A variety of molecular modeling, molecular docking, and first-principles electronic structure calculations were performed to study how the alpha 4 beta 2 nicotinic acetylcholine receptor (nAChR) binds with different species of two typical agonists, (S)-(-)-nicotine and (R)-(-)-deschloroepibatidine, each of which is distinguished by different free bases and protonation states. On the basis of these results, predictions were made regarding the corresponding microscopic binding free energies. Hydrogen-bonding and cation-pi interactions between the receptor and the respective ligands were found to be the dominant factors differentiating the binding strengths of different microscopic binding species. The calculated results and analyses demonstrate that, for each agonist, all the species are interchangeable and can quickly achieve a thermodynamic equilibrium in solution and at the nAChR binding site. This allows quantitation of the equilibrium concentration distributions of the free ligand species and the corresponding microscopic ligand-receptor binding species, their pH dependence, and their contributions to the phenomenological binding affinity. The predicted equilibrium concentration distributions, pK(a) values, absolute phenomenological binding affinities, and their pH dependence are all in good agreement with available experimental data, suggesting that the computational strategy from the microscopic binding species and affinities to the phenomenological binding affinity is reliable for studying alpha 4 beta 2 nAChR-ligand binding. This should provide valuable information for future rational design of drugs targeting nAChRs. The general strategy of the "from-microscopic-tophenomenological" approach for studying interactions of alpha 4 beta 2 nAChRs with (S)-(-)-nicotine and (R)-()-deschloroepibatidine may also be useful in studying other types of ligand-protein interactions involving multiple molecular species of a ligand and in associated rational drug design.