The electron transfer mechanisms of a series of nucleic acid precursors (nucleobase, nucleotide and their modified structures) and ctDNA with protein enzymes and several fluorescence probes in both static and excited states were investigated using fluorescence quenching, transient absorption spectroscopic techniques and Rehm- Weller theory. The results indicate that their static and transient fluorescence quenching interactions are very strong. Both quenching rate constants k~q(s) and k~q(d) reach the magnitude of 10^10(mol/L)^- ^1.S^-^1, close to diffusion-controlled rate. This illustrates that the nucleic acid precursors in the ground states can behave as either electron donor or acceptor to undergo electron transfer interactions with electron-accepting fluorescence probes and tryptophan-containing serum albumins (HAS, BSA). The transient absorption measurement by laser flash photolysis for the modified tricyclic derivatives of guanine base was carried out. Several transient intermediates were detected and the photoinduced ET and EnT mechanism was discussed.

Key words: NUCLEIC ACIDS, PRECURSOR, FLUOROSPECTROPHOTOMETER, NUCLEIC ACID BASE, DEOXYRIBONUCLEIC ACID, NUCLEOSIDE, QUENCHING EFFECTS, PROTEASE, ELECTRON TRANSFER REACTION