The charge and deformation of protein molecules

Abstract

AbstractThe density of like‐charged groups distributed along the molecules of proteins is much lower than in molecules of polyelectrolytes. From this it follows that the degree of deformation of the protein molecules resulting from the charge is much less. In buffered solutions—if the correction for hydration (measured by the compressibility method) is taken into account—the shape of gelatin particles (as measured by viscosity) practically does not depend on the pH, while in unbuffered solutions it varies roughly in proportion to the fourth to fifth root of the number of charges. For molecules of serum albumin and egg albumin the deformation is proportional to the 25th to 30th root of the number of charges, i.e., completely negligible. This comparison also shows that the deformation of the threadlike molecules of gelatin occurs much more easily than with the firmly molecules of the globular proteins. For globular proteins the following expression has been derived where E is the modulus of elasticity, n the number of charges inthe molecule, m the relative elongation of the molecule, a0, b0, and S the semi‐axes and surface of the ellipsoidal molecule and κ the reciprocal thickness of the ionic atmosphere. A value of E = 10–40 kg./mm.2 was calculated according to this relation for the molecules of these globular proteins (for soft rubber E = 0.14 kg./mm.2, for polyamides 70–80 kg./mm.2, for ebonite 50–80 kg./mm.2, etc.). These results for the first time quantitatively characterize the high rigidity of the molecules of globular proteins.