Insolubilization mechanisms

The second limitation stems from the insolubilization mechanism operant in these resists. Photoinitiated cross-linking converts the polymer film... 

A methacrylate terpolymer containing 35 mol% methacrylic acid units was mixed with diepoxides (Fig. 131) and triphenylsulfonium triflate and imaged in a negative mode by exposure to ArF excimer laser followed by development with 0.06% (weak) TMAH solution [383]. The aromatic epoxide was selected for imaging because the more transparent aliphatic epoxide was liquid and thus lowered the Tg of the resist film to an unacceptable level. The insolubilization mechanism was speculated to be crosslinking through acid-catalyzed esterification between the carboxylic acid and epoxide. 

Ueno, H. Shiriashi, and S. Nonogaki, Insolubilization mechanism and lithographic character istics of a negative electron beam resist iodinated polystyrene, J. Appl. Polym. Sci. 29, 223 (1984). "ibid. 

Y. Kojima, F. Oshima, Y. Matsuyama, M. Moriya, Clarification of Insolubilization Mechanism of... 

Hargreaves has suggested that the insolubilization of some closely related polymers is due to photolytic homolysis of the endoperoxide 0-0 bond and subsequent generation of carbon-centered radicals from the O radicals (19). There are several facts that make this an extremely unlikely explanation for the data described here these include the quantitative insufficiency of the maximum amount of endoperoxide reaction obtainable with a few hundred mJ/cm2 dose (homolysis quantum yield <0.5 (46), and extinction coefficient 1 (M cm)-1 (47)), and the synthetic utility of such homolysis reactions in related molecules in the presence of good hydrogen atom donors (implying facile epoxide formation) (48). Clearly the crosslinking observed under N2 is not accounted for by this mechanism. 

As pointed out by Heller (2), polymer erosion can be controlled by the following three types of mechanisms (1) water-soluble polymers insolubilized by hydrolytically unstable cross-links (2) water-insoluble polymers solubilized by hydrolysis, ionization, or protonation of pendant groups (3) hydrophobic polymers solubilized by backbone cleavage to small water soluble molecules. These mechanisms represent extreme cases the actual erosion may occur by a combination of mechanisms. In addition to poly (lactic acid), poly (glycolic acid), and lactic/glycolic acid copolymers, other commonly used bioerodible/biodegradable polymers include polyorthoesters, polycaprolactone, polyaminoacids, polyanhydrides, and half esters of methyl vinyl ether-maleic anhydride copolymers (3). 

Figure I. Effect of heating of soy milk before drying and effect of addition of N-ethylmaleimide (NEMI) to heated soy milk on the insolubilization of protein after drying. The curves are (a), dried without adding NEMI (b), dried after adding SEMI and (C), the values of (a) minus the values of (b). Curve (a) indicates total amount of insolubilized protein curve (b) indicates the amount of protein insolubilized by mechanisms other than by intermolecular disulfide bond formation and curve (c) indicates the amount of protein insolubilized through disulfide bond polymerization (3).

Thus, the complicated phenomena observed for insolubilization of the protein of heated soy milk may be explained by these mechanisms. 

There is another phenomenon, regarded as a deteriorative change in the protein of soy milk, caused also by the evaporation of water. This is a film formation on the surface of soy milk, which occurs when heated soy milk is kept open to the air. This phenomenon is observed not only in heated soy milk but also in heated cow s milk. Film formation of soy milk occurs only when the soy milk is heated above 60°C and there is evaporation of water from the surface of the soy milk. The mechanism of protein insolubilization is basically the same as that of soy milk powder produced from heated soy milk (10. When water is removed from the surface of heated soy milk by evaporation, the molecular concentration of protein near the surface increases locally and the exposed reactive groups of the denatured molecules come close enough to interact intermolecularly both by hydrophobic interactions and through the sulfhydryl/disulfide interchange reaction to form a polymerization (film) on the surface. The upper side of the film contains more hydrophobic amino acids because of orientation of the hydrophobic portions of the unfolded molecules to the atmosphere rather than into the aqueous solution. 

Figure 12. Schematic diagram for the mechanisms of reversible and irreversible insolubilization of soybean protein (7)...

The original interpretation of these results was in terms of crosslinking caused by radicals produced by peroxide photolysis (41). It was subsequently shown that a more likely mechanism is polymerization of residual monomer (39). Irradiation failed to insolubilize the polymer, which should have happened even if only a small amount of crosslinking had occurred. The residual monomer theory is also consistent with the irreproducibility of the process, which is a severe inconvenience. 

The model in Figure 5 includes formation of both soluble and insoluble complexes of sHsp and substrate. The formation of insoluble sHsp/substrate complexes is consistent with the in vivo transition of sHsps to an insoluble, structure-bound form under many stress conditions as discussed above. At present we can provide only speculative explanations for this insolubility in the context of the chaperone model of sHsp function. From in vitro studies, it is clear that the ability of sHsps to keep substrates soluble is dependent on the sHsp-to-substrate ratio, the rate of substrate denaturation, and other factors in vitro conditions can be manipulated to cause precipitation of sHsp and substrate, as well as to maintain substrate solubility. Thus, insolubilization could result from a type of overload of the soluble binding capacity of the sHsps. Since in vivo there is good evidence that the insolubilization is reversible, this leads to the intriguing question of the mechanism of resolubilization, and whether this is also a function of Hsp70 systems, or if additional components are required. Alternatively, sHsp insolubilization in vivo could result from interaction with insoluble components in the cell. 

Irradiation of purified natural rubber films results in gas evolution with a quantum yield [53] of about 10 3 and insolubilization of the polymer. However, the major photochemical processes in vacuo are cis—trans isomerization, loss of unsaturation and formation of cyclopropyl groups [49] with quantum yields estimated at 0.041, 0.083 and 0.018, respectively [47]. An over all mechanism identical to that already... 

Affinity chromatography is similar in principle to the use of immuno-adsorbents for antibody and antigen purification (see Section IX, p. 375) and of insolubilized nucleic acids for purification of nucleic acids and related enzymes (see Section X, p. 384). The subject of affinity chromatography in general has been reviewed, ° and a mechanism... 

Based on the crystallographic data, detailed mechanisms for the carboxypeptidase A enzymic reaction have been proposed. These mechanisms and recent work relating to them have been reviewed.Although probably correct in general, these mechanistic conclusions are based on the assumption that the kinetic and chemical properties are conserved on crystallization. In general coordination chemistry examples abound where the structures of species in the crystd and in solution are markedly different and indeed it has been shown that the detailed kinetics of carboxypeptidase A solutions differ from those of the enzyme crystals. It has been suggested that different conformations of the active site exist in the two physical states,Detailed kinetic studies on crystals over a range of enzyme concentrations, substrate concentrations and crystal sizes have been carried out and the results interpreted in terms of a recent theory for insolubilized enzymes. The marked differences... 

Cold stabilization is also partially effective in preventing other types of colloidal precipitation. It helps to prevent ferric casse by insolubilizing ferric phosphate in white wines and ferric tannate in reds. However, even after aeration to promote the formation of the Fe + ions involved in these mechanisms, only small quantities of iron are eliminated. Fining at the same time as cold stabilization improves treatment effectiveness but is never sufficient to prevent ferric casse completely. 

---