Tocopherol and a-Tocopherylquinone

The chemistry and biochemistry of the tocopherylquinones and tocopherols has been recently reviewed in some detail. The first recognized function of vitamin E was as an antisterility factor for the laboratory rat. However, it is now known to have multiple functions in vivo. For example, it is required for the maintenance of structural and functional integrity of skeletal, cardiac, and smooth muscle and, in some animals, the peripheral vascular system. Tocopherols also function as intracellular antioxidants, particularly with respect to the stabilization of ingested fats and perhaps products arising in the metabolic synthesis and degradation of lipids. It has also been proposed that the tocopherylquinones may have some functional role in electron transport systems. 

The relationship between the electrochemistry of tocopherols and toco-pherylquinones and their biological redox reactions is not as obvious as with the other systems discussed in this chapter. However, the electrochemistry serves to show the complexity of their redox behavior and to point out some important points in the redox mechanisms of quinones. 

The first reports dealing with the electrochemical behavior of vitamin E-type compounds are those of Smith and co-workers who studied the polarographic oxidation waves of a-tocopherol and a number of tocopherol model compounds in methanol-water (1 1, v/v). These workers concluded that the electrooxidation of tocopherols consisted of an initial, reversible le-lYi reaction of the tocopherol [(I, Eq. (7)] to give an unstable quinonoid intermediate [(II, Eq. (7)] which was rapidly transformed into the corresponding tocopherylquinone [(III, Eq. (7)]. These first reports pointed out the possibility of an unstable species intermediate between tocopherol and tocopherylquinone in the oxidation reaction. Controversy still remains regarding the nature of this putative intermediate species. Among the proposed alternative structures for this intermediate are a-tocopheroxide (12a or and 

Marcus and Hawley have also studied the electrochemical reduction of a-tocopherylquinone and 2,3j5-trimethyl-6-(3 -methyl-3 -hydroxy-butyl)quinone (2,3 5-TMHQ, 15) in nonaqueous solvents. The electrochemical 

The electrochemical behavior of a-tocopherylquinone and related model compounds points out a number of interesting facts. For example, in aprotic media in the absence of protons or proton donors essentially all biological quinones, including a-tocopherylquinone, are electrochemically reduced in stepwise e processes giving first an anion radical and then a dianion. Both these processes are electrochemically reversible. However, addition of protons or a proton donor to the a-tocopherylquinone or other bioquinones in an 

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