EXCITED-STATE DEACTIVATION MECHANISM IN METHYLENE BLUE-NUCLEOTIDE COMPLEXES - A PICOSECOND TRANSIENT ABSORPTION STUDY

Methylene blue (MB(+)) complexed with guanosine 5'-monophosphate was s ubmitted to subpicosecond (0.8 ps fwhm) laser excitation of wavelength 628 nm, and the decay of the transient difference spectra obtained wa s analyzed in the 660-840 nm spectral range. The presence of a moderat e special shift of the stimulated emission band during the decay was u sed as an argument for proposing that the strong lifetime shortening o f the MB(+) excited singlet (S-1) state in the complex is due to the c reation of a double-well structure of the energy surface of the S-1 st ate by coupling of the locally excited (LE) and charge-transfer (CT) s tates. Support for this hypothesis was obtained from the observation o f a correlation between the S-1 lifetime and the nucleotide oxidation potential for the MB(+) complexes with the 5'-monophosphates of guanos ine, xanthosine, inosine, and adenosine, respectively The absence in a ll these cases of any detectable population of an intermediate state d uring the decay of the S-1 state shows that the CT state is extremely short-lived. Study of the decay of the transient spectra of the MB(+)- tryptophan complex confirmed the correlation between the Sr-state life time and the LE-CT energy gap, In D2O, the S-1 state of the MB(+)-guan osine complex was found to decay more slowly than in H2O. This result was interpreted as a dynamical solvent effect characteristic of LE --> CT transitions.