OH Radical Induced One-Electron Oxidation of Serotonin and Tryptamine

Abstract

The interactions of the one-electron oxidants, N 3 • and Br 2 •radicals, with serotonin, tryptamine and their analogues 5-methoxytryptamine, N-methyltryptamine and N(1)-methyltryptophan were studied using the technique of pulse radiolysis with spectrophotometric detection. One-electron oxidation of serotonin results in the formation of an indoloxyl radical with a pK a value ,3. With tryptamine, N-methyltryptamine and 5-methoxytryptamine, their one-electron oxidation gives indolyl radicals with pK a values of 4.2, 4.3 and 4.8, respectively. The reactions of OH radicals ( • OH) with serotonin and tryptamine lead to the formation of the respective • OH-adducts, which decay by acid catalyzed water elimination to give almost quantitatively the corresponding indoloxyl and indolyl radicals, respectively. The first-order rate constants determined for water elimination are pH dependent, suggesting that the dehydration reaction is acid and base catalyzed. The • OHadduct of serotonin reacts with oxygen in competition with the dehydration reaction to yield a peroxyl radical adduct, which is tentatively suggested to eliminate HO 2 • . On the basis of the above findings, the mechanisms for the • OH -induced formation of the indoloxyl and indolyl radicals from serotonin and tryptamine respectively are proposed.

Reactions of 2,4,6-tripyridyl-1,3,5-s-triazine and its Fe(ii) complex with OH· and eaq-

“…Moreover, the spectra do not exhibit any red shift, which indicates that the nitrogen atoms are still complexed to the metal center. All these results are consistent with our previous results [9] obtained for Fe(terpy) 2 2+ , and also with those reported in literature for complexes of bipy [14][15][16] and phen [17]. In case of Fe(tptz) 2 2+ , a decrease in apparent molar extinction coefficient was observed at the characteristic peak, 596 nm.…”

“…In our previous communication with terpy and its Fe(II) and Co(II) [9] complexes with OH • and e aq − , we found the formation of OH and H-adducts, respectively, both with the free ligand as well as with the complexes. All these adducts decayed by second order processes, which suggest dimerization or disproportionation depending on the nature of the ligand.…”

“…The free ligand tptz, similar to the complex Fe(tptz) 2 2+ , reacts with OH • or e aq − to form an adduct similar to terpy [9], bipy [6][7][8] and phen. Hence, the reaction may lead to an adduct, tptz(OH) • or Htptz(OH) •+ or H 2 tptz(OH) •2+ , depending on the pK a of tptz (2.19 and 3.40) [2].…”

On the generation and quenching of reactive-oxygen-species by aqueous vitamin B2 and serotonin under visible-light irradiation

“…The derivative towards thermally generated radical species, including O ÅÀ 2 and OH Å , where the main oxidation products of Sero are oxygenated tryptamine derivatives [36][37][38][39].…”

Identification and reactivity of the triplet excited state of 5-hydroxytryptophan

“…Figure 1A shows the transient absorption spectra at 200 ns and 8 μs after the laser pulse in oxygenated solutions of 5HT (0.2 mmol dm -3 ) at pH 7.0. The main feature of these spectra is the absorption maximum at 400-420 nm which has previously been identified at the neutral 5-indoloxyl radical (16,(35)(36)(37) formed by electron and proton loss from the 5-hydroxyindole (HO-Ind) part of 5HT (reaction [1]). The 5-indoloxyl radical appears not to react with oxygen and is comparatively stable with a lifetime of hundreds of microseconds under the conditions of our experiment and has previously been shown to decay through a second order process [36].…”