Sulfate monoesters hydrolysis

The hydrolysis of sulfate monoesters has been studied increasingly in relation to sulfate group transfer in vivo. In general, the rate-enhancing effect on the sulfate cleavage is small even with hydroxamate- or imidazole-functionalized cationic micelles which are extremely effective for the hydrolysis of phenyl esters. Recently, Kunitake and Sakamoto (1979a) found that zwitterionic hydroxamate [47] cleaved 2,4-dinitrophenyl sulfate effectively in cationic and... 

Phosphoric and sulfuric acid derivatives possess crucial properties that allow them to uniquely fill their many roles in biochemistry. Phosphoric acid may be esterified to form a monoester, diester, or triester (Figure 1). Sulfuric acid may be esterified at one or two positions, to form a monoester or a diester. Sulfate diesters are highly reactive, and have not been found in nature nor do phosphate triesters occur naturally. The hydrolysis of both phosphate and sulfate esters are thermodynamically favorable, but nucleophiles are repelled by the negative charge of the ionized forms. The resulting kinetic stability of phosphate monoesters and diesters, and of sulfate monoesters, is a major factor in their suitability for biological roles. For example, the half-life for hydrolysis of alkyl phosphate dianions by water is approximately 1.1 X 10 years k=2 x 10 s ) at 25°C. Such species... 

Figure 20 Possible mechanisms for sulfate ester hydrolysis catalyzed by members of the AS family. Formation of the sulfoenzyme (a) nucleophilic attack of the hydrated FGly oxygen on the substrate (b) nucleophilic attack of the sulfate oxygen of the substrate on the aldehyde FGly yields a diester sulfoenzyme, which hydrolyzes into the sulfoenzyme monoester. Enzyme desulfurylation (c) elimination of inorganic sulfate leaves an aldehyde moiety, which is hydrated to give a gerr7-diol (d) nucleophilic attack of a nucleophile, such as water, yields the hydrated gerr7-diol FGly.

Sulfates having alkyl groups from methyl to pentyl have been examined. With methyl as an example, the hydrolysis rate of dimethyl sulfate iacreases with the concentration of the sulfate. Typical rates ia neutral water are first order and are 1.66 x lO " at 25°C and 6.14 x lO " at 35°C (46,47). Rates with alkaH or acid depend on conditions (42,48). Rates for the monomethyl sulfate [512-42-5] are much slower, and are nearly second order ia base. Values of the rate constant ia dilute solution are 6.5 X 10 L/(mol-s) at 100°C and 4.64 X 10 L/(mol-s) at 138°C (44). At 138°C, first-order solvolysis is ca 2% of the total. Hydrolysis of the monoester is markedly promoted by increasing acid strength and it is first order. The rate at 80°C is 3.65 x lO " ... 

Alkaline solvolysis has been studied by a calorimetric method (49). Heat of hydrolysis of dimethyl sulfate to the monoester under alkaline conditions is 106 kj/mol (25 kcal/mol) (51). 

In contrast, the acid-catalyzed hydrolysis of alkyl selenates is A-2158. The actual species which undergoes decomposition to alcohol and sulfur trioxide is probably the zwitterion as in the case of phosphate monoester monoanions. Evidence for sulfur trioxide as the reactive initial product of the A-1 solvolysis is obtained from the product compositions arising with mixed alcohol-water solvents. The product distribution is identical to that found for sulfur trioxide solvolysis, with the latter exhibiting a three-fold selectivity for methanol. Although the above entropies of activation and solvent deuterium isotope effects do not distinguish between the conventional A-l mechanism and one involving rate-limiting proton transfer, a simple calculation, based on the pKa of the sulfate moiety and the fact that its deprotonation is diffusion controlled. 

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