Structural characterization of free and membrane-bound nisin by infrared spectroscopy

This study reports two new trends about nisin affinity for lipid membranes. First, there is a very strong dependence of nisin binding on the membrane surface charge. As illustrated in this work, the binding of nisin is much greater for phosphatidylglycerol (PG) than for phosphatidylcholine (PC) membranes. This can be rationalized by electrostatic attraction between the positively charged peptide and the negatively charged PG. Second, the affinity of nisin shows a very weak dependence on the lipid phase, the binding to fluid or gel phase membranes being nearly equivalent. Therefore, our results suggest that nisin behaves as an extrinsic peptide. This work also presents the first piece of information relative to the structure of membrane-bound nisin. The Amide I band of the peptide is different for free nisin in water and for membrane-bound nisin. By analyzing this region using self-deconvolution and band fitting, and by comparing with results obtained from nisin dissolved in various H2O/trifluoroethanol mixtures, it can be inferred that the binding of nisin to phospholipid membranes leads to an increased proportion of beta-turns.