Semiquinone formation influence

A systematic survey of the influence exerted by the substituents upon the stability of semiquinone forms was undertaken by Tozer and Dallas Tuck, in order to investigate the effect of the substituent upon the biological activity of phenothiazine derivatives, an activity correlated by many authors with in vivo formation of radical species. If the disproportionation reaction shown in Eq. (3) (Section IV, B, 1) were to reach equilibrium, the value K defined in Eq. (5) would express the stability of substituted S+ species. 

The oxidation potential of 3-hydroxyphenothiazine is lowered in the presence of j3-dextrin this was interpreted as evidence for the formation of inclusion compounds, the spatial alkalinity of the inner hole of j8-dextrin molecule influencing the prototropic equilibria, which influence the stability of the semiquinone. Oxidation of 3-hydroxyphenothiazine to phenothiazone is accompanied by loss of two protons and any base facilitates this process. 

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