Protein Chemistry. Structure, Properties, Research Methods - Shendryk A.N. 2022

Protein Structure
Osmosis and Membrane Equilibrium

Many biologically important Properties of Proteins in solutions are associated with their large molecular size. They do not diffuse through certain membranes that allow Water and low-molecular-weight compounds to pass through. This makes it possible, for example, to use cellophane membranes to separate proteins from low-molecular-weight impurities. This Separation process is called dialysis. Most Introduction/36.html">Biological Membranes are also impermeable to proteins.

If a protein solution is separated from distilled water by a semipermeable partition, or membrane (which is permeable to water and other small molecules and impermeable to proteins), water molecules will begin to diffuse through the membrane into the protein solution. Conversely, low-molecular-weight compounds will rush through the membrane into the water reservoir. This phenomenon is called osmosis. It is driven by the tendency of any thermodynamic system to equalize chemical potentials across all its parts.

Let us examine this issue in more detail. By definition, the chemical potential (μ_i) of a separate substance or a component of a mixture (thermodynamic system) in any aggregate state is the partial derivative of the Gibbs Free energy (G) with respect to the quantity (concentration) of this component while keeping all natural parameters for G constant:

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In other words, the chemical potential is a partial molar state function. It is very important to understand that the chemical potential is essentially a generalized force. It allows us to predict the direction in which energy or matter exchange will occur between two thermodynamic systems or PARTS OF THE same thermodynamic system. This direction is always the same: from a system with a higher value of μ_i to a system with a lower μ_i, exactly as in the mechanical case: in the direction from a greater mechanical force to a smaller one.

From the perspective of chemical potential, The phenomenon of osmosis is also easy to understand. To do this, let us refer to Fig. 2.9.

Fig. 2.9 State of osmotic equilibrium in a solvent-solution system separated by a semipermeable partition. p represents atmospheric pressure and П represents osmotic pressure

Suppose the left arm of a vessel divided by a semipermeable partition (Fig. 2.9) contains a pure solvent, while the right arm contains a dilute solution of a substance whose molecules cannot penetrate the membrane, represented by dark spheres. Fig. 2.9 illustrates The equilibrium state when the liquid levels in the left arm (A), containing the pure solvent, and the right arm (B), containing the solution, are different due to the arising osmotic pressure, П. We will discuss it shortly. In the initial state, when osmotic pressure П is absent, the chemical potential of the solvent in the left part of the vessel is determined solely by The properties of the solvent itself, its quantity, and the external (atmospheric) pressure, p. In the right part of the vessel, the chemical potential of the solvent will be somewhat lower because its partial content in the solution is lower than in the pure substance. Striving to equalize the chemical potentials on both sides of the semipermeable partition, solvent molecules will begin to diffuse through the membrane from the left side of the vessel to the right. As a result, the liquid level in arm (B) will begin to rise, creating additional pressure, P. This is the osmotic pressure, which increases the chemical potential of the solvent in the right part of the vessel. The osmotic pressure will continue to rise until the chemical potentials of the solvent become equal on both sides of the membrane. This state is shown in Fig. 2.9. From this moment on, the rates of solvent molecule diffusion through the semipermeable partition from left to right and vice versa will become equal, and osmotic pumping of the solvent will cease. Without delving into the details of osmosis, we note that the magnitude of the osmotic pressure exerted by the dilute solution on the semipermeable partition does not depend on The Nature of the solute, but is determined solely by its concentration. This law is described by the van 't Hoff equation, which in form and essence coincides with the ideal gas state equation:

where n_s, R, T, and V are the number of moles of solute, the universal gas constant, Temperature, and solution volume, respectively.

This equation expresses a very simple and practically important property of osmotic pressure, The Essence of which is as follows. A solute that cannot penetrate the semipermeable membrane creates an osmotic pressure in dilute solutions of the same magnitude as if it were in the gas phase and occupied a volume equal to the volume of the solution. In other words, the solvent in osmotic phenomena acts like a vacuum in ideal gas-phase systems.

The phenomenon of osmosis is utilized by virtually all Cells and cellular Organelles. Furthermore, it forms The basis of A number of experimental Methods FOR STUDYING proteins, particularly in determining their molecular weight.