Charge Description of Base-Catalyzed Alcoholysis of Aryl Phosphodiesters: A Ribonuclease Model
The release of substituted phenol from aryl uridine-3'-phosphates is catalyzed by bases and involves cyclization to form the 2',3'-cyclic nucleotide.The rate constants for imidazole and hydroxide ion catalysis (kim and kOH, respectively) obey the Broensted equations (25 deg C and 0.25 M ionic strength) log kim = -0.59 pKArOH + 1.40 (n = 7, r = 0.955) and log kOH = -0.54 pKArOH + 6.68 (n = 9, r = 0.967).General-base-catalyzed release of 4-nitrophenol from the 4-nitrophenyl ester (kB) obeys the Broensted relationship log kB = 0.67 pKBH - 7.50 (n = 7, r = 0.989).Charge changes on base and leaving group atoms as determined from the corresponding β and βeq values do not balance.Comparison with data in the literature indicates that the difference in charge may be assigned to the attacking oxygen (2'-hydroxyl) rather than to the phosphoryl oxygens in the O...PO2... group of atoms.Both P-O bond forming and bond fission components of the reaction are considered to be only weakly advanced in a transition state that lies on a concerted pathway.
Davis, Andrew M.,Hall, Adrian D.,Williams, Andrew
p. 5105 - 5108
(2007/10/02)
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