Diffusion controlled hydrolysis

Neighbouring group participation by carbonyl groups in ester hydrolysis, 28, 171 Nitration, nitrosation, and halogenation, diffusion control and pre-association in, 16, 1 Nitrosation, mechanisms, 19, 381... 

The cholinesterases, acetylcholinesterase and butyrylcholinesterase, are serine hydrolase enzymes. The biological role of acetylcholinesterase (AChE, EC 3.1.1.7) is to hydrolyze the neurotransmitter acetylcholine (ACh) to acetate and choline (Scheme 6.1). This plays a role in impulse termination of transmissions at cholinergic synapses within the nervous system (Fig. 6.7) [12,13]. Butyrylcholinesterase (BChE, EC 3.1.1.8), on the other hand, has yet not been ascribed a function. It tolerates a large variety of esters and is more active with butyryl and propio-nyl choline than with acetyl choline [14]. Structure-activity relationship studies have shown that different steric restrictions in the acyl pockets of AChE and BChE cause the difference in their specificity with respect to the acyl moiety of the substrate [15]. AChE hydrolyzes ACh at a very high rate. The maximal rate for hydrolysis of ACh and its thio analog acetyl-thiocholine are around 10 M s , approaching the diffusion-controlled limit [16]. 

Facilitated transport of penicilHn-G in a SLM system using tetrabutyl ammonium hydrogen sulfate and various amines as carriers and dichloromethane, butyl acetate, etc., as the solvents has been reported [57,58]. Tertiary and secondary amines were found to be more efficient carriers in view of their easy accessibility for back extraction, the extraction being faciUtated by co-transport of a proton. The effects of flow rates, carrier concentrations, initial penicilHn-G concentration, and pH of feed and stripping phases on transport rate of penicillin-G was investigated. Under optimized pH conditions, i. e., extraction at pH 6.0-6.5 and re-extraction at pH 7.0, no decomposition of peniciUin-G occurred. The same SLM system has been applied for selective separation of penicilHn-G from a mixture containing phenyl acetic acid with a maximum separation factor of 1.8 under a liquid membrane diffusion controlled mechanism [59]. Tsikas et al. [60] studied the combined extraction of peniciUin-G and enzymatic hydrolysis of 6-aminopenicillanic acid (6-APA) in a hollow fiber carrier (Amberlite LA-2) mediated SLM system. 

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. 

It may be concluded from the known rate coefficients for the hydrolysis of acetic anhydride, for example, that the hypothetical k0H value is larger than a diffusion-controlled rate constant. If Xd is taken as about 107 for acetic anhydride, then from k0n = h2o where kHiQ is the second-order water... 

The reaction of barbiturate and 1,3 -dimethylbarbiturate ions with 2- and 4-nitrobenzaldehyde and 2,4-dinitrobenzaldehyde represented generally in Scheme 5 involves a diffusion-controlled (viscosity effects on rates) proton transfer from hydronium ion to an addition intermediate T in the slow step.14 The addition of water and ring-opening reactions of the protonated benzoxazines (14) involves the cyclic intermediate (15). At low buffer concentrations buffer-catalysed collapse of the intermediate is rate limiting but, at high buffer concentrations, the addition of water is the rate-limiting step.15 The anionic tetrahedral intermediate (16) is involved in the hydrolysis of the 2, 2, 2,-trifluoroethyl monoester of 1,8-naphthalic acid (17).16... 

A combined XPS-SIMS study of dealuminated HFAU samples shows a significant gradient in the Si/Al ratio in the crystallites50. This gradient was ascribed both to a gradient of dealumination (i.e. dealumination is diffusion controlled) and to a selective deposit of silica on the outer surface of the crystallites. Silica was proposed to be formed from hydrolysis of SiF4 that results from the decomposition of hexafluorosilicate. 

Cholinesterases, e.g., acetylcholinesterase (AChE, EC 3.1.1.7) and butyrylcholi-nesterase (BChE, EC 3.1.1.8), are serine hydrolases that break down the neurotransmitter acetylcholine and other choline esters [5]. In the neurotransmission processes at the neuromuscular junction, the cationic neurotransmitter acetylcholine (ACh) is released from the presynaptic nerve, diffuses across the synapse and binds to the ACh receptor in the postsynaptic nerve (Fig. 1). Acetylcholinesterase is located between the synaptic nerves and functions as the terminator of impulse transmissions by hydrolysis of acetylcholine to acetic acid and choline as shown in Scheme 4. The process is very efficient, and the hydrolysis rate is close to diffusion controlled [6, 7]. 

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