Uridyl esters

The common nucleophile in ribonuclease enzymes, and thus in relevant models, is the 2 -OH group of the central nucleotide. The work of the Williams group [31] confirmed the mechanism of hydrolysis of uridyl esters (Scheme 2.14, base = U) with good, substituted-phenol leaving groups as a relatively simple process, described by the simple general base catalysis mechanism (Bronsted p = 0.67), with 2.10 2.11 as the rate determining step (Scheme 2.14), followed by rapid breakdown of the presumed phosphorane (pentacovalent addition) intermediate dianion... 

Imidazolides of adenylic acid (ImpA) or uridylic acid (ImpU) are polycondensed to oligonucleotides by means of Zn2+ ions. 1673 The resulting phosphordiester bond was found to be of the 2, 5 type. In the reaction of nucleoside 5 -phosphoric acid methyl ester with ImpA in the presence of MgC, 2, 5 -dinucleotides are formed six to nine times more frequently than the corresponding 3, 5 compounds. 63 Polycondensations of ImpA in aqueous solution in the presence of various divalent metal ions lead to short oligo-adenylic acids (pA) (n = 1—5) mainly with 2, 5 -intemucleotide linkages. With Pb2+, for example, the total yield of oligomers was as high as 57%. 1683 1693... 

A methylesterase which catalyzes the hydrolysis of y-glu-tamyl methyl esters of membrane bound proteins in Salmonella typhimurium and JJ. coli has recently been identified (25). Apparently, these membrane-bound proteins undergo methylation by a S-adenosylmethionine requiring methyltransferase (similar to transferase II). In this case the methylation and demethylation are directly associated with the chemotactic mobility of the microorganisms. When the cells are exposed to a chemotactic attractant, methylation of the membrane-bound proteins increases and straight-line movement up the gradient is induced. When the attractant is removed or a repellent substituted, the esterase decreases the methylation and random movement results. These control mechanisms are analogous in regulation to some of the reversible processes such as adenylation (26), uridylation (27) and phosphorylation (28). 

Hydrolytic rate data in the range Ho = —2 to pH 12 have been reported for a series of phosphorus triesters, the asymmetric dialkyl esters of 5 -0-pivaloyl-3 -uridylic acid (245 R = Et, CH2CH20Me, CH2CHF2). This provided scope for comparison with similar data for a series of symmetrical dialkyl esters of 5 -0-pivaloyl-3 -uridylic acid (246 R = Me, Et, IV, CH2CH20Me) reported previously by the same group and... 

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