QUENCHING OF CHEMIEXCITED TRIPLET ACETONE BY BIOLOGICALLY IMPORTANT COMPOUNDS IN AQUEOUS MEDIUM

Abstract— Thermolysis of tetramethyl-l,2-dioxetane is a convenient source of triplet acetone, which can be monitored in aerated solutions by the sensitized fluorescence of 9,10-dibromoanthracene. We have investigated the quenching of chemiexcited triplet acetone in air-equilibrated aqueous solutions containing the 9,10-dibromoanthracene-2-sulfonate ion by five classes of compounds: indoles, tyrosine derivatives, quinones, riboflavin, and xanthene dyes. Quenching rates for indoles, tyrosine and its 3,5-dihalogenoderivatives, and xanthene dyes (k_q= 10⁸-10⁹ M^-1 s^-1) are considerably smaller than the diffusion controlled rate, whereas those for quenchers with high electroaffinities, such as quinones (IP = 10–11 eV), approach the diffusion controlled rate (k_q= 10¹⁰ M^-1 s^-1). Energy transfer for riboflavin probably occurs by a triplet-singlet Förster type process.
A comparison of the present data with previous studies of quenching of enzymically generated triplet acetone (isobutanal/O₂/horseradish peroxidase) by the same classes of quenchers (except riboflavin) reveals that, independent of the nature of the quencher and the deactivation mechanism, the Stern-Volmer quenching constants ( k_q t⁰) are systematically about one order of magnitude higher in the enzymatic system. The difference is attributed to a longer lifetime of triplet acetone in the latter case, "protected" in an enzyme cavity against collisions with dissolved oxygen.