High sensitivity differential scanning calorimetry is applied to the study of the thermotropic behavior of mixtures of synthetic phospholipids in multilamellar aqueous suspensions. The systems dimyristoylphosphatidylcholinedipalmitoylphosphatidylcholine, dimyristoylphosphatidylcholine-distearoylphosphatidylcholine, and dimyristoylphosphatidylethanolamine istearoyphosphatidylcholine, although definitely nonideal, exhibit essentially complete miscibility in both gel and liquid crystalline states, while the system dilau- amount of weight on heating at 500 in a vacuum oven for several hours, and they were thus assumed to be anhydrous. Our best current values for the transition properties of these lipids are given in Table 1, and differ significantly in some respects from the values previously reported from this laboratory (10, 11), presumably because of varying small amounts of impurities.Lipid mixtures were prepared by dissolving the weighed components in a small volume of chloroform and removing the solvent at 40-50°in a vacuum oven. The appropriate amount of 0.01 M sodium phosphate buffer, pH 7.0, was added and the lipids were suspended by 2-3 min of shaking on a vortex mixer at about 60°under nitrogen. For mixtures containing DLPC it was necessary to use buffer containing 15% (vol/vol) of ethylene glycol to prevent freezing. The transition behavior of DSPC in this solvent was found to be very similar to that of DSPC in aqueous buffer, with just a slight broadening of the main transition. A small amount of settling observed with some of the mixtures seemed to have no effect on the calorimetric results.All calorimetric scans were performed with the Privalov calorimeter (9), usually at 0.50 min1. Scan rates as low as 0.1°m inI were occasionally used, and on the basis of these experiments we conclude that the systems were close to equilibrium throughout their phase transitions. With pure lipids, concentrations of 0.2-0.4 mg ml-l were used; with mixtures, concentrations as high as 2-4 mg ml-l were used. As would be expected with heterogeneous systems, there were no indications of any concentration dependence.RESULTS AND DISCUSSION Pure Lipids. The transitions of DLPC and DMPE, which have not previously been studied by high sensitivity differential scanning calorimetry, are shown in Fig. 1. It has been reported (12) that the latent heat of the transition of DLPC is 4.3 kcal moh'. The total area of the curve for DLPC in Fig. 1 from -

Six possible sources of the large heat capacity and entropy changes frequently observed for processes involving proteins are identified. Of these the conformational, hydrophobic, and vibrational effects seem likely to be of greatest importance. A method is proposed for estimating the magnitudes of the hydrophobic and vibrational contributions. Application of this method to several protein processes appears to achieve significant clarification of previously confusing and apparently contradictory data.In recent years direct calorimetric measurements have shown that many processes involving biopolymers, particularly proteins, take place with large changes in the apparent heat capacities of the reacting species. It now appears that these large values for ACp constitute the rule rather than the exception. (27) was among the first to point out the effect of nonpolar groups in raising the apparent heat capacities of solutes in aqueous solutions. Since then the general nature of the hydrophobic effect (2831) has been well established as resulting from the formation of cages of structured water of abnormally high heat capacity and low entropy around nonpolar groups.Data are available in the literature which permit evaluation of ACp for the transfer of many nonpolar substances or groups from nonpolar to aqueous medium, but in only 17 cases (Table 2)

No abstract

A highly sensitive and stable scanning microcalorimeter is employed in a reinvestigation of the effect of cholesterol on multilamellar suspensions of dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC). Below 20 mol % cholesterol the DPPC mixtures give heat-capacity curves each of which can be resolved into a narrow and a broad peak, suggesting the coexistence of two immiscible solid phases; above 20 mol % only the broad peak is observed and this disappears at about 50 mol %. The DMPC mixtures show a more complicated behavior; from about 13.5 to 20 mol % cholesterol the observed curves appear to be the sum of three component peaks. As with the DPPC mixtures, only a single broad peak is observed above 20 mol % cholesterol, and this broad peak becomes undetectable above about 50 mol %. These results are discussed.

In this paper we show that the usual assumption in studies of the temperature variation of equilibrium constants for equilibria of the form A + B T± AB that a plot of In K vs. 1/T (K = equilibrium constant, T = temperature in degrees kelvin) is a straight line with slope equal to -AHVH/R (AHVH = van't Hoff or apparent enthalpy, R = gas constant) is not valid in many cases. In all the cases considered here, AHVH is temperature dependent and is significantly different from the true or calorimetrically measured enthalpy, and the respective values for ACp are also significantly different.The ready availability of sensitive isothermal titration calorimeters makes possible the accurate determination of both enthalpies and equilibrium constants on the same sample for a wide variety of processes. In view of this situation, detailed comparisons of calorimetrically determined enthalpies of reaction (AHcal) and enthalpies derived from equilibrium constants by means of the van't Hoff equation (AHVH) highlighted by the fact that ACp as evaluated from the equilibrium constants is -0.287 kcal-K-1 mol-1, which is 45% larger in magnitude than that derived from the observed enthalpies-namely, -0.198 kcal K-1 mol-'.To check on the possibility that a significant contribution to AHcal might arise from an exchange of protons between the protein and the buffer, the two reactants were mixed in the absence of buffer at pH 5.5 at -25°C. The results indicated that the reaction leads to the liberation of 0.025 ± 0.05 mol of HI per mol of protein, corresponding in acetate buffer to -0.05 kcal mol-1 due to buffer protonation and probably a similar, perhaps compensating, contribution due to deprotonation of the protein.