Radical activation

Figure 6.11 Comparison of the number distribution of n-mers for polymers prepared from anionic and free-radical active centers, both with f = 50.

Table 7.2 lists a few cross-propagation constants calculated by Eq. (7.20). Far more extensive tabulations than this have been prepared by correlating copolymerization and homopolymerization data for additional systems. Examination of Table 7.2 shows that the general order of increasing radical activity is... 

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

Oiganometallic usage is shown in the piepaiation of titanium- oi vanadium-containing catalysts foi the polymerisation of styrene or butadiene by the reaction of dimethyl sulfate with the metal chloride (145). Free-radical activity is proposed for the quaternary product from dimethylaruline and dimethyl sulfate and for the product from l,l,4,4-tetramethyl-2-tetra2ene and dimethyl sulfate (146,147). 

Polymerisation does not continue until all of the monomer is used up because the free radicals involved are so reactive that they find a variety of ways of losing their radical activity. The two methods of termination in radical polymerisations are combination and disproportionation. The first of these occurs when two radical species react together to form a single bond and one reaction product as in Reaction 2.5. 

Globus MY, Busto R, Lin B, Schnippering H, Ginsberg MD. Detection of free radical activity during transient global ischemia and recirculation effects of intraischemic brain temperature modulation. J Neurochem 1995 65 1250-1256. 

With the above considerations in mind, the task for the analytical chemist is one of deciding which methods are likely to yield acceptable indices of free radical activity. This problem is further complicated by cost restraints and by the fact that the method to be employed is, in general, critically dependent on the nature of the radical investigated and the sample to be tested. 

Endogenous or exogenous aromatic compounds such as phenols and phenolic acids react extremely rapidly with OH radicals to form a mixture of hydroxylated products (Halliwell et /., 1988). Indeed, aromatic hydroxylation serves as an efiective method for evaluating OH radical activity both in vitro (Moorhouse et al., 1985 Grootveld and Halliwell, 1986a) and in vivo (Grootveld and Halliwell, 1986b). 

If an aromatic compound reacts with an OH radical to form a specific set of hydroxylated products that can be accurately identified and quantified in biological samples, and one or more of these products are not identical to naturally occurring hydroxylated species, i.e. not produced by normal metabolic processes, then the identification of these unnatural products can be used to monitor OH radical activity therein. This is likely to be the case if the aromatic detector molecule is present at the sites of OH radical generation at concentrations sufficient to compete with any other molecules that might scavenge OH radical. 

Patients in which oxidative damage may be an important aetiological factor cataract formation include those with Down s syndrome, since there is now evidence that they have increased indices of free-radical activity and lipid peroxidation. It has been su ested that this is due to the increased levels of Cu/Zn-SOD (carried on chromosome 21) generating increased concentrations of hydrogen peroxide (Bras etal., 1989). In the presence of superoxide radicals these produce highly reactive hydroxyl radicals. 

Although there is no data relating to free-radical activity in humans, several animal models have been developed (Parks etal., 1983). Pitt etal. (1991) showed that more severe injury occurred in the proximal small intestine, where levels of XO were highest. SOD has been reported to be protective in a rat model (Dalsing et al., 1983) and more recently a tungsten-supplemented diet was also found to be of benefit (Pitt et al., 1991). However, whether such treatment could modify the human disease, which usually develops within 10 days of birth, remains to be seen. 

Chemiluminescence has been used to demonstrate increased free-radical activity after induction of caerulein pancreatitis, with levels peaking at about 20 min and decreasing rapidly to control values thereafter (Gough et al., 1990). Electron spin resonance has been used to demonstrate increased hydroxyl radical activity in choline-deficient diet pancreatitis in the mouse (Nonaka etal., 1989a). 

McKeever, J., Qureshi, A., Gorey, T., Kay, E., Byrne, P. and Hennessey, T.P.J. (1992). Azoxymethane induced free radical activity in rat colon carcinoma. Gastroenterology 102, A378. 

The myopathy associated with chronic alcohol abuse has also been associated with increased free-radical activity (Martin and Peters, 1985) as have various other toxicity syndromes affecting muscle, such as cocaine toxity (Kloss et al., 1983). Little work appears to have been undertaken on the possible role of free radicals in the inflammatory myopathies, although, by analogy with other inflammatory disorders, this is likely to be an area worthy of further study. 

