We studied the interaction of spectrin dimers from human erythrocytes with (bilayer and monolayer) model membranes of mixtures of dimyristoylphosphatidylethanolamine, dimyristoylphosphatidylcholine, and dimyristoylphosphatidylserine (DMPS) by densitometric evaluation of phase transitions and phase boundaries, film balance experiments, and microfluorescence. We demonstrate that spectrin readily adsorbs to mixed bilayers and monolayers even in the presence of small DMPS concentrations (30 mol %) whereas no appreciable interaction with lamellae containing zwitterionic lipids alone is observed. The selectivity of the DMPS/spectrin interaction is established by quantitative evaluation of the shifts of the phase boundaries (liquidus and solidus line) caused by the lipid/protein interaction as a function of the composition of the binary lipid mixtures. Quantitative information about the free energy of the lipid/protein interaction is obtained by computer simulation of the phase diagram of the lipid mixture in the absence or in the presence of a very small molar fraction of the protein and comparison of calculated and measured shifts. A binding energy of about 10(-17) J per spectrin molecule is found. The present perturbation method can be generalized to study selective lipid/protein interaction mechanisms in ternary or higher component mixtures. The present results provide evidence that in addition to the binding to band III, spectrin may also couple directly to the lipid moiety of the inner monolayer of erythrocytes.(ABSTRACT TRUNCATED AT 250 WORDS)
The temperature dependence of the apparent expansibility of lysozyme and ovalbumin in solution has been measured as a function of pH. This temperature dependence is explained in terms of suppressed fluctuations in bound water due to the protein. It is shown that the thermal expansion coefficient of bound water is different from bulk water. The pH dependence can be explained by increased hydration of side chains at lower pH. The amount in volume of hydration water in a typical protein-water system varies from 0.16 to 0.7. How the intrinsic thermal expansion coefficient of proteins can be derived from the apparent quantity is discussed. Intrinsic values of the thermal expansion coefficient for lysozyme at room temperature are between 1.7 and 4.4 x 10(-4) K-1 for a 10% solution.
a/A-isotherms of catalase monolayers established at the air/water-interface are discussed quantitatively on the basis of molecular data: A relationship between a critical value of the surface pressure, the corresponding molecular area, and the molecular dimension of the molecules at the interface is proposed. It is shown that the unfolding of molecules at the water surface is pH-dependent. For each pH-value there is a distinct degree of unfolding; the molecules keep their globular state at neutral pH. Establishment at the surface of bulk solutions corresponding to globular and partly unfolded states, respectively, catalase molecules keep their original configuration on changing the pHvalue of bulk-phase. The monolayers are confirmed to show reversibility with regard to lateral changes of state as well as irreversibility with respect to desorption of molecules. A model is proposed to explain the nature of the critical re/A-value occurring in the ~r/A-isotherms: on compression beyond ~rc, molecular segments are transferred from the surface into the bulkphase via a subsurface layer. From the experiments it is concluded that the surface pressure ~ is determined, not only by the surface itself, but also by this subsurface layer.
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