M. Enescu et L. Lindqvist, EXCITED-STATE DEACTIVATION MECHANISM IN METHYLENE BLUE-NUCLEOTIDE COMPLEXES - A PICOSECOND TRANSIENT ABSORPTION STUDY, Journal of physical chemistry, 99(20), 1995, pp. 8405-8411
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.