Radicals overall free-energy change

A catalyst is a substance that participates in a chemical reaction and thereby increases the reaction rate but without a net change in the amount of that substance in the system. The catalyst is used and regenerated during each set of microscopic events leading from reactants to products, and the overall free energy of the reaction is unaltered by the presence of the catalyst which must, therefore, be in small amount relative to reactants. In radical chemistry, a catalyst can be the initiator of a chain reaction or a carrier (see Chapter 10). 

Free radical polymerization is generally exothermic because it involves conversion of n bonds to ct bonds. Thus, the change in enthalpy AH is negative. Also, because there is a decrease in randomness in conversion of monomers to polymer, the change in entropy AS is also negative. The overall change in free energy of the free radical polymerization process is,... 

Overall results suggest that NO acts by inducing biophysical parameter changes in chloroplast membrane lipid. Moreover, since NO is a relatively weak free radical, it probably does not bind co-valently to membrane constituents, but may loosely associate with the n bond in C=C sections which are comparatively prolific in MGDG fatty acyl side chains and may thus reduce the energy of cis-trans isomerisation of the C=C bond [8]. If so, this implies that NO should hinder LOX activity, a contention experimentally borne out [Table 1]. 

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