Protein Chemistry. Structure, Properties, Research Methods - Shendryk A.N. 2022
Methods for Experimental Investigation of Protein Structure
Methods for Determining the Molecular Weight of Proteins
Determination of Molecular Weight by Saturated Vapor Pressure
It is known from Thermodynamics that the saturated vapor pressure of a solution is equal to the sum of the partial pressures of all solution components:
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where PΣ and pi are the total saturated vapor pressure and the partial vapor pressures of each solution component, respectively.
In ideal (extremely dilute) solutions, the partial pressures of the components are proportional to their respective mole fractions. The saturated vapor pressure of polymers (particularly Proteins) at room temperatures is extremely low, and its contribution to PΣ can be neglected. Thus, for an ideal binary solution of a protein in a solvent, we obtain:
An increase in protein concentration in the solution lowers the mole fraction of the solvent and, consequently, its partial pressure. In other words, the saturated vapor pressure above the solution will decrease in this case, which makes it possible to determine the Molecular Weight of the dissolved polymer.
The vapor pressure above The surface of dilute non-electrolyte solutions can be compared with that of pure solvent using a tensimeter (see fig.). For simplicity, it is assumed that the pressure is generated solely by the solvent vapors, i.e., the vapor pressure of the solutes is zero. It has been established that the vapor pressure of a solution decreases as its titer (s) increases, calculated by the formula:
where m' and m are the mass of the solute and the solvent, respectively.
The relative decrease in vapor pressure above the solution ((P - P')/P) is related to its titer and the molecular weights of the solute (M') and the solvent (M) by the following simple relation:
This is the fundamental equation of tonometry, which allows the calculation of the molecular weight of the solute. The molecular weight of the solvent is a tonometric constant.
The tonometry method does not always yield reliable results. First, because protein solutions rapidly deviate from ideality as concentration increases. Second, at extremely high dilutions, it is difficult to accurately measure the minute decreases in saturated vapor pressure caused by small additions of protein.
Last update: 06/08/2026
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