Titration calorimetry was employed to measure the critical micelle concentration (cmc) and the heat of demicellization A/fdemic of the four surfactants octyl glucoside, sodium dodecyl sulfate (SDS), sodium cholate, and sodium deoxycholate at temperatures between 10 and 70-80 °C. From these data, the thermodynamic parameters AGdemic, Anemic, and ACp,demic associated with the demicellization process were calculated. Titration calorimetry has the advantage that the cmc and the thermodynamic parameter AFÍáemic can be directly measured, whereas with other methods /fdemic has to be calculated from the temperature dependence of the cmc, which requires high precision for the cmc data. Changes in temperature caused large variations of A//demic and ASdemic, whereas AGdemic remained virtually constant. Therefore, the changes in enthalpy and entropy almost completely compensate each other. At room temperature, the entropy was found to be the dominant factor responsible for micellization, whereas at elevated temperatures contributions from enthalpy dominate. These observations are in agreement with data of other processes where hydrophobic effects play a major role and were used to discuss the nature of the driving forces that rule micelle formation at various temperatures. Furthermore, predictions regarding the degree of hydration of the micelle interior were made. It is shown that titration calorimetry is an easy and fast method to determine the cmc and the demicellization enthalpy from a single experiment. For surfactants with low aggregation numbers the titration curves could be simulated using a mass action model.

Fourier transform infrared spectroscopy has been used to characterize the carbonyl stretching vibration of DMPC, DMPE, DMPG, and DMPA, all labeled with 13C at the carbonyl group of the sn-2 chain. Due to the vibrational isotope effect, the 13C = O and the 12C = O vibrational bands are separated by ca. 40-43 cm-1. This frequency difference does not change when the labeling is reversed with the 13C = O group at the sn-1 chain. For lipids in organic solvents possible conformational differences between the sn-1 and sn-2 ester groups have no effect on the vibrational frequency of the C = O groups. In aqueous dispersion unlabeled phospholipids always show a superposition of two bands for the C = O vibration located at ca. 1740 and 1727 cm-1. These two bands have previously been assigned to the sn-1 and sn-2 C = O groups. FT-IR spectra of 13C-labeled phospholipids show that the vibrational bands of both, the sn-1 as well as the sn-2 C = O group, are clearly superpositions of at least two underlying components of different frequency and intensity. Band frequencies were determined by Fourier self-deconvolution and second-derivative spectroscopy. The difference between the component bands is ca. 11-17 cm-1. Again, the conformational effect as shown by reversed labeling is negligible with only 1-2 cm-1. The splitting of the C = O vibrational bands in H2O and D2O is caused by hydrogen bonding of water molecules to both C = O groups as shown by a comparison with spectra of model ester compounds in different solvents. To extract quantitative information about changes in hydration, band profiles were stimulated with Gaussian-Lorentzian functions. The chemical nature of the head group and its electronic charge have distinctive effects on the extent of hydration of the carbonyl groups. In the gel and liquid-crystalline phase of DMPC the sn-2 C = O group is more hydrated than the sn-1 C = O. This is accord with the conformation determined by X-ray analysis. In DMPG the sn-1 C = O group seems to be more accessible to water, indicating a different conformation of the glycerol backbone.

The critical micellar concentration (cmc) and the demicellization enthalpy ΔH demic of the primary aggregates of sodium cholate (NaC) and sodium deoxycholate (NaDC) in water and 0.1 M NaCl at pH 7.5 were determined by isothermal titration calorimetry (ITC). The cmc of NaC and NaDC in water and 0.1 M NaCl at pH 7.5 shows a minimum between 295 and 300 K. With increasing ionic strength, the cmc of the bile salts decreases. ΔH demic is strongly temperature-dependent but shows almost no dependence on the ionic strength. For comparison with other systems, the thermodynamic parameters ΔG demic and ΔS demic associated with the demicellization process were calculated using the pseudo-phase-separation model. From the temperature dependence of ΔH demic, the change in heat capacity ΔCp demic for the demicellization process was determined. The data obtained for ΔCp demic are positive and at 298 K have values of 250 J·mol-1·K-1 for NaC and 350 J·mol-1·K-1 for NaDC. These values correspond to changes in the exposed hydrophobic surface area of 1.1−1.5 nm2 per molecule. For NaDC, ΔCp demic decreases at 343 K to ∼250 J·mol-1·K-1, whereas ΔCp demic for NaC remains essentially unchanged. The calorimetric titration curves were simulated using a mass action model including counterion condensation for the aggregation process. The simulation of the titration curves yielded values for the aggregation number n. In the concentration region of the cmc, n is approximately 4−6 for NaC in water or 0.1 M NaCl and independent of temperature. For NaDC in water values of n of 7 and 12 were obtained at low temperature (284 K) in water and 0.1 M NaCl, respectively. For NaDC in water and 0.1 M NaCl, the aggregation number n decreases to 5 and 7, respectively, at 328 K.

