Catalyzed degradation

The anodized surface is often subjected to additional treatment before the radiation-sensitive coating is appHed. The use of aqueous sodium siUcate is well known and is claimed to improve the adhesion of diazo-based compositions ia particular (62), to reduce aluminum metal-catalyzed degradation of the coating, and to assist ia release after exposure and on development. Poly(viQyl phosphonic acid) (63) and copolymers (64) are also used. SiUcate is normally employed for negative-workiag coatings but rarely for positive ones. The latter are reported (65) to benefit from the use of potassium flu o r o zirc onate. 

The most common catalyst used in urethane adhesives is a tin(lV) salt, dibutyltin dilaurate. Tin(IV) salts are known to catalyze degradation reactions at high temperatures [30J. Tin(II) salts, such as stannous octoate, are excellent urethane catalysts but can hydrolyze easily in the presence of water and deactivate. More recently, bismuth carboxylates, such as bismuth neodecanoate, have been found to be active urethane catalysts with good selectivity toward the hydroxyl/isocyanate reaction, as opposed to catalyzing the water/isocyanate reaction, which, in turn, could cause foaming in an adhesive bond line [31]. 

One of the first decisions to be made when designing an experiment is the method of detection to be used with a particular solute. If radiolabeled material is available, a simple method of analysis is to count the radiolabel appearing in the receiver compartment as a function of time. While convenient, this can be a dangerous practice. Depending upon the type of radioisotope, its position in the molecule, and its specific activity, radiolabeled compounds can be subject to a variety of chemical and solution-catalyzed degradation pathways. If the stock solution contains a significant amount of radioactive impurities or generates them as a result of solution instability, then the possibility for preferential transport of... 

Karunakaran C, Dhanalakshmi R (2008) Semiconductor-catalyzed degradation of phenols with sunlight. Sol Energy Mater Sol Cells 92 1315-1321... 

Allen, B.L. et al. (2009) Mechanistic investigations of horseradish peroxidase-catalyzed degradation of single-walled carbon nanotubes. Journal of the American Chemical Society,... 

Landry D., Zhao K., Yang G.-Q., Glickman M., Georgiadis T. Antibody-catalyzed degradation of cocaine. Science. 259 1899, 1993. 

DGBP Lactalbumins Lysozymes Galactose activation (369) Lactose biosynthesis Catalyze degradation of peptidoglycan, e.g. of bacterial cell walls (370,371)... 

It is generally believed that proteases enhance cancer spread primarily by catalyzing degradation of the extracellular matrix. Since multiple substrates are encountered in this matrix, a number of different proteases are likely to be required to complete the metastatic process. Multiple proteases may also be required to activate different inactive precursor forms. Thus, in vitro, plasmin (D7), cathepsin B (D7), and a trypsin-like protease (K12) can all activate pro-uPA, while plasmin, which results from the action of uPA on plasminogen, can activate certain metalloproteases (M4). As mentioned earlier, completion of the metastatic process may require a cascade of different proteases operating, as shown in Fig. 2. 

The rate of acid-catalyzed degradation of the penicillins also depends largely on the nature of their acylamido side chain. Structure-activity-relationship studies undertaken for the rational design of orally active penicillins have shown that the stability in gastric juice increases with the sum of Taft s inductive substituent constants (of values) of the 6-amino side chain [95]. 

Fig. 5.19. Nucleophilic phosphate-catalyzed degradation of penicillins involving the formation of a penicilloyl phosphate intermediate [131]...

The failure of anionic micelles to promote HCT-catalyzed hydrolysis can be attributed to electrostatic repulsion between /3-lactams and micelles. The same explanation holds for the inhibition of acid-catalyzed degradation by cationic micelles [140] [144],... 

H. Bundgaard, J. Hansen, Nucleophilic Phosphate-Catalyzed Degradation of Penicilloyl Phosphate Intermediate and the Transformation of Ampicillin to Piperazinedione , Int. J. Pharm. 1981, 9, 273-283. 

Diketopiperazine formation was also examined in the peptides 6.50, 6.51, and 6.52 shown in Fig. 6.17 [76a], Clear evidence for the formation of a diketopiperazine product was obtained only for Arg-Trp-Phe (6.50, R=OH). In this case, the product was cyclo(Arg-Trp) (Reaction b). The diketopiperazine formed from Phe-Trp-Arg (6.51, R=OH) was not seen directly, but the presence of Trp-Phe together with Phe-Trp afforded indirect evidence. Interestingly, diketopiperazine formation occurred during acid-catalyzed degradation but not under basic conditions, and, as explained, was restricted to the two deamidated tripeptides. 

That derivatization may increase rather than decrease peptidase-catalyzed degradation is illustrated with aspartame (6.79, R = MeO), the C-terminal methyl ester of the dipeptide Asp-Phe. The metabolism of this artificial sweetener was compared to that of the underivatized dipeptide (6.79, R = H) and of the corresponding amide Asp-Phe-NH2 (6.79, R = NH2) in microvillar membranes obtained from human duodenum, jejunum, and ileum [189]. The activities monitored were clearly those of peptidases as shown by the effects of inhibitors. Whereas the peptide bond in Asp-Phe and Asp-Phe-NH2 was hydrolyzed at a comparable rate, that in aspartame was hydrolyzed approximately twice as fast. This is an interesting and favorable situation, given that aspartame is expected to be degraded once it has elicited its effect in the buccal cavity. 

Pierotti, C., Deal, C., and Derr, E. Activity coefficient and molecular structure, Ind. Chem. Eng. Fundam., 51 95-101, 1959. Pignatello, J.J. Dark and photoassisted Fe -catalyzed degradation of chlorophenoxy herbicides by hydrogen peroxide. Environ. 

Surface-catalyzed degradation of pesticides has been examined in the context of research on contaminant-clay interactions. Such interactions were observed initially when clay minerals were used as carriers and diluents in the crop protection industry (Fowker et al. 1960). Later specific studies on the persistence of potential organic contaminants in the subsurface defined the mechanism of clay-induced transformation of organophosphate insecticides (Saltzman et al. 1974 Mingelgrin and Saltzman 1977) and s-triazine herbicides (Brown and White 1969). In both cases, contaminant degradation was attributed to the surface acidity of clay minerals, controlled by the hydration status of the system. 

Zhou M, Takayanagi M, Yoshida Y, Ishii S, Noguchi H (1999) Enzyme-catalyzed degradation of aliphatic polycarbonates prepared from epoxides and carbon dioxide. Polym Bull 42(4) 419 24... 

Utilize a metal chelating additive to help minimize the effect of copper-catalyzed degradation. 

From the base-catalyzed degradation of D-fructose (pH 8.0), Shaw and coworkers147 identified 18 compounds, none of which was (a) isomeric with the starting material, or (b) a simple dehydration product. Among the products, the hydroxy-2-butanones and 1-hydroxy-2-propanone (acetol) were shown to participate in forming the carbo-cyclic products identified, but the mechanism of their formation was not elucidated. Several furan derivatives were isolated, but no lactic acid was isolated. In a similar study but with weak acid,41 most of the products were formed by a combination of enolization and dehydration steps, with little fragmentation. 

Figure 8 General structure of polyorthoesters. The functional group R can be varied to impart different properties to the polymer. Presence of lactic acid oligomer segments at R catalyzes degradation of the polymer.

---