Catalysis buffer

Figure 6-4 is a plot according to Eq. (6-46) for the hydrolysis of trans-cinnamic anhydride in the presence of carbonate buffers. The nonzero slopes indicate the existence of buffer catalysis, and the increasing slope value with increasing pH shows that /cb must be larger than k. From the intercepts ko is obtained at each pH, and the plot according to Eq. (6-47) is shown in Fig. 6-5. This plot shows that k is negligible. With this information, Eq. (6-46) can be simplified to...

Poly(L-malate) decomposes spontaneously to L-ma-late by ester hydrolysis [2,4,5]. Hydrolytic degradation of the polymer sodium salt at pH 7.0 and 37°C results in a random cleavage of the polymer, the molecular mass decreasing by 50% after a period of 10 h [2]. The rate of hydrolysis is accelerated in acidic and alkaline solutions. This was first noted by changes in the activity of the polymer to inhibit DNA polymerase a of P. polycephalum [4]. The explanation of this phenomenon was that the degradation was slowest between pH 5-9 (Fig. 2) as would be expected if it were acid/base-catalyzed. In choosing a buffer, one should be aware of specific buffer catalysis. We found that the polymer was more stable in phosphate buffer than in Tris/HCl-buffer. 

In the two examples of buffer catalysis of proton transfer from an intramolecularly hydrogen-bonded acid which have been discussed, it seems reasonably certain that the mechanism in Scheme 7 applies. The reactions are of the first order with respect to the catalyst B and it therefore follows that proton removal from the non-hydrogen-bonded species is rate-limiting k j > 2[B]- If this step consists of diffusion-controlled proton removal from a low concentration intermediate, the value k2 lx 109dm3 moP s-1 will apply for proton transfer to an amine. In the case of proton removal by hydroxide ion from 4-(3-nitrophenylazo)salicylate ion, the reaction was found to be of the first order in hydroxide ion up to the highest concentrations which could be studied (0.003 mol dm-3) with a rate... 

In 50% dioxan (Fife and Przystas, 1977). The reference intermolecular reaction is the hydrolysis of the acetal group of the corresponding methyl ester by a carboxylic acid of p/f. 5.56. The value of k2 was calculated from the buffer catalysis data given for three carboxylic acids using buffer pK. s measured in 50% dioxan (at 50°) by Fife and Brod (1970)... 

In previous chapters, we discussed the hydrolysis of a number of esters of A-(hydroxymethyl)phcnytoin, namely esters of organic acids (7V-acyloxy-methyl derivatives, Sect. 8.7.3) or inorganic acids (Sect. 9.3.2). Hydrolysis of these potential prodrugs released 3-(hydroxymethyl)phenytoin (11.45), whose breakdown to phenytoin and formaldehyde was also investigated per se [79], The latter reaction followed pseudo-first-order kinetics. At pH 7.4, the f1/2 values were 4.7 and 1.6 s at 25° and 37°, respectively. The tm values decreased tenfold for each increase of pH by one unit, which, together with the absence of any buffer catalysis, indicates catalysis by the HO- anion. 

The acid-catalysed hydrolysis of the acylal, 1-phenoxyethyl propionate (13), has been studied using the PM3 method in the gas phase. The kinetics and mechanism of the hydrolysis of tetrahydro-2-furyl and tetrahydropyran-2-yl alkanoates (14) in water and water-20% ethanol have been reported. In acidic and neutral media, kinetics, activation parameters, isotope-exchange studies, substituent effects, solvent effects and the lack of buffer catalysis pointed clearly to an Aai-1 mechanism with formation of the tetrahydro-2-furyl or tetrahydropyran-2-yl carbocation as the rate-limiting step (Scheme 1). There is no evidence of a base-promoted Bac2 mechanism up to pH 12. ... 

Perdue EM, Wolfe NL (1983) Prediction of buffer catalysis in field and laboratory studies of pollutant hydrolysis reactions. Environ Sci Technol 17 635-642 Pierqinski GM, Sims JT, Vance GF (2000) Soils and environmental quahty, 2nd edn. Lewis Publications, Chelsea, Michigan... 

PoweU BA, Duff MC, Kaplan Dl, Field RA, Newville M, Hunter BD, Bertsch PM, Coates JT, Serkiz SM, Sutton RS, Triay IR, Vaniman DT (2006) Plutonium oxidation and subsequent reduction by Mn(lV) minerals in Yucca Mountain tuff Environ Sci Technol 40 3508-3514 Power LE, Aral Y, Sparks DL (2005) Zinc adsorption effect on arsenite oxidation kinetics at the bimessite water interface. Environ Sci Technol 39 181-187 Purdue EM, Wolfe NL (1983) Prediction of buffer catalysis in field and laboratory studies. Environ Sci Technol 17 635-642... 

Buffer catalysis is observed in the intramolecular tranesterification of ethyl 2-hydroxymethyl benzoate to phthalide [equation (24)] (Fife and Benjamin, 1973). There is general base catalysis by... 

