PH- AND BUFFER-INDEPENDENT CLEAVAGE AND MUTUAL ISOMERIZATION OF URIDINE 2'-ALKYL AND 3'-ALKYL PHOSPHODIESTERS - IMPLICATIONS FOR THE BUFFERCATALYZED CLEAVAGE OF RNA

Concurrent isomerization of the ethyl, 2-ethoxyethyl, 2,2-dichloroethy l and 2,2,2-trichloroethyl esters of uridine 3'-phosphate to their 2'- counterparts and cleavage to uridine 2',3'-cyclic phosphate have been studied over a wide pH-range (H-0 -0.2 to pH 9) at 363.2 K, The buffer -independent pa-rate profiles obtained show involvement of four distin ct kinetic terms in the cleavage reaction, viz. dependence of rate on [H+][SH], [SH], [S-] and [S-][H+](-1) (S- denotes the diester monoanio n). The beta(lg) values (lg = leaving group) for the partial reactions are -0.04 +/- 0.04, -0.19 +/- 0.12, -0.59 +/- 0.12 and -1.10 +/- 0.05 , respectively. The isomerization, in turn, shows dependence of rate o n [H+][SH], [SH] and [S-], the beta(rg) values (rg = remnant group) be ing -0.23 +/- 0.04, -0.23 +/- 0.11 and -0.03 +/- 0.01. The mechanisms of various partial reactions are evaluated by comparing these values w ith those of specific and buffer catalyzed reactions of the correspond ing 3'-phosphotriesters, regarded as mimetics of the neutral ionic for m of diesters, Furthermore, the mechanistic significance of the dissim ilar competition between the cleavage and isomerization with diesters and triesters is discussed.