Hydrolytic stability of 2',3'-O-methyleneadenos-5'-yl 2',5'-di-O-methylurid-3'-yl 5'-O-methylurid-3'(2')-yl phosphate: implications to feasibility of existence of phosphate-branched RNA under physiological conditions.
Hydrolytic reactions of 2',3'-O-methyleneadenos-5'-yl 2',5'-di-O-methylurid-3'-yl 5'-O-methylurid-3'(2')-yl phosphate (1a,b) have been followed by RP-HPLC over a wide pH range to evaluate the feasibility of occurrence of phosphate-branched RNA under physiological conditions. At pH <2, where the decomposition of is first order in [H3O+], the P-O5' bond is cleaved 1.5 times as rapidly as the P-O3' bond. Under these conditions, the reaction probably proceeds by an attack of the 2'-OH on the phosphotriester monocation. Over a relatively wide range from pH 2 to 5, the hydrolysis is pH-independent, referring to rapid initial deprotonation of the attacking 2'-OH followed by general acid catalyzed departure of the leaving nucleoside. The P-O5' bond is cleaved 3 times as rapidly as the P-O3' bond. At pH 6, the reaction becomes first order in [HO-], consistent with an attack of the 2'-oxyanion on neutral phosphate. The product distribution is gradually inversed: in 10 mmol L(-1) aqueous sodium hydroxide, cleavage of the P-O3' bond is favored over P-O5' by a factor of 7.3. The results of the present study suggest that the half-life for the cleavage of under physiological conditions is only 100 s. Even at pH 2, where is most stable, the half-life for its cleavage is less than one hour and the isomerization between and is even more rapid than cleavage. The mechanisms of the partial reactions are discussed.
Lönnberg
Tuomas
T
Department of Chemistry, University of Turku, FIN-20014 Turku, Finland. tuanlo@utu.fi
Kiiski
Johanna
J
Mikkola
Satu
S
D002851:Chromatography, High Pressure Liquid; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D007700:Kinetics; D009705:Nucleosides / Q000737:chemistry*; D010755:Organophosphates / Q000737:chemistry*; D010710:Phosphates / Q000737:chemistry*; D012313:RNA / Q000737:chemistry*; D013237:Stereoisomerism; D013816:Thermodynamics; D014529:Uridine / Q000031:analogs & derivatives*
D009705:Nucleosides; D010755:Organophosphates; D010710:Phosphates; D012313:RNA; D014529:Uridine