McClelland and co-workers identified the initial adduct detected in laser flash photolysis experiments involving the reaction of 75g with d-G as 111 (Ar = 2-fluorenyl, Y = H, R = 2 -deoxyribose)." This identification was based on the absorption spectrum of the intermediate, which extends out to 400 nm suggesting a highly conjugated species, by the observed pXa of 3.9 of the intermediate, which is consistent with deprotonation of 111 to form 112, by the lack of dependence of the rate constant for decomposition of the intermediate on the nature of Ar for the intermediates derived from 75g, 75n, 75p, and 75q, and by the kinetics of the decomposition of the intermediate into the stable C-8 adduct 102, which includes a pH-rate profile that showed both ionization states were reactive, buffer catalysis of decomposition of the... 

Buffer catalysis of the hydrolysis of phenyl (311 R = Ph) and methyl (311 R = Me) benzenesulfinates to give the sulfinic acid (312) and alcohol ROH is strongly accelerated by both carboxylate and amine components of the buffer which give Bronsted /i values of approximately unity on separate lines. The carboxylates are about 44 tunes more effective than amines of similar basicity. A concerted. S n2 mechanism with a hypervalent intermediate (313) is proposed for the nucleophilic reaction of these esters.286 The reaction of the thiosulfinate esters (314) with sulfenyl chlorides RSCI and sulfenate esters (315) to give sulfinyl chlorides and disulfides and sulfinate esters and disulfides, respectively, has been studied.287 Hydrolysis of 2-(3-aminophenyl)sulfonyl-ethanol hydrogensulfate gives under different conditions various products such as the ether (316) and the sulfone (317).288... 

Buffer catalysis has been applied to induce chiral induction by enantio-selective protonation remarkable enantiomeric excess was achieved in the photodeconjugation of a,/3-unsaturated ketones and esters by using chiral catalysts for the ketonization of photoenols in aprotic solvents.29... 

In Guo, after the very fast protonation of the electron adduct by water at the heteroatom [k > 107 s 1, von Sonntag 1991 Candeias et al. 1992 at 0(6), N(3) or N(7), cf. reaction (180)], a rapid transformation occurs [reaction (181) k in H20) = 1.2 x 106 s k(in D20) = 1.5 x 10s s 1] which is also catalyzed by phosphate buffer (k = 5.9 x 107 dm3 mol-1 s 1) which has been attributed to a protonation at C(8) (Candeias et al. 1992). This assignment is based upon solid-state EPR data, where C(8)-H--adduct is the thermodynamically most stable H -adducl radical (Rakvin et al. 1987 for DFT calculations see Naumov and von Sonntag, unpubl. results). The high solvent kinetic isotope effect of ku/ko = 8 is a strong indication that a proton is transferred in the rate-determining step. The magnitude of the rate of phosphate buffer catalysis points to a protonation at carbon (for a similar reaction observed with the Thy radical anion see Table 10.20). The C(8)-H -ad-duct has a pKa value of 5.4 [equilibrium (182)]. 

While buffering systems are ideally considered to be unreactive with pharmaceuticals, buffers are known to enhance certain reactions (buffer catalysis) and there are documented cases of covalent reactions with general chemical compounds and with pharmaceuticals. For example, TRIS [tris... 

The situation is complex. In another study we examined the cyclization of compound 54 catalyzed by cyclodextrin bis-imidazoles [140]. This dialdehyde can perform the intramolecular aldol reaction using the enol of either aldehyde to add to the other aldehyde, forming either 55 or 56. In solution with simple buffer catalysis both compounds are formed almost randomly, but with the A,B isomer 46 of the bis-imidazole cyclodextrin there was a 97 % preference for product 56. This is consistent with the previous findings that the catalyst promotes enolization near the bound phenyl ring, but in this case the cyclization is most selective with the A,B isomer 46, not the A,D that we saw previously. Again the enolization is reversible, and the selectivity reflects the addition of an enol to an aldehyde group. The predominant product is a mixture of two stereoisomers, 56A and 56B. Both were formed, and were racemic despite the chirality of the cyclodextrin ring. 

The rates of hydrolysis of compounds 12 (X = H), 13 and 14 are independent of pH from about pH 6 to 1, and buffer catalysis is not seen. The interpretation of these results is the same as it is for series 5, namely rate-controlling attack by water on the iminium ion. At these pH values the enamine is present in a protonated form and equation 23 is the rate law. As is clear from Figure 2, the morpholine-derived iminium ion is by far the most electrophilic, while the pyrrolidino iminium ion is least reactive. This order is exactly the same as for compounds 1-3 the reasons were discussed in Section III.A.2. Rate constants for nucleophilic attack upon the iminium ions are given in Table 8. 

The interpretation of the results given so far, namely Scheme 2, is a reasonable one, but should be taken with some caution for two reasons. One is that the experiments which would allow or exclude a mechanism more exactly like that of Scheme 1 (C-protonation), that is, a thorough search for buffer catalysis and kinetic determinations... 

Catalysis of cation-pseudobase equilibration has been observed by a number of buffer species.92 This catalysis appears to be reasonably complex, and a more detailed study than is available to date will be required to allow a complete description of the catalytic steps involved. In general, it is found92 that the effects of buffer catalysis tend to be leveled out by working at constant buffer ionic strength (e.g., / = 0.1). 

---