Islet amyloid polypeptide (IAPP) is a pancreatic hormone and one of a number of proteins that are involved in the formation of amyloid deposits in the islets of Langerhans of type II diabetes mellitus patients. Though IAPP-membrane interactions are known to play a major role in the fibrillation process, the mechanism and the peptide's conformational changes involved are still largely unknown. To obtain new insights into the conformational dynamics of IAPP upon its aggregation at membrane interfaces and to relate these structures to its fibril formation, we studied the association of IAPP at various interfaces including neutral as well as charged phospholipids using infrared reflection absorption spectroscopy. The results obtained reveal that the interaction of human IAPP with the lipid interface is driven by the N-terminal part of the peptide and is largely driven by electrostatic interactions, as the protein is able to associate strongly with negatively charged lipids only. A two-step process is observed upon peptide binding, involving a conformational transition from a largely alpha-helical to a beta-sheet conformation, finally forming ordered fibrillar structures. As revealed by simulations of the infrared reflection absorption spectra and complementary atomic force microscopy studies, the fibrillar structures formed consist of parallel intermolecular beta-sheets lying parallel to the lipid interface but still contain a significant number of turn structures. We may assume that these dynamical conformational changes observed for negatively charged lipid interfaces play an important role as the first steps of IAPP-induced membrane damage in type II diabetes.

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A detailed study on the structure, dynamics, and thermodynamic behavior of phosphatidylcholine/cholesterol (PC/CHOL) mixtures was undertaken using differential scanning calorimetry (DSC) and solid-state nuclear magnetic resonance (NMR) spectroscopy. DSC thermograms of mixtures of cholesterol (CHOL) with 1,2-dipalmitoyl-sn-phosphatidylcholine (DPPC), 1,2-distearoyl-sn-phosphatidylcholine (DSPC), and 1,2-diarachidoyl-sn-phosphatidylcholine (DAPC) showed a broadening of the first-order gel-->liquid crystalline transition and a decrease in the transition enthalpy, indicating a gradual loss of cooperativity for high CHOL concentrations. DPPC and DSPC were labeled with 13C at the carbonyl group of the sn-2 chain and 2H was introduced into the middle of the sn-2 chain at the 6- and 12-position for DPPC and DSPC, respectively. The 13C and 2H NMR spectra of each labeled lipid were studied as a function of temperature and CHOL concentration. The residual quadrupole splitting in the 2H NMR spectra, delta nu Q perpendicular, was analyzed as a function of temperature and composition. For CHOL concentrations less than 30 mol %, a precipitous change in delta nu Q perpendicular occurs near the chain melting temperature of the phospholipid. Further increases in CHOL concentration broaden the transition and shift the midpoint to higher temperature, indicating the presence of a new phase at higher CHOL contents. Moreover, at a given temperature, delta nu Q perpendicular increases with increasing cholesterol content, which indicates a more ordered structure. The 13C NMR spectra in the gel state consisted of a superposition of two components which can be attributed to both gel-like and fluid phospholipid domains in the bilayer. This two-component spectrum can be simulated quantitatively with a two-parameter chemical exchange model, which permits the fraction of each form and the exchange rate to be determined as a function of temperature and composition. At high CHOL contents the line width of the fluid component broadens, suggesting an increase in the exchange rate between the domains. These results were interpreted in terms of a temperature composition diagram with one region L beta', two regions LGI and LGII, and one liquid crystalline region L alpha, with LG denoting "liquid-gel" type phases. Liquid-gel phases correspond to phases with increased order in the hydrocarbon chains (in comparison to that of the pure PC bilayer in the L alpha phase) combined with fast limit axial diffusion that averages the 13C NMR spectrum to a "fluidlike" line.(ABSTRACT TRUNCATED AT 400 WORDS)

Osmotic and diffusive water permeability coefficients Pf and Pd were measured for lipid vesicles of 100-250 nm diameter composed of a variety of phospholipids with different head groups and fatty acyl chains. Two different methods were applied: the H2O/D2O exchange technique for diffusive water flow, and the osmotic technique for water flux driven by an osmotic gradient. For phosphatidylcholines in the liquid-crystalline state at 70 degrees C, permeability constants Pd between 3.0 and 5.2.10(-4) cm/s and ratios Pf/Pd 7 and 23 were observed. The observation of a permeability maximum in the phase transition region and the fact that osmotically driven water flux is higher than diffusive water exchange suggest that water is diffusing through small transient pores arising from density fluctuations in the bilayers. The Pd values depend on the nature of the head group, on the chemical structure of the chains, and on the type of chain linkage. In the case of charged lipids, the ionic strength of the solution has a strong influence. For phosphatidylethanolamines, phosphatidic acids, and ether phosphatidylcholines, permeability constants Pd were considerably lower (2-4.10(-6) cm/s at 70 degrees C). For liquid-crystalline phosphatidylcholines, a strong reduction of Pd after addition of ethanol was observed (2-4.10(-6) cm/s at 70 degrees C). The experimental values are discussed in connection with different permeation models.