Negative cooperativity and aggregation in biphasic binding of mastoparan Xpeptide to membranes with acidic lipids

The change of Trp fluorescence intensity when large vesicles with 10% acidi c lipid are added to mastoparar X solutions reflects a fast and a slow bind ing process. By means of a novel procedure of data analysis that takes adva ntage of so-called mass conservation plots we have separated association is otherms related to: (i) the apparent fast pre-equilibrium; and (ii) the fin al equilibrium, respectively. This approach also reveals that the intrinsic fluorescence signal of the slow binding is considerably raised against tha t of the fast binding, presumably indicating a penetration of bound peptide from the lipid/water interface into the apolar lipid core. The shape of ei ther binding curve discloses a pronounced tendency of aggregation. Furtherm ore, it turns out that in the slow process the final binding ratio decrease s markedly compared with the initial fast binding ratio. Accordingly the oc cupation of final binding sites must exert a substantial effect of negative cooperativity on the affinity of the interfacial binding states. (C) 2001 Elsevier Science B.V. All rights reserved.