Experimental theromodynamics of the helix-random coil transition. I. Influence of polymer concentration and solvent composition in the PBG-DCA-EDC system

Interaction of poly(L-arginine) with negatively charged DPPG membranes: calorimetric and monolayer studies

The interaction of poly(L-arginine) (PLA) with dipalmitoyl-phosphatidylglycerol (DPPG) bilayer membranes and monolayers was studied by differential scanning calorimetry (DSC), isothermal titration calorimetry (ITC), and monolayer experiments. The binding of PLA affected the main transition temperature of lipid bilayers (T(m)) only marginally. Depending on the PLA chain length, T(m) was slightly increased or decreased. This finding was attributed to the superposition of two counteracting effects on the transition after PLA binding. The main transition enthalpy (DeltaH(m)) was decreased upon PLA binding and the formation of a ripple phase (P(beta)') was suppressed. ITC experiments showed that two distinct processes are involved in binding of PLA to gel phase (L(beta)') membranes. At low peptide content the binding reaction is endothermic, and at high peptide concentration the binding becomes exothermic. However, the enthalpy of binding to fluid (L(alpha)) membranes was exothermic for all peptide-to-lipid ratios. The temperature dependence of PLA binding to fluid palmitoyl-oleoyl-phosphatidylglycerol (POPG) membranes showed a decrease in binding enthalpy with increasing temperature (Delta(R)C(p) < 0), indicating hydrophobic contributions to the free energy of binding. For longer PLA chains, the binding enthalpy for L(alpha) membranes was more exothermic than for shorter chains. Monolayer adsorption experiments showed two consecutive binding processes. At low initial surface pressures (pi(0)) a condensation of the lipid film (Deltapi < 0) is first observed after PLA injection into the subphase, followed by an increase in film pressure (Deltapi > 0) due to insertion of peptide side chains into the monolayer. At higher pi(0) only an increase in film pressure can be observed due to the insertion of the side chains. Deltapi increases with increasing pi(0). The insertion of the peptide into the monolayer is corroborated by the observed shift of pi-A isotherms to higher molecular areas. All presented experiments show that the binding of PLA to DPPG membranes has not only electrostatic but also nonelectrostatic contributions.

[A calorimetric study of the triple helix formation: interaction of poly(C) - guanosine - protonated poly(C)]

The triple helix formation of poly(C) - guanosine - poly(C+) was investigated by the help of an LKB scanning micro-calorimeter. The existence of the triple helix could also be shown by recording the melting curves. The ultraviolet absorption at different wave lengths namely 275 nm, 260 nm, and 245 nm was plotted as a function of the temperature. Furthermore formation of the triple helix was shown by plotting the ultraviolet absorption at 245 nm during the increasing addition of guanosine solution to a fixed amount of poly(C) in the solution. Finally the formation of the triple helix was demonstrated by plotting the ultraviolet absorption at 245 nm of a certain mixture of the components while the pH value of the solution was continuously lowered. All these methods show that the monomer interacts with the polymer double helix to form a triple helix. The calorimetric measurements show that the reaction enthalpy is concentration dependent. Above a threshold concentration a rapid increase of the reaction enthalpy is observed. This increase occurs in a very narrow concentration interval. Above this interval a final value of the reaction enthalpy is reached. The amount of the reaction enthalpy for the interaction of guanosine with poly(C) - poly(C+) double helix is 5.5 Kcal (mol base triplet)-1.