Spectroscopic properties and reactivity of free radical forms of A2E

A pyridinium bisretinoid (A2E) is the only identified blue-absorbing chromophore of retinal lipofuscin that has been linked to its aerobic photoreactivity and phototoxicity. Pulse radiolysis has been used to study both the one-electron oxidation and the one-electron reduction of A2E in aqueous micellar solutions. The reduction to the semireduced A2E (λmax broad and between 500 and 540 nm) was achieved with formate radicals and the subsequent decay of A2E was slow (over hundreds of milliseconds) via complex kinetics. The long lifetime of the A2E should facilitate its reactions with other biomolecules. For example, with oxygen, the A2E produced the superoxide radical anion with a rate constant of 3 × 108 M−1 s−1. The A2E was also reduced by the NAD radical, the corresponding rate constant being 2.3 × 108 M−1 s−1. Other experiments showed that the one-electron reduction potential of A2E lies in the range −640 to −940 mV. The semioxidized form of A2E (λmax 590 nm) was formed via oxidation with the Br2− radical and had a much shorter lifetime than the semireduced form. With strongly oxidizing peroxyl radicals (CCl3O2) our kinetic data suggest the formation of a radical adduct followed by dissociation to the semioxidized A2E. With milder oxidizing peroxyl radicals such as that from methanol, our results were inconclusive. In benzene we observed an efficient oxidation of zeaxanthin to its radical cation by the A2E radical cation; this may be relevant to a detrimental effect of A2E in vision.