--%>

Donnan Membrane Equilibria

The electric charge acquired by macromolecules affects the equilibrium set up across a semipermeable membrane.

Laboratory studies of macromolecule solutions as in osmotic pressure and dialysis studies confine the macromolecules to one compartment while allowing passage of small ions or solvent in or out compartment. Much of the transport occurring in cells and cell compartments in living systems can be similarly described. In all such cases, the equilibrium state that would be reached as a result of the net transport of the small ions can be markedly affected if the macromolecule carries a charge, as is generally the case.

Except at the isoionic pH, proteins and nucleic acids carry a charge as a result of a net gain or loss of protons. Additional charges are acquired by the binding of other species, e.g. the binding of Mg2+ ions by phosphate groups. Thus, macromolecules in laboratory or biological systems generally carry a charge. The overall electrical neutrality of the solution results from a corresponding opposite charge contributed by ions, called counterions, included in the remaining ionic make up of the solution.

Suppose such a macromolecule or, specifically, a protein solution is separated from pure water by a semipermeable membrane that allows passage of small ions but prohibits the passage of protein molecules. Such a situation could arise in an osmotic pressure study or in the dialysis of the protein solution. Suppose the protein carries a net negative charge and that Na+ ions are the counterions. The Na+ ions will tend to diffuse to the low concentration region of initially pure water. Electrical neutrality would be lost and this process prevented if it were not for the dissociation of water. This occurs, and H+ ions tend to accumulate on the proteins side of the membrane while the corresponding OH- ions accumulate, along with the buffered, pH charges will occur to upper the osmotic pressure or dialysis experiment.

In such ways are led to deal with the equilibrium between protein solutions, which are often themselves buffered, and buffer solutions. The complication arise can be illustrated by considering the simplest situation of the protein-sodium-ion solution separated by a semipermeable membrane from a sodium chloride solution.

Suppose the proteins species P carries a negative charge of -z. the neutrality of the solution is achieved by the presence of z positive charges, Na+ ions for example, for each protein concentration is cP, as the initial Na+ concentration in the protein compartment is zeP.

Species concentration in a Donnan-membrane equilibrium study:

368_donnan membrane.png 



Rearrangement leads to x, the concentration of chloride that develops in the protein compartment:

At large salt concentrations, the effect of the protein is overwhelmed and x = 1/2cs. The two compartments achieve equal salt concentrations. At large a protein concentration, however, the passage of salt into the protein compartment is prevented, even though this rejection of the chloride ion by a solution that contains none of that ion.

Donnan-membrane equilibrium calculated from the above equation for z = 1:

2230_donnan membrane1.png 

The effects of various concentrations of protein and electrolyte are shown in the table. Only at high concentration relative to the protein concentration is the effect of the confined charged protein small. Therefore many studies of proteins or other polyelectrolytes in solution are made at high electrolyte concentration and at a pH near the isoionic point.  

   Related Questions in Chemistry

  • Q : Determining highest normality What is

    What is the correct answer. Which of the given solutions contains highest normality: (i) 8 gm of KOH/litre (ii) N phosphoric acid (iii) 6 gm of NaOH /100 ml (iv) 0.5M H2SO4

  • Q : Finding Normality Can someone please

    Can someone please help me in getting through this problem. Concentrated H2SO4 has a density of 1.98 gm/ml and is 98% H2SO4 by weight. The normality is: (a) 2 N  (b) 19.8 N  (c) 39.6 N  (d) 98

  • Q : Problem on partial pressure i) Show

    i) Show that the equilibrium constant Kp for the reaction CaCo3(s) ↔ CaO(s) +CO2(g)is about unity (i.e. = 1.0) at 895 °C.ii) If two grams of calcium carbonate are pl

  • Q : How molecule-molecule collisions takes

    An extension of the kinetic molecular theory of gases recognizes that molecules have an appreciable size and deals with molecule-molecule collisions. We begin studies of elementary reactions by investigating the collisions b

  • Q : What is ortho effect? Orthosubstituted

    Orthosubstituted anilines are generally weaker bases than aniline irrespective of the electron releasing or electron withdrawing nature of the substituent. This is known as ortho effect and may probably be due to combined electronic and steric factors.The overall basic strength of ort

  • Q : HCl is an acid or a base Illustrate is

    Illustrate is HCl an acid or a base ?

  • Q : Molarity of sodium hydroxide Can

    Can someone please help me in getting through this problem. Determine the molarity of a solution having 5g of sodium hydroxide in 250ml  solution is: (i) 0.5  (ii) 1.0  (iii) 2.0   (d) 0.1Answer: The right answer i

  • Q : Mole fraction and Molality Select the

    Select the right answer of the following question.What does not change on changing temperature : (a) Mole fraction (b) Normality (c) Molality (d) None of these

  • Q : Pressure and power for adiabatic

    a) Air flowing at 1 m3/s enters an adiabatic compressor at 20°C and 1 bar. It exits at 200°C. The isentropic efficiency of the compressor is 80%. Calculate the exit pressure and the power required. b) Steam enter

  • Q : What are electromotive force in

    The main objective of this particular aspect of Physical Chemistry is to examine the relation between free energies and the mechanical energy of electromotive force of electrochemical cells. The ionic components of aqueous solutions can be treated on the basis of the