NUCLEOBASE-SPECIFIC QUENCHING OF FLUORESCENT DYES .1. NUCLEOBASE ONE-ELECTRON REDOX POTENTIALS AND THEIR CORRELATION WITH STATIC AND DYNAMIC QUENCHING EFFICIENCIES

Intermolecular static and dynamic fluorescence quenching constants of eight coumarin derivatives by nucleobase derivatives have been determi ned in aqueous media. One common sequence of the quenching efficiency has been found for the nucleobases. The feasibility of a photoinduced electron transfer reaction for the nucleobase-specific quenching of fl uorescent dyes is investigated by the calculation of the standard free energy changes with the Rehm-Weller equation. A complete set of one-e lectron redox potential data for the nucleobases are determined electr ochemically in aprotic solvents for the first time, which are compared with values obtained by various other methods. Depending on the redox properties of the fluorescent dyes, the sequences of the quenching ef ficiencies can be rationalized by the orders of electrochemical oxidat ion potentials (vs NHE) of nucleosides (dG (+1.47 V) < dA ( dC approxi mate to dT < U (greater than or equal to +2.39 V)) and reduction poten tials (de (( -2.76 V) < dA < dC approximate to dT < U (-2.07 V)). The correlation between the intermolecular dynamic quenching constants and the standard free energy of photoinduced electron transfer according to the classical Marcus equation indicates that photoinduced electron transfer is the rate-limiting step. However, an additional, water-spec ific gain of free energy between -0.5 and -0.9 eV shows that additiona l effects, like a coupled proton transfer and a hydrophobic effect, ha ve to be considered, too. Furthermore, the capability of the nucleobas es to form ground state complexes with fluorescent dyes is influenced by their redox potentials. The relevance of these observations to curr ent efforts for DNA sequencing with a detection by laser-induced fluor escence and their application to other dyes are discussed.