Type I Photosensitization of Chain Reactions

This sequence may be thought of as a chain reaction because the radical activity is continually transferred and kept alive. Except in very unusual structures, free radicals are considered high reactivity species, but a suitable donor or acceptor in the near vicinity is needed. Secondary alcohols are examples of molecules with readily abstractable hydrogens. Thus, Lo-propanol, mannitol, and ascorbic acid are very good scavengers of free radicals and can be used to protect the therapeutic substance while they undergo oxidation. 

The limit to the chain reaction is determined by the relative values of the rate constants for the propagation step and the branching or transfer reactions involving solvent or inhibitor molecules. As the concentration of the oxidizable molecule falls in the solution, the reaction rate also falls. The reaction is characterized by a steady-state or maximum rate represented by the linear portion of the sigmoidal reaction progress curve. This is achieved when the rate of generation of new initiating radicals is equal to their termination rate. Here, the kinetics is simplified by the steady-state approximation, and the maximum rate is first order with respect to the benzaldehyde concentration. 

Inhibition of chain processes is achieved by the addition of free radical scavengers, which react by chain transfer more rapidly than the propagation step. The product of chain transfer is also a free radical, but the key to the transfer agent being a good inhibitor is that it must be a very unreactive radical, e.g., sterically hindered radicals formed from the widely used antioxidants BHT (2,6-di-t-butyl-hydroxy-toluene) and BHA (2,6-di-t-butyl-hydroxy-anisole). 

Chain reactions are the major pathway by which hydrocarbon polymers as used in packaging are broken down, with the radicals for initiation arising from photoinduced 

In biological systems, free radicals can react with cellular macromolecules in a variety of ways, the most important of which is hydrogen abstraction from DNA leading to chain scission or cross-linking. In proteins, tryptophan is the amino acid residue most susceptible to free radical attack. Lipid peroxidation by free radicals in turn is liable to cause alteration in cell membranes. 

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