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We investigate, by means of ab initio calculations, electronic transport in molecular junctions composed of a biphenyl molecule attached to metallic carbon nanotubes. We find that the conductance is proportional to cos 2 , with the angle between phenyl rings, when the Fermi level of the contacts lies within the frontier molecular orbitals energy gap. This result, which agrees with experiments in biphenyl junctions with nonorganic contacts, suggests that the cos 2 law has a more general applicability, irrespective of the nature of the electrodes. We calculate the geometrical degree of chirality of the junction, which only depends on the atomic positions, and demonstrate that it is not only proportional to cos 2 but also is strongly correlated with the current through the system. These results indicate that molecular conformation plays the preponderant role in determining transport properties of biphenyl-carbon nanotubes molecular junctions.

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