Photophysics of Methylene Blue‐polynucleotide Complexes in the Picosecond Time Range: Influence of the Local Structure of the Polynucleotide

The photophysics of methylene blue (MB) complexed with 2'‐deoxycytidylyl‐2'‐deoxygua‐nosine, polydeoxyguanylic‐deoxycytidylic acid, polydeoxyguanylic‐polydeoxycytidylic acid and poly‐guanylic acid, respectively, was investigated by transient absorption spectroscopy with a time resolution of ˜1 ps. The decays of the transient difference spectra indicate that the radiationless process responsible for the strong shortening of the S, state lifetime of MB associated with a guanine in the polynucleotide chain is similar to that already observed in the case of the MB—guanosine‐5'‐monophosphate complex, that is, the excited state decays by return of molecules to the ground state without formation of a detectable intermediate state. The local structure of the polynucleotide apparently does not change the nature of the deactivation process but influences significantly the deactivation rate. Thus, the intercalation of MB between guanosine and cytosine increases this rate constant (as compared to that of the MB—guanosine‐5'‐monophosphate complex) while the interaction of MB intercalated with a supplementary guanine has an opposite effect. The results are discussed in connection with the photosensitizing effect of MB in nucleic acid damage.