane hydrogens in O3P-BH33- was detected in D2O by the presence of a P-31 isotope shift. The P-31 resonances shifted downfield by 0.14 ppm upon substitution of each of three H-1 atoms by H-2. Exchange of The borane hydrogens with D2O occurs as sequential processes superimposed upon hydrolysis of O3P-BH33-. The hydrolysis and deuteration steps were characterized in terms of pseudo-first-order rate constants. Hydrolysis of O3P-BH33- is an order of magnitude slower than deuterium substitution.