Electronically excited intermediate from peroxynitrite: evaluation by chemiluminescence and by the isomerization of β-carotene

Abstract

A set of hydrophobic triplet energy acceptors, chemiluminescence (CL) sensitizers and β-carotene, has been used to evaluate electronically excited intermediate formation during peroxynitrite (ONOO −) decay to nitrate in tetrahydrofuran. The yield of spontaneous CL ( Φ CL = 1.3 × 10 −9 photons/peroxynitrite) in tetrahydrofuran is close to the value obtained for aqueous solution. CL is strongly enhanced by sensitizers of triplet carbonyl chemiluminescence, coumarin 525 and chlorophyll- a, most likely by the energy transfer mechanism. Maximal yields of sensitized CL have been calculated to be 10 −6 and 2 × 10 −6 photons/peroxynitrite, respectively. The acceptor of high-energy triplets, 9,10-dibromoanthracene, is a weak sensitizer, so the energy of the electronically excited intermediate of peroxynitrite is within the range 56 ≤ Δ E ≤ 67 kcal mol −1. The yield of the excited intermediate of peroxynitrite in tetrahydrofuran is estimated from sensitized chemiluminescence to be about 0.2 mole per mole of peroxynitrite decomposed. This estimation is in accordance with the observed maximal yields of all- trans- and 13- cis-β-carotene trans/cis or cis/trans isomerization, which is believed to proceed via energy transfer from the excited intermediate of peroxynitrite to β-carotene, 0.06 and 0.14 (mole of isomerized β-carotene per mole of peroxynitrite), respectively. These results indicate that an electronically excited intermediate of peroxynitrite is produced with considerable yield and may be involved in peroxynitrite-mediated reactions, e.g., β-carotene isomerization.