Biomolecules organic radicals

Marcus RA (1999) Electron transfer past and future. In Jortner J, Bixon M (eds) Electron transfer -from isolated molecules to biomolecules, part 1. Wiley, New York, pp 1-6 Martin RF, Anderson RF (1998) Pulse radiolysis studies indicate that electron transfer is involved in radioprotection by Floechst 33342 and methylproamine. Int J Radiat Oncol Biol Phys42 827-831 Maruthamuthu P (1980) Absolute rate constants for the reactions of sulfate, phosphate and hydroxyl radicals with monomers. Macromol Chem Rapid Commun 1 23-25 Maruthamuthu P, Neta P (1977) Reactions of phosphate radicals with organic compounds. J Phys Chem 81 1622-1625... 

Although the term lipoprotein can describe any protein that is covalently linked to lipid groups (e.g., fatty acids or prenyl groups), it is most often used for a group of molecular complexes found in the blood plasma of mammals (especially humans). Plasma lipoproteins transport lipid molecules (triacylglycerols, phospholipids, and cholesterol) through the bloodstream from one organ to another. Lipoproteins also contain several types of lipid-soluble antioxidant molecules (e.g., a-tocopherol and several carotenoids). (The function of antioxidants, substances that protect biomolecules from free radicals, is described in Chapter 10.) The protein components of lipoproteins are called apolipoproteins or apoproteins. 

The hydrogen peroxide is normally destroyed by catalase and peroxidases, but some may be transformed to OH-, the hydroxyl radical, which is extremely reactive and modifies all biomolecules in its vicinity. Membrane lipids are destroyed. The mode of action of copper salts and paraquat is in many ways the same, although their target organisms are quite different. 

Nitroxides are persistent free radicals [1] which can often be isolated and handled as kinetically stable species. Nitroxides react rapidly with carbon free-radical intermediates [2] with well-characterized rate constants [3], and can thus be used as kinetic and mechanistic probes, as well as to trap carbon radicals in synthetic processes. They are easily oxidized or reduced, and thus have a rich redox chemistry that has been utilized for a variety of oxidations. As nitroxides have an unpaired electron, they are paramagnetic and thus ESR active, making them valuable as spin labels for biomolecules [4] and as spin traps for transient radicals [5]. In addition, nitroxides have been developed as organic ferromagnetic materials [6]. The synthesis of nitroxides has been reviewed in 1994 [7]. This review will focus on the synthetic applications of nitroxides. 

It is obvious that two main factors control the stability of organic molecules. First, the intrinsic stability of organic molecules stabilizes aromatic substances and lipids. Second, the cross linkage between biomolecules inhibits the interaction of enzymes. Only reactions forming small radicals are able to break bonds of cross linked structures and release breakdown products. 

Chemical analysis of the biomolecules introduced in the preceding sections is often radically different from the classical analysis of relatively small organic molecules such as pesticides, drugs or explosives residues. 

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