Exploring the picosecond time domain of the solvation dynamics of Coumarine-153 in beta-cyclodextrins

RODRIGUEZ, J; MARTÍ J, GUARDIA, E.; LARIA D.

JOURNAL OF PHYSICAL CHEMISTRY B

AMERICAL CHEMICAL SOCIETY

Año: 2008

1089-5647

We report molecular dynamics simulation results of equilibrium anddynamical characteristics  pertaining to the solvation ofthe dye coumarine 153 (C153) trapped within hydrophobic cavities of di- andtri-methylated beta-cyclodextrins (CD) in aqueous solutions.We found that stable configurations of the encapsulated probe arecharacterized by a slanted docking, in which the plane of theC153 lies mostly parallel to one of the glucose units of the CD.``In and out" dynamical modes of the encapsulated probe present very smallamplitudes. The rotational dynamics of the trapped coumarine can be casted in terms ofa simple model that includes diffusive motions within alocal restrictive environmentcoupled to the overall rotational motion of the CD.%which, in turn, would  dictate the long time behavior of the fluorescence%anisotropy decay of the dye.We have examined the early stages of the solvation response ofthe environment following a vertical excitation of the probe.Regardless of the degree of the CD methylation,the water dynamical response seems to be completedwithin 2-3 ps, and does not differ substantially from the one observedfor non-encapsulated probes. The CD response is characterized by asingle, sub-picosecond relaxation that involves intramolecular motions.We also explored dynamical modesthat could account for the recently reported  persistence of Stokes shiftsin the nanosecond time domain.  In all cases, the only sources ofultraslow dynamics that we detected were those associated toisomerization processes of the CD, which do not seem to be directly connectedto the electronic excitation of the probe.