The influence of a nonlinear spring and nonlinearity in exchange integrals in molecular chains are theoretically investigated in the presence of intramolecular vibrations of two different types. Considering a cubic term in the potential, we show that the anharmonic effect leads to a set of three coupled nonlinear equations (one Boussinesq equation and two nonlinear Schr\"odinger equations) for which we propose an exact soliton solution. The nonlinearity in the exchange integrals results in two coupled nonlinear Schr\"odinger equations with saturable nonlinearity. \textcopyright{} 1996 The American Physical Society.