We have observed for the first time coherent oscillations in the fs fluorescence (Fl) dynamics of wild-type (w-t) photoactive yellow protein (PYP) and its site-directed mutants. Two oscillatory modes coupled with the ultrafast Fl decay due to the twisting of the excited chromophore were identified, a high frequency mode (similar to 140 cm(-1)) with 700 fs damping time and a low frequency mode (similar to 50 cm(-1)) with 500 fs damping time, respectively. The latter becomes overdamped when the structure of the protein nanospace (PNS) surrounding the chromophore becomes more disordered by mutation. Both modes disappear in the Fl dynamics of denatured PYP. Similarities between PYP and rhodopsins (Rh) are discussed. (C) 2002 Elsevier Science B.V, All rights reserved.