BINDING OF BASIC AMPHIPATHIC PEPTIDES TO NEUTRAL PHOSPHOLIPID-MEMBRANES - A THERMODYNAMIC STUDY APPLIED TO DANSYL-LABELED MELITTIN AND SUBSTANCE-P ANALOGS

A thermodynamic approach is proposed to quantitatively analyze the bin ding isotherms of peptides to model membranes as a function of one adj ustable parameter the actual peptide charge in solution z(p)(+). The m ain features of this approach are a theoretical expression for the par tition coefficient calculated from the molar free energies of the pept ide in the aqueous and lipid phases, an equation proposed by S. Stanko wski [(1991) Biophysical Journal, Vol. 60, p. 341] to evaluate the act ivity coefficient of the peptide in the lipid phase, and the Debye-Huc kel equation that quantifies the activity coefficient of the peptide i n the aqueous phase. To assess the validity of this approach we have s tudied by means of steady-state fluorescence spectroscopy, the interac tion of basic amphipathic peptides such as melittin and its dansylcada verine analogue (DNC-melittin), as well as a new fluorescent analogue of substance P, SP (DNC-SP) with neutral phospholipid membranes. A con sistent quantitative analysis of each binding curve was achieved. The z(p)(+) values obtained were always found to be lower than the physica l charge of the peptide. These z(p)(+) values can be rationalized by c onsidering that the peptide charged groups are strongly associated wit h counterions in buffer solution at a given ionic strength. The partit ion coefficients theoretically derived using the z(p)(+) values were i n agreement with those deduced from the Gouy-Chapman formalism. Ultima tely, from the z(p)(+) values the molar free energies for the free and lipid-bound states of the peptides have been calculated. (C) 1997 Joh n Wiley & Sons, Inc.