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Nisin is an antibiotic polypeptide naturally sinthesized by acid lactic bacteria.1 Citoplasmic membrane of Gram positive bacteria is the primary target of its bactericidal action.2 This polypeptide has a positive net charge (+5) and an amphiphilic behavior that permit high affinity interaction with anionic lipid membranes.The aim of this work is to perform a vibrational spectroscopic study (Raman and IR) of pure nisin and of phosphatidylglycerol (PG)/nisin mixtures in different lipid phases. Structural alterations on anionic model membranes induced by the peptide addition were evaluated. In turn, conformational changes in the peptide backbone upon its incorporation to the lipid media were also studied.This is the first complete vibrational characterization of nisin. Spectral analysis was performed by quantum-chemical calculations of a peptide fragment (Ile1-Ala7) and by taking into account published assignments for amino acids and related peptide systems3,4 Vibrational bands considered as spectral markers of the lipid interchain coupling as well as the hydration degree of membrane polar headgroups evidence the peptide effects in the different bilayer regions . Changes in the secondary structure of nisin embedded in lipid membranes were evaluated by the Amide I band in both the IR and Raman spectra.Results permit to conclude that nisin penetrates anionic bilayers driving out water molecules from the polar region and decreasing the chain coupling in the hydrophobic region. At the same time, an increment in -turns is observed when nisin interacts with model membranes. Referencias(1) Mattick, A. T. R. & Hirsch, A. Nature (London). 154, 551 (1944)(2) Ruhr, E. & Sahl, H-G. Antimicrob. Agents Ch. 27, 841-845 (1985)(3) Tuma, R. J. Raman Spectrosc. 36, 307-319 (2005)(4) Zhu, G., Zhu, X., Fan, Q. & Wan, X. Spectrochim. Acta, Part A. 78, 1187-1195 (2011) AgradecimientosWe thank to Dr. Eejfan Breukink for purified nisin donation and to CONICET for finnancial support