The mechanism by which ORs detect odorants is also uncertain, but if isotopologues can be distinguished then vibrational modes (phonons) may play a role. Apart from their vibrational spectra, which depend sensitively on the mass of their nuclei, two isotopologues have nearly identical chemical properties. ORs, then, might detect olfactants by coupling to their vibrational modes, as proposed as long ago as 1938 . This has a certain intuitive appeal for explaining the dependence of odor on specific functional groups, which leave easily identifiable vibrational traces in infrared (IR) and Raman spectra . Turin proposed a mechanism, inelastic electron tunneling (IET), for vibrational detection of olfactants by receptors. In IET, an electron tunnels from a donor to an acceptor site in the active site of an OR. When there is no olfactant present, the donor and acceptor are separated in both distance and in active energy levels, and electron transfer is suppressed. The olfactant fills the space required for tunneling, and is also hypothesized to have a phonon whose energy approximates the donor-acceptor gap, suppressing the barrier to electron transfer . In biological systems, this mechanism is plausible only if the active site is well separated from the ambient water, implying a much smaller Marcus theoretic reorganization energy than is usual for an aqueous solution at 310 K . IET has been observed at physiological temperatures in artificial circuits , but it is not yet clear whether it occurs in living creatures .