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'-deoxycytidyl yl-2'-deoxyguanosine, polydeoxyguanylic-deoxycytidylic acid, polydeoxy guanylic-polydeoxycytidylic acid and polyguanylic acid, respectively, was investigated by transient absorption spectroscopy with a time reso lution of similar to 1 ps. The decays of the transient difference spec tra indicate that the radiationless process responsible for the strong shortening of the S-1 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 excit ed state decays by return of molecules to the ground state without for mation of a detectable intermediate state. The local structure of the polynucleotide apparently does not change the nature of the deactivati on process but influences significantly the deactivation rate. Thus, t he intercalation of MB between guanosine and cytosine increases this r ate constant (as compared to that of the MB-guanosine-5'-monophosphate complex) while the interaction of MB intercalated with a supplementar y guanine has an opposite effect. The results are discussed in connect ion with the photosensitizing effect of MB in nucleic acid damage.