PHOTOCHEMISTRY OF INTERCALATED METHYLENE-BLUE - PHOTOINDUCED HYDROGEN-ATOM ABSTRACTION FROM GUANINE AND ADENINE

Methylene blue intercalates more effectively into poly[dGdC] (K = (1.6 +/- 0.4) X 10(6) M-1) than poly[dAdT] (K = (1.6 +/- 0.5) X 10(4) M-1); in the latter case, intercalation competes with surface binding (K = (2.3 +/- 0.6) x 10(4) M-1). Intercalation is accompanied by a dramatic reduction in the lifetime of the first excited singlet state, causing decreases in quantum yields for both fluorescence and formation of the triplet state. In the case of methylene blue intercalated into poly[dGdC], transient absorption spectroscopic measurements have shown that the excited singlet state of the dye abstracts a hydrogen atom (or an electron) from an adjacent nucleic acid base, presumed to be guanine on thermodynamic grounds. The rate constants for forward and reverse transfers, respectively, are (2.5 +/- 0.3) X 10(11) and (3.3 +/- 0.4) X 10(10) s-1. The rate constant for the forward transfer in poly[dAdT] is (1.4 +/- 0.5) x 10(10) s-1 and is presumed to be slower than that of the reverse transfer. Using thermodynamic arguments, it is concluded that the quenching processes involve hydrogen atom transfer from guanine or adenine to the excited singlet state of methylene blue.