Acid enhancement

Ammonium acetate has limited commercial uses. It serves as an analytical reagent, and in the production of foam mbber and vinyl plastics it is also used as a diaphoretic and diuretic in pharmaceutical appHcations. The salt has some importance as a mordant in textile dyeing. In a hot dye bath, gradual volatilization of ammonia from the ammonium acetate causes the dye solution to become progressively more acidic. This increase in acidity enhances the color and permanence of the dyeing process. 

The health effects of sorbic acid and sorbates have been reviewed (165—167). The extremely low toxicity of sorbic acid enhances its desirabiHty as a food preservative. The oral LD q for sorbic acid in rats is 7—10 g/kg body weight compared to 5 g/kg for sodium chloride (165—169). In subacute and chronic toxicity tests in rats, 5% sorbic acid in the diet results in no abnormal effects after 90 days or lifetime feeding studies. A level of 10% in rat diets results in a slight enlargement of the Hver, kidneys, and thyroid gland (170). This same dietary level fed to mice also resulted in an increase in Hver and kidney weight... 

Tnethanolaimne dansyl amino acids, enhancement dansyl amides... 

Experiments have shown that sulfuric acid enhances the rate of substitution of alcohols sufficiently to make this a practical reaction, but substitution of amines is not practical under these conditions, and substitution rates for fluorides and chlorides are not significantly affected by H2SO4. Why ... 

It was found that acid enhances grafting and homopolymer formation. Analysis of homopolymers shows that acid reduces the chain length but increases the number of grafted chains. 

Chlorogenic acid enhanced the colors of strawberry and chokeberry juices in concentrations higher than those of the anthocyanins present. However, the effects on purified pigments were lower, indicating the presence of other stabilizing compounds in the juices. ... 

Addifion of oils and oilseeds to the diet is another method that has been used to reduce CH4 emissions (Beauchemin et ah, 2008 Eckard et ah, 2010). Lipid addition to the diet may reduce CH4 emissions by hydrogenation of unsaturated fatty acids, enhanced propionic acid production, and protozoal inhibition (Johnson and Johnson, 1995). Reductions in CH4 of >40% have been demonstrated with lipid supplementation... 

Wang, X. C., C. Allen et al. (2007). Retinoic acid enhances the production of IL-10 while reducing the synthesis of IL-12 and TNF-alpha from LPS-stimulated monocytes/macrophages. J. Clin. Immunol. 27(2) 193-200. 

Yoshida M, Babensee JE (2004) Polydactic-co-glycolic acid) enhances maturation of human monocyte-derived dendritic cells. J Biomed Mater Res 71 45-54... 

In 1998, Schlotte et al. [259] showed that uric acid inhibited LDL oxidation. However, subsequent studies showed that in the case of copper-initiated LDL oxidation uric acid behaves itself as prooxidant [260,261]. It has been suggested that in this case uric acid enhances LDL oxidation by the reduction of cupric into cuprous ions and that the prooxidant effect of uric acid may be prevented by ascorbate. On the other hand, urate radicals formed during the interaction of uric acid with peroxyl radicals are able to react with other compounds, for example, flavonoids [262], and by that participate in the propagation of free radical damaging reactions. In addition to the inhibition of oxygen radical-mediated processes, uric acid is an effective scavenger of peroxynitrite [263]. 

Amino acids enhance the oxidation peak of Cu(0) obtained with a carbon paste electrode incorporating Cu(II) cyclohexylbutyrate. The increased current is proportional to the amino acid concentration at trace levels in the pM range373. The behavior of such electrodes was investigated for cysteine (115). On scanning potentials in the positive direction, the amino acid is accumulated on the electrode as the Cu(I) complex at +0.90 V vs a standard calomel electrode (SCE), in acetate buffer at pH 4.5 linear range is 2 x 10 9 to 1 x 10-7 M, 1 min accumulation, RSD 3% (n = 5)374,375. 

A theory for this acid effect has been developed essentially from the wool and cellulose work (3,4). Recently, in a brief communication, we reported analogous acid enhancement effects in the radiation grafting of monomers such as styrene in methanol to nonpolar synthetic backbone polymers like polypropylene and polyethylene (5). In the present work, detailed studies of this acid enhancement effect are discussed for the radiation grafting of styrene in various solvents to polyethylene. The results are fundamentally important since most of the experiments reported here have been performed in solvents such as the low molecular weight alcohols which, unlike cellulose and wool systems, do not swell polyethylene. 

Efficient copolymerisation can also be achieved in solvents other than the alcohols (Table VI). Thus the order of effectiveness for the present copolymerisation of these additional solvents is DMSO>DMF>dioxan>acetone>>chloroform>hexane. Acid enhancement is also observed in the first of these four solvents (Table VI). Characteristically (5), acid increases the intensity of the Tromms-dorff peak if it is already present in the system (dioxan) or alternatively induces the formation of the gel peak if it is not present in the solutions prior to acid addition (DMSO). 

