Table 3. Parameters of the vibrationally assisted mechanism of olfaction.

Listed are values of the five physical parameters Δε, ε, λ, σ, and γ expected for an odorant bound to an olfactory receptor to enable vibrationally assisted electron tunneling. The physical meaning and supporting references for each parameter are indicated. The range of values for the Huang-Rhys factor is estimated from our study of three odorants (acetophenone, citronellyl nitrile and octanol), and, in principle, can be wider for other odorant compounds.

param.	physical meaning	expected values	how estimated	role in mechanism
Δε	energy difference between donor and acceptor states in olfactory receptor	0 – 1 eV	matches odorant vibrational excitation energies; see Eq. (12)	selectivity filter for odorant vibrations
ε	odorant vibrational energy added to electron transfer	0 – 1 eV ε ≈ Δε	obtained from ab initio calculations; see Eq. (13)	discriminates vibrational fingerprint of an odorant
λ	general coupling of electron transfer to bath modes	0 – 0.1 eV λ ≪ ε, Δε	generalized from Rhodobacter capsulatus consistent with [4, 20]; see Eq. (11)	smears out detection of individual odorant vibrations
σ	coupling of odorant vibrations to electron transfer, Huang-Rhys factor	0 – 0.35	obtained from ab initio calculations; see Eq. (10)	enhances recognition of particular vibrations
γ	intrinsic strength of receptor electron transfer	~ 10−3 eV	estimated based on general assumptions; consistent with [4]	defines time scale of odor-free electron tunneling