Indirect characterizations of mOR-EG: Modeling analysis of five concentration-olfactory response curves via an advanced monolayer adsorption model.

The investigation of the adsorption process putatively involved in the olfactory perception of apocynin, guaiacylacetone, homovanillyl alcohol, 4-ethylguaiacol and homoguaiacol molecules on the mouse eugenol olfactory receptor mOR-EG was a very useful tool for comprising olfaction process at a molecular level. Indeed, the experimental data were correlated by using an advanced monolayer adsorption model with identical and independent binding sites. Thanks to the grand canonical ensemble in statistical physics formalism, the physico-chemical interpretations of modeling results indicated that the five odorants were adsorbed via a multi-molecular mechanism. Hence, the calculation of adsorption energies, that described the interaction between the odorant molecules and the olfactory receptor binding cavities, indicated that weak bonds were made between apocynin, guaiacylacetone, homovanillyl alcohol, 4-ethylguaiacol and homoguaiacol molecules and mOR-EG binding pockets amino acid residues. In addition, theoretical stereographic and energetic characterizations of mOR-EG were made via the determination of the olfactory receptor site size distributions (RSDs) and the adsorption energy distributions (AEDs) relative to apocynin, guaiacylacetone, homovanillyl alcohol, 4-ethylguaiacol, homoguaiacol molecules. The RSD provided the size of different binding cavities of mOR-EG. Indeed, the five RSDs spectrums situated between 0.5 and 10 nm were spread out around an average size each one. The mean values obtained from the peaks of the distributions were 2.14 nm, 2.20 nm, 2 nm, 2.10 nm and 1.83 nm for apocynin, guaiacylacetone, homovanillyl alcohol, 4-ethylguaiacol and homoguaiacol molecules, respectively. The AED gave a whole spectrum of adsorption energies that was activated by the odorant molecule. Thus, the apocynin, guaiacylacetone, homovanillyl alcohol, 4-ethylguaiacol and homoguaiacol AEDs were spread out from 5 to 27.5 kJ/mol, from 5 to 30 kJ/mol, from 5 to 35 kJ/mol, from 0 to 22.5 kJ/mol, 5 to 25 kJ/mol, respectively. The thermodynamic study, via the establishment of the adsorption entropy, indicated that the peak of the disorder was obtained when half of the binding sites were occupied. In addition, the Gibbs free enthalpy and the internal energy were determined and their negative values indicated that the adsorption phenomenon involved in the olfactory perception was spontaneous and exothermic physisorption phenomenon.