Acid enhancement in the radiation grafting of styrene in methanol to cellulose (4), wool (3) and in preliminary work with the polyolefins (5 6) has been proposed as being predominantly due to such reactions. 

Further work (10) with acid effects in the radiolysis of binary mixtures such as benzene-methanol and pyridine-methanol indicates that the acid phenomenon is more complicated than the simple H atom model originally developed ( ). These more recent experiments (10) show that whilst increased hydrogen atom yields in the presence of acid enhance the overall grafting yield, other mechanisms also contribute to this acid effect. Thus the acid stability of intermediate radicals (I-III) and also analogous species involving the trunk polymer are important. With radicals (I-III), at low styrene concentrations in methanol, these intermediates (MR-) will predominantly react with other available... 

The fact that acid enhances grafting also indicates the possibility that ionic processes may also contribute to the present grafting mechanism. In this context, acid may be considered to be a catalyst for the cationic process especially since ionising radiation is the initiator for the reaction and both free radicals and ions are known to be species formed from interaction between molecules and such radiation. However, the ionic mechanism for grafting is favoured by anhydrous conditions, thus, in the present system, acid enhancement via the ionic pathway would not appear to be a predominant process. 

Because of their previous findings that a,/l-unsaturated thioesters were more reactive than their ester counterparts in Diels-Alder reactions85, Hart and coworkers86 performed a systematic study of the cycloaddition reactions of a,/l-unsaturated thioesters and a,ft-unsaturated selenoesters with several dienes. Thermal reactions were compared with Lewis acid catalyzed reactions at room temperature (equation 28 and Table 2). The results clearly demonstrated that use of a Lewis acid enhanced the regioselectivity (entries 1 vs 2, 3 vs 4, 5 vs 6 and 7 vs 8) as well as the endo (with respect to the thioester or selenoester group) selectivity (entries 5 vs 6 and 7 vs 8). 

Often Lewis acids are added to the system as a cocatalyst. It could be envisaged that Lewis acids enhance the cationic nature of the nickel species and increase the rate of reductive elimination. Indeed, the Lewis acidity mainly determines the activity of the catalyst. It may influence the regioselectivity of the catalyst in such a way as to give more linear product, but this seems not to be the case. Lewis acids are particularly important in the addition of the second molecule of HCN to molecules 2 and 4. Stoichiometrically, Lewis acids (boron compounds, triethyl aluminium) accelerate reductive elimination of RCN (R=CH2Si(CH3)3) from palladium complexes P2Pd(R)(CN) (P2= e g. dppp) [7], This may involve complexation of the Lewis acid to the cyanide anion, thus decreasing the electron density at the metal and accelerating the reductive elimination. 

Baltes S, Nau H, and Lampen A [2004] All-trans retinoic acid enhances differentiation and influences permeability of intestinal Caco-2 cells under serum-free conditions. Dev Growth Differentiation 46 503-514... 

The first step in the esterase activity of carbonic anhydrase (Fig. 3.15, e) is analogous to the first step in C02 hydration (Fig. 3.15,a). The tetrahedral intermediate so formed (Fig. 3.15,f) necessitates the participation of a proton donor for the departure of the leaving alcohol group (Fig. 3.15,f and g). It is possible that the Thr200 residue plays an important role in the esterase activity of carbonic anhydrase. Indeed, its replacement by other amino acids enhances the esterase activity but has no significant effect on the rate of C02 hydration [106],... 

Keough, T. DeStefano, A.J. Acid-Enhanced Field Desorption Mass Spectrometry of Zwitterions. Amd. Chem. 1981, 53, 25-29. 

Cui SS, Yang CP, Bowen RC, Bai O, Li XM, Jiang W, Zhang X (2003) Valproic acid enhances axonal regeneration and recovery of motor function after sciatic nerve axotomy in adult rats. Brain Res 975(l-2) 229-236... 

The formation and degradation of planktonic POC and PIC influence pH and C.A. as follows. The remineralization of POC produces CO2, which is rapidly hydrolyzed to carbonic acid, bicarbonate, and carbonate via the reactions given in Eqs. 5.53 through 5.57. Carbonic acid and bicarbonate are both weak acids, so their dissociation generates H. This acid enhances the dissolution of PIC through the following reaction ... 

Very interestingly, the regioselectivity of the palladium-catalyzed hydrophosphinylation can be switched by addition of a tiny quantity of diphenylphosphinic acid or other acidic compounds (Scheme 39) [35]. Thus, the linear product (19 8), which is the major product under normal conditions (i.e., without the phosphinic acid), becomes the minor regioisomer, and instead, the branched isomer (19a) is formed as the prevailing product. Furthermore, the addition of the phosphinic acid enhances the catalytic activity. Note that the quantity of the palladium catalyst is 5 mol% relative to the reactant, and that the addition of only 1 mol% (relative to the reactant) of the phosphinic acid results in a substantial change in the regioselectivity. New palladium species, which are much more active in the catalysis, appear to be generated upon addition of the phosphinic acid or other acidic additives. 

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