The HIV-1 envelope glycoprotein 41 (gp41) undergoes large-scale conformational changes in order to induce the fusion of the virus and cell membranes Thus we investigated a possible structure transit at the air-water interface for the tryptophan-rich peptide of gp41 (gp41W) The synthetic peptide (KWASLWNWFNITNWLWYIK) corresponding to gp41W shows interfacial properties on pure water and Tris buffer at pH 8 5 Isotherm measurements and Brewster angle microscopy (BAM) imaging showed that the behavior of the peptide monolayer was dependent on the subphase composition A homogenous film was formed on buffer during the peptide monolayer compression while the appearance of condensed domains on pure water could indicate the oligomerization of gp41W during the surface pressure increase Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) showed that whatever the subphase gp41W adopts an alpha helix structure at the air-water interface and does not transit for any other structure even at high surface pressures (C) 2010 Elsevier Inc All rights reserved