  Combined studies on fluorescence quantum yield in coenzymes NADH and FAD in
water-methanol, water-ethanol and water-propylene glycol mixtures and on
time-resolved fluorescence of the same molecules under excitation at 450 and
355~nm by means of the TCSPC method have been carried out. The dependence of
quantum yield in NADH on alcohol concentration was found to be similar in
methanol and ethanol and different from that in propylene glycol. The behavior
of quantum yield in FAD was found to be almost independent of the type of
alcohol and exhibited a dramatic 5--6 times increase with alcohol
concentration. A model describing molecular excited state relaxation dynamics
in pico- and nanosecond time domain was developed on the basis on the quantum
mechanical theory and used for analysis of the experimental results. In
particular, the model provided a new insight into the nature of the Decay
Associated Spectra (DAS) phenomenon. The analysis of the role of pico- and
nanosecond quenching in NADH suggested that the picosecond decay in methanol
and ethanol likely does not occur through electron transfer in the stacking
configuration of the nicotinamide and adenine moieties, but through other
mechanisms. However in propylene glycol the picosecond quenching in NADH
depended strongly on propylene glycol concentration and sharp increase of the
measured quantum yield was associated with the contribution of both picosecond
and nanosecond quenching mechanisms. The picosecond quenching to FAD was found
to depend significantly on the any alcohol concentration suggesting that it
gives a profound contribution to the rise of the measured quantum yield with
alcohol concentration. This conclusion supports the established mechanism of
the fluorescence quenching in FAD through electron transfer reaction in the
$\pi$-stacked conformation between isoalloxazine and adenine moieties.
