  We study a simple quantum mechanical symmetric donor-acceptor model for
electron transfer (ET) with coupling to internal deformations. The model
contains several basic properties found in biological ET in enzymes and
photosynthetic centers; it produces tunnelling with hysteresis thus providing a
simple explanation for the slowness of the reversed rate and the near 100%
efficiency of ET in many biological systems. The model also provides a
conceptual framework for the development of molecular electronics memory
elements based on electrostatic architectures.
