  We present the results of experimental and theoretical studies of fast
anisotropic relaxation and rotational diffusion in the first electron excited
state of biological coenzyme NADH in water-ethanol solutions. The experiments
have been carried out by means of a novel polarization-modulation transient
method and fluorescence polarization spectroscopy. For interpretation of the
experimental results a model of the anisotropic relaxation in terms of scalar
and vector properties of transition dipole moments and based on the
Born-Oppenheimer approximation has been developed. The results obtained suggest
that the dynamics of anisotropic rovibronic relaxation in NADH under excitation
with 100~fs pump laser pulses can be characterised by a single vibration
relaxation time $\tau_v$ laying in the range 2--15~ps and a single rotation
diffusion time $\tau_r$ laying in the range 100--450~ps a subject of ethanol
concentration. The dependence of the times $\tau_v$ and $\tau_r$ on the
solution polarity (static permittivity) and viscosity has been determined and
analyzed. Limiting values of an important parameter $\langle
P_2(\cos\theta(t))\rangle$ describing the rotation of the transition dipole
moment in the course of vibrational relaxation has been determined from
experiment as function of the ethanol concentration and analyzed.
