Preparation and cleavage reactions of 3 '-thiouridylyl-(3 '-> 5 ')-uridine

3'-Thiouridylyl-(3'--> 5')-uridine [(Us)pU] 3 is prepared by coupling toget her the disulfide 14 and the 5'-H-phosphonate 18, and then removing the pro tecting groups. (Us)pU 3 readily undergoes cleavage in 0.05 mol dm(-3) sodi um glycinate buffer (pH 10.06) at 50 degrees C to give, in the first instan ce, uridine 4 and 3'-thiouridine 2',3'-cyclic phosphorothioate 21; in glaci al acetic acid at 30 degrees C, it rapidly undergoes cleavage in essentiall y the same way. The behaviour of (Us)pU 3 is compared with that of uridylyl -(3'--> 5')-uridine (UpU) 1a under the same basic and acidic reaction condi tions. (Us)pU 3 and 3'-thiouridine 2',3'-cyclic phosphorothioate 21 are bot h substrates for ribonuclease A; (Us)pU 3 is a substrate for Crotalus adama nteus snake venom phosphodiesterase but not for calf spleen phosphodiestera se.