ISOLATION AND IDENTIFICATION OF TRYPTOPHAN PHOTOPRODUCTS FROM AQUEOUS SOLUTIONS OF TRYPTOPHAN EXPOSED TO NEAR‐UV LIGHT*

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Abstract
—One of the previously unidentified photoproducts isolated from the photolysate of aqueous tryptophan solutions was identified as 2‐carboxy‐3a‐hydroxy‐1,2,3,3a,8,8a‐hexahydropyrrolo(2,3b)‐indole by direct comparison with the authentic reference compound synthesized using the established procedure. This pyrroloindole alcohol has been shown to be the reduction product of the 3a‐hydroperoxy intermediate (structure 4 in Fig. 1) by Nakagawa et al. (1977). The isolation and identification of this derivative and the detection of the peroxy intermediate 3a‐hydroperoxypyrrolidinoindole (4). from irradiated aqueous tryptophan solutions suggests that the direct photooxidation of l‐tryptophan to fromylkynurenine may follow a pathway via a tricyclic intermediate instead of the energetically unfavorable dioxotane intermediate. This scheme is similar to the mechanistic model proposed by Nakagawa et al. (1977) for the rose bengal sensitized photooxidation of tryptophan.