The M2 protein of influenza A virus performs the crucial function of transporting protons to the interior of virions enclosed in the endosome. Adamantane drugs, amantadine (AMN) and rimantidine (RMN), block the proton conduction in some strains, and have been used for the treatment and prophylaxis of influenza A infections. The structures of the transmembrane (TM) region of M2 that have been solved in micelles using NMR (residues 23-60) (Schnell and Chou, 2008) and by X-ray crystallography (residues 22-46) (Stouffer et al., 2008) suggest different drug binding sites: external and internal for RMN and AMN, respectively. We have used molecular dynamics (MD) simulations to investigate the nature of the binding site and binding mode of adamantane drugs on the membrane-bound tetrameric M2-TM peptide bundles using as initial conformations the low-pH AMN-bound crystal structure, a high-pH model derived from the drug-free crystal structure, and the high-pH NMR structure. The MD simulations indicate that under both low- and high-pH conditions, AMN is stable inside the tetrameric bundle, spanning the region between residues Val27 to Gly34. At low pH the polar group of AMN is oriented toward the His37 gate, while under high-pH conditions its orientation exhibits large fluctuations. The present MD simulations also suggest that AMN and RMN molecules do not show strong affinity to the external binding sites. (C) 2010 Elsevier B.V. All rights reserved.