SYNTHESIS AND CHARACTERIZATION OF AN RNA DINUCLEOTIDE CONTAINING A 3'-S-PHOSPHOROTHIOLATE LINKAGE

The synthesis of an RNA dinucleotide (IspU) containing a 3'-S-phosphor othiolate linkage is described. The compound is prepared from -(3-deox y-3-iodo-beta-D-xylofuranosyl)hyperanthine with installation of the ph osphorothiolate group via an Arbusov reaction and protection of the ri bose 2'-hydroxyl as a silyl ether. IspU is found to be a substrate for several enzymes including T4 polynucleotide kinase, snake venom phosp hodiesterase, and ribonuclease T-2. Base-catalyzed cleavage of the din ucleotide is accelerated (similar to 2000-fold) relative to that of th e phosphate-linked compound IpU. Product characterization and kinetic analysis show that IspU is cleaved through the same mechanism as IpU. The observed rate acceleration is argued to reflect stabilization of t he anionic transition state by the polarizable sulfur atom.