The interaction of four structurally related somatostatin analogues (effective electric charge +0.4 < or = < or = +3) with lipid membranes was studied with titration calorimetry and was compared with the functional activity of the peptides. Surface activity measurements provided average cross-sections of 70 or 135 A2, indicating that the cyclic molecules orient at the air-water interface with their ring system either parallel (z = +3) or perpendicular (z = +1) to the surface or switching between the two orientations according to the surface density (z = +2). The nonspecific binding of the peptides to sonified lipid vesicles was enthalpy-driven with a delta H of -4 to -7.5 kcal/mol. A consistent quantitative analysis of the binding isotherms was achieved by combining electrostatic attractions, calculated via the Gouy-Chapman theory, with a nonspecific surface partition equilibrium for the nonpolar interactions. The electrostatic attraction of the cationic peptides varied strongly according to the peptide charge. Due to the flat ring structure of the cyclic peptides, their true physical charge was sensed at the membrane surface, and no "charge screening" was observed. Peptide binding to the negative charged membrane was accompanied by a proton-uptake of the N-terminal amino group of 0.23-0.38 H+/peptide. Deviations from the theoretical prediction of 0.39 H+/peptide can be explained by a preferential binding of the nonprotonated species. The nonpolar interactions, as described by the surface partition coefficients of the four peptides, fell into a narrow range of K congruent to 50-230 M-1 whereas the apparent overall binding constants were between 200 and 5000 M-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Membrane fusion induced by the hemagglutinin glycoprotein of influenza virus has been extensively characterized, but the mechanism whereby the protein achieves the merger of the viral and target membrane lipids remains enigmatic. Various lipid intermediate structures have been proposed, and the energies required for their formation predicted. Here, we have analyzed the enthalpies of fusion of influenza with liposomes by titration calorimetry. If a small sample of virus in a weak neutral pH buffer was added to an excess of liposomes at low pH, a two-component reaction was seen, composed of an exothermic reaction and a slower endothermic reaction. The exothermic reaction was the result of acid-base reactions between the neutral pH virus sample and low pH buffer and low-pH-induced changes in the virus. The endothermic reaction was not observed in the absence of liposomes and much reduced if acid-inactivated virus, which had lost its fusion but not its binding activity, was added to liposomes. The endothermic reaction was more temperature dependent than the exothermic reaction; its pH dependence corresponded with that of fusion and its enthalpy was higher if fusion was more extensive. These data indicate that most of the endothermic reaction was due to membrane fusion. The experimentally determined enthalpy of fusion, 0.6-0.7 kcal per mol of viral phospholipids, is much higher than expected on the basis of current theories about the formation of lipid intermediates during membrane fusion.

Closed lipid vesicles act as osmometers increasing or decreasing their volume under the influence of osmotic gradients. The enthalpy changes accompanying membrane compression or expansion have not been measured yet, and first results obtained with high-sensitivity titration calorimetry are reported here. Phospholipid vesicles suspended in and in equilibrium with an electrolyte or nonelectrolyte with a defined initial concentration of c(i), were injected into a solution with a final concentration of c(f), and the heat changes were monitored with a titration microcalorimeter. Osmotic compression (delta c = c(f) - c(i) > 0) produced an exothermic heat change with deltaH approximately -500 +/- 100 cal/mol and osmotic expansion (delta c < 0) an endothermic heat change with deltaH approximately 1000 +/- 200 cal/mol; both results normalized to a concentration gradient of delta c = 1 M NaCl. The heats of compression and expansion varied linearly with the lipid content and the size of the osmotic gradient but were independent of the vesicle size. The cubic thermal expansion coefficient alpha(v) which equals (1/V)(deltaV/deltaT)p could be derived and was found to be 1.25 x 10(-3) and 2.5 x 10(-3) K(-1) for the compressed and expanded bilayer vesicles, respectively. The entropy changes associated with compression and expansion could be estimated. Compression of the membrane led to a negative entropy change and increased the hydrocarbon chain order. Expansion of the membrane was accompanied by a positive entropy change which can be explained, in part, by more disordered hydrocarbon chains. Vesicle expansion and compression thus appear to be asymmetric as far as the thermodynamic driving force is concerned.