  The electrical properties of a set of seven-helix transmembrane proteins,
whose space arrangement (3D structure) is known, are investigated by using
regular arrays of the amino acids. These structures, specifically cubes, have
topological features similar to those shown by the chosen proteins. The
theoretical results show a good agreement between the predicted current-voltage
characteristics obtained from a cubic array and those obtained from a detailed
3D structure. The agreement is confirmed by available experiments on
bacteriorhodopsin. Furthermore, all the analyzed proteins are found to share
the same critical behaviour of the voltage-dependent conductance and of its
variance. In particular, the cubic arrangement evidences a short plateau of the
excess conductance and its variance at high voltages. The results of the
present investigation show the possibility to predict the I-V characteristics
of multiple-protein sample even in the absence of a detailed knowledge of their
3D structure.
