THERMODYNAMIC CHARACTERIZATION OF THE ASSOCIATION OF SMALL BASIC PEPTIDES WITH MEMBRANES CONTAINING ACIDIC LIPIDS

We measured the binding of the peptide acetyl-Trp-Lys7-amide to membranes for-med from mixtures of the zwitterionic lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (PC) and the acidic lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (PG). Direct filtration and equilibrium dialysis measurements demonstrate that binding increases sigmoidally with the mole fraction of PG as predicted from a simple Gouy-Chapman/mass action theoretical model. We used these measurements to calibrate two binding assays, one based on the increase in Trp fluorescence that occurs when the peptide binds to the membrane, the other on the quenching of Trp fluorescence that occurs when the peptide binds to membranes containing fluorescent lipids. Both fluorescence assays demonstrate that binding does not depend strongly on temperature, which suggests the enthalpy change, DELTAH, is small. Calorimetric measurements demonstrate this directly for the analogous basic peptide Lys 5: DELTAH congruent-to + 1 kcal/mol for the binding of Lys5 to sonicated phospholipid vesicles and DELTAH congruent-to 0 kcal/mol for its binding to large unilamellar vesicles. Thus, the decrease in the free energy that occurs when these peptides bind to the membrane is due to a positive change in the entropy of the system. Fluorescence measurements demonstrate the binding of the Trp-containing peptide to 4:1 PC/PG membranes is independent of pressure up to 2 kbar, which suggests that binding occurs without a significant change in volume.