The effect of protein dynamics on the long-distance biological electron transfer reactions is discussed, Computer simulations reported recently by our group [Daizadeh, Medvedev, and Stuchebrukhov, Proc. Natl. Acad. Sci. USA 94, 3703 (1997)] have shown that in some cases a strong dynamic coupling of a tunneling electron to vibrational motions of the protein matrix can exist. This results in a modification of the conventional picture of electron transfer in proteins, The new element in the modified theory is that the tunneling electron is capable of emitting or absorbing vibrational energy (phonons) from the medium. As a result, some biological reactions may occur in an activationless fashion. In the present paper we study analytically the probabilities of such inelastic tunneling events and show how they affect the overall dependence of the reaction rate on the driving force, temperature, and the strength of electron-phonon coupling. Harmonic and anharmonic models are proposed for vibrational dynamics of the intervening medium. (C) 1997 American Institute of Physics.