Photochemical electron transfer mechanism in the quenching of FMN by N,N,N′,N′-tetramethyl-p-phenylene diamine

Abstract

The mechanisms of fluorescence quenching of Flavin Mononucleotide (FMN) by N,N,N′,N′-tetramethyl- p-phenylenediamine (TMPD) and oxygen have been studied in non-aqueous and aqueous media. The rate constant k Q , for the quenching reaction has been measured by monitoring the fluorescence intensity of FMN ( λ max = 530 nm) as a function of the quencher concentration; the lifetime of the excited singlet state of FMN is estimated using a pulsed nitrogen laser. On the basis of the data obtained in this study the following electron transfer mechanism is proposed for the quenching reaction in non-aqueous media ▪ Unlike the case of fluorescence quenching of aromatic hydrocarbons by amines where triplet states of hydrocarbons are produced through the radical reaction, the electron transfer reaction of FMN − and TMPD + does not produce the triplet states of either of the species ( E T(FMN) = 2.05 eV and E T(TMPD) = 2.96 eV). This difference is explained by the reaction free energy being less than the energy of the triplet states. Further, experiments carried out in electrochemiluminescence by generating FMN − and TMPD + does not produce emission by radical ion annihilation reaction. Marcus theory of electron transfer reactions has been used to understand more about the quenching reaction. In aqueous media, exciplex formation is postulated in the fluorescence quenching of FMN by oxygen.