An expression for the electronic coupling in electron transfer reactions between a donor and an acceptor separated by a bridge is obtained using an approach based upon the multiple scattering of an electron in a disordered medium. For homogeneous systems the coupling depends exponentially upon the distance separating the donor and the acceptor. In the weak scattering limit k(o)l* > 1 (where k(o) = 2 pi/lambda(e) with lambda(e) the wavelength of the tunneling electron and l* the transport mean free path), the exponent beta, which determines the range of the electronic coupling, is found to be a function of the energy of the tunneling electron and of the density of each kind of atom in the protein. The results for beta from the application of this approach to proteins are compared with those that are found in the literature.