| Abstract
| - The hydrolytic reactions of the phosphorodithioate analogue of uridylyl(3‘,5‘)uridine [3‘,5‘-Up(s)2U]were followed by HPLC over a wide pH range at 363.2 K. Under acidic and neutral conditions,three reactions compete: (i) desulfurization to a mixture of the (RP)- and (SP)-diastereomers of thecorresponding 3‘,5‘- and 2‘,5‘-phosphoromonothioates [3‘,5‘- and 2‘,5‘-Up(s)U], which are subsequently desulfurized to a mixture of uridylyl(3‘,5‘)- and -(2‘,5‘)uridine [3‘,5‘- and 2‘,5‘-UpU], (ii)isomerization to 2‘,5‘-Up(s)2U, and (iii) cleavage to uridine, in all likelihood via a 2‘,3‘-cyclicphosphorodithioate (2‘,3‘-cUMPS2). Under alkaline conditions (pH > 8), only a hydroxide ioncatalyzed hydrolysis to uridine via 2‘,3‘-cUMPS2 takes place. At pH 3−7, all three reactions arepH-independent, the desulfurization being approximately 1 order of magnitude faster than thecleavage and isomerization. At pH < 3, all the reactions are hydronium ion catalyzed. On going tovery acidic solutions, the cleavage gradually takes over the desulfurization and isomerization.Accordingly, the cleavage overwhelmingly predominates at pH < 0. The overall hydrolytic stabilityof 3‘,5‘-Up(s)2U is comparable to that of (SP)- and (RP)-3‘,5‘-Up(s)U (and to that of 3‘,5‘-UpU, exceptat pH < 2). The rate of the hydroxide ion catalyzed hydrolysis of 3‘,5‘-Up(s)2U is 37% and 53% ofthat of (SP)- and (RP)-3‘,5‘-Up(s)U, respectively. The reactions, however, differ with the respect ofthe product accumulation. While the phosphoromonothioates produce a mixture of 2‘- and3‘-thiophosphates as stable products, 3‘,5‘-Up(s)2U is hydrolyzed to uridine without accumulationof the corresponding dithiophosphates. At pH < 3, where the hydrolysis is hydronium ion catalyzed,the kinetic thio-effect of the second thio substitution is small: under very acidic conditions (Ho−0.69), (SP)-3‘,5‘-Up(s)U reacts 1.6 times as fast as 3‘,5‘-Up(s)2U, but the reactivity differencedecreases on going to less acidic solutions. In summary, the hydrolytic stability of 3‘,5‘-Up(s)2Uclosely resembles that of the corresponding phosphoromonothioate. While replacing one of thenonbridging phosphate oxygens of 3‘,5‘-UpU with sulfur stabilizes the phosphodiester bond underacidic conditions by more than 1 order of magnitude, the replacement of the remaining nonbridgingoxygen has only a minor influence on the overall hydrolytic stability.
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