A more recent study, which measured three established markers of free-radical activity in addition to serum ascorbic acid in two groups of elderly diabetic patients (with and without retinopathy), found no significant differences in any of the markers between patients and age-matched controls despite significant depletion of ascorbic acid in patients with diabetes, especially those with retinopathy (Sinclair et al., 1992). These rather paradoxical findings suggest the existence of a complex interrelationship between the levels of individual antioxidant molecules in cells and tissues. 

It should be remembered that some of the established antioxidants have other metabolic roles apart from free-radical scavenging. The finding of reduced antioxidant defences in diabetes, for example, may not be prima fascie evidence of increased oxidative stress, since alternative explanations may operate. For example, this may reflect a response to reduced free-radical activity as su ested by the results of a previous study (Collier et al., 1988). In the case of ascorbate, an alternative explanation has been proposed by Davis etal. (1983), who demonstrated competitive inhibition of ascorbate uptake by glucose into human lymphocytes. This view is supported by the similar molecular structure of glucose and ascorbic acid (see Fig. 12.4) and by a report of an inverse relationship between glycaemic control and ascorbate concentrations in experimental diabetes in rats. Other investigators, however, have not demonstrated this relationship (Som etal., 1981 Sinclair etal., 1991). 

Further research is required to establish whether free-radical-induced damage is a primary event in diabetes. Tissue damage, which is associated with inactivation of antioxidants and release of metal ions that are potent catalysts of free radical reactions, can lead to lipid peroxidation. This raises the possibility that the diabetic process itself or other frctors may increase free-radical activity following direct tissue damage. 

Essential hypertension, whose prevalence is increased nearly two-fold in the diabetic population, may be another source of free-radical activity. The vascular lesions of hypertension can be produced by free-radical reactions (Selwign, 1983). In the recent Kuopio Ischaemic Heart Risk Factor Study in Finnish men, a marked elevation of blood pressure was associated with low levels of both plasma ascorbate and serum selenium (Salonen etal., 1988). A few studies report a hypotensive effect of supplementary ascorbate in patients with hypertension, but the actual changes in both systolic and diastolic pressure after ascorbate were not statistically significant in comparison with placebo (Trout, 1991). 

Stmte ies to Modulate Free-radical Activity in Diabetic Vascular Disease (Table 12.4)... 

Table 12.4 Strategies to modulate free radical activity in diabetic vascular disease...

It is important that future studies of antioxidant treatment in patients with specific disorders should be well designed (randomized, double-blind, placebo-controlled) prospective studies that utilize the most up-to-date methodology to assess free-radical activity. 

Future research should also focus its attention on the factors/mechanisms that regulate free-radical activity in vivo. The complex interrelationship between cellular and extracellular levels of antioxidants needs to be clarified, and factors that govern the synthetic rate of the scavenging enzymes, for example, SOD or catalase will provide further insight into cellular redox control. 

Collier, A., Jackson, M., Dawkes, R.M., Bell, D. and Clarke, B.F. (1988). Reduced free radical activity detected by decreased diene conjugates in insulin-dependent diabetic patients. Diabetic Med. 5, 747-749. 

Pitkanen, O.M., Martin, J.M., Hallman, M., Akerblom, H.K., Sariola. H. and Andersson, S.M. (1991). Free radical activity during development of insulin-dependent diabetes mellitus in the rat. Life Sci. 50, 335-339. 

Sinclair, A.J., Lunec, J., Girling, A.J. and Barnett, A.H. (1992). Modulators of free radical activity in diabetes mellitus role of ascorbic acid. In Free Radicals and Aging (eds. I. Emerit and B. Chance) pp. 342-352. BirkhauserVerlagBasel. 

Thompson, S. and Smith, M.T. (1985). Measurement of the diene conjugated from of linoleic acid in plasma by high performance liquid chromatography. A questionable non-invasive assay of free radical activity. Chem. Biol. Interactions 55, 357-366. 

Albano, E., Tomasi, A., Persson, J.O., Terelius, Y., Goria-Gatti, L., Ingelman-Sundberg, M. and Dianzani, M.U. (1991). Role of ethanol inducible cytochrome P-450 (P450IIE1) in catalysing the free radical activation of aliphatic alcohols. Biochem. Pharmacol. 41, 1895-1902. 

---