METHODS OF INVESTIGATION OF THE STRUCTURE AND ACTION MECHANISMS OF ACTIVE SITES OFENZYMES - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 12

frequency and amplitude of the probe motion in an organized medium (Likhtenshtein et
al., 1996).
The traditional fluorescence and electron-spin resonance methods for recording
molecular collisions do not allow the study of translational diffusion and rare encounters
of molecules in a viscous media because of the short characteristic times of these
methods. To measure the rate constants of rare encounters between macromolecules and
to investigate the translation diffusion of labelled proteins and probes in a medium of
high viscosity (like biomembranes), a new triplet-photochrome labeling technique has
been developed (Mekler and Likhtenshtein, 1986; Mekler and Umarova, 1988;
Likhtenshtein, 1993; Papper and Likhtenshtein, 2001).
The stilbene photoisomerisation through the triplet potential surface can be sensitized
by a donor molecule excited to their triplet state, which is close energetically to the
stilbene excited triplet level (Hammond et al., 1962). The sensitizers (donors) with
triplet energies of at least 255 kJ/mole (in a case of unsubstituted stilbene) transfer their
energies to both trans and cis isomers of the stilbene molecule in the ground state in a
diffusion-controlled process. The reaction proceeds from an initial donor-acceptor
encounter complex, which generates the stilbene excited triplet states without change of
spin. From the excited triplet states of stilbene molecule, a relaxation process takes place
on the triplet potential energy surface, leading to the deactivation transition occurrence.
Finally, the triplet-triplet energy transfer drives the stilbene photoisomerisation through
the triplet pathway (Fig. 1.5).
The triplet-photochrome method is based on the above mentioned cascade scheme.
Starting from cis -stilbene, which is not fluorescent at the steady-state conditions of our