It is shown that methylene blue (MB+) photosensitises DNA in either aerated or deaerated solutions, causing direct cleavage of phosphodiester bonds and rendering additional bonds labile to alkali. Evidence from unwinding and fluorimetric studies indicates that MB+ binds to DNA in at least 2 ways. Intercalation, which optimally induces helical unwinding of 24 degrees +/- 2 degrees per MB+, is markedly reduced upon neutralisation by Mg2+ of the DNA phosphates, while significant non-intercalative binding persists as shown by substantial fluorescence quenching at Mg2+ concentrations where there is little unwinding. MB+ induces photolysis at both low and high Mg2+ concentration - intercalation is apparently not required for photolysis. The quantum yield for strand breakage varies from 1-3 X 10(-7) under different conditions and is oxygen enhanced. The DNA cleavage is guanine specific. The 3' termini of the primary MB+-induced DNA photoproducts, unlike those generated by chemical sequencing retain an alkali labile adduct on the terminal phosphate.

Absorption spectra, fluorescence spectra and quantum yields, triplet state absorption spectra, yields and rates of decay have been obtained for methylene blue (MB+), MB+/CT‐DNA complexes and complexes of MB+ with the synthetic polynucleotides poly[d(G‐C)] and poly[d(A‐T)]. Strong fluorescence quenching is observed in the complexes of the dye with CT‐DNA and poly[d(G‐C)] concomitant with a decrease in the triplet yield. The fluorescence and triplet yield of MB+ with poly[d(A‐T)] are similar to those of the free dye. The triplet decay times are increased in all three polymer/dye complexes and show a decreased sensitivity to oxygen quenching. These results are interpreted in terms of the binding of the dye to these polymers and the implications of polymer binding on the photosensitizing properties of the dye are discussed.

Pyridin-2'-yl)-l,2,4-triazoles. X-Ray Crystal Structure of Bis(2,2'-bipyridine)-[3-methyl-5-( pyridin-2'-yl)-I ,2,4-triazolato-N1] ruthenium

Transient absorption signals observed after picosecond laser excitation of Methylene Blue (MB+) bound to double-stranded polydeoxyguanylic-deoxycytidylic acid { poly[d(G-C)].poly [d(G-C)]} are assigned to the dye's singlet excited state reacting with a lifetime of 4 ps to form the electron-transfer product (MB'.G'+) which itself decays with a lifetime of 130 ps.

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