The Fourier Transform Infrared Spectroscopy was utilized in the study to investigate the thermodynamics of the main phase transition of dipalmitoylphosphatidylcholine (DPPC) model membrane in the vicinity of the antibiotic peptide gramicidin S (GS). Based on the two-state thermotropic phase behavior of DPPC, linear extrapolation method was employed to monitor the stability of GS-DPPC interacting systems at different peptide concentrations. The main phase transition temperature (T-m) and the enthalpy change (DeltaH(m)) were observed as GS concentration was raised. In addition, an evident reduction in the energy at maximum stability (DeltaG(s)) was obtained with the incorporation of GS. These thermodynamics results together with FTIR data indicated a destabilization effect imposed on the acyl chains of DPPC by GS presence. All these results showed the strong interactions of this peptide with DPPC model membrane. The results obtained by employing the thermodynamics approach using the FTIR spectral data gave a quantitative description and some additional information such as the values of DeltaG(s),DeltaH(m) and better estimates of T-m of DPPC and gramicidin containing DPPC systems. (C) 2001. Elsevier Science B.V. All rights reserved.