Diffusionskoeffizient und Wassergehalt globulärer Proteinmoleküle

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Abstract
Crystalline protein molecules normally contain water, and complete removal of the water usually leads to denaturation. Svedberg''s ultrafiltration method gives the mol. wt. of the protein fraction of the molecule, but no information concerning its water content. When a substance of low mol. wt., such as a sugar or a salt, is dissolved in a protein soln., the part of the water bound to the protein cannot act as a solvent, and is designated NSS (non-soluble space). The NSS for dissolved proteins may be detd. by measuring the lowering of the vapor pressure of the soln. by the dissolved low mol. wt. substance, or by determining the partition of the dissolved substance between the protein soln. and water separated by a dialyzing membrane. The NSS for protein crystals may be calculated by crystallizing the protein from a soln. containing (NH4)2SO4, and determining the water and (NH4)2SO4 contents of the filtrate and the filtered residue. The protein crystals may also be rapidly dried over concentrated H2SO4 without denaturation and the amt. of water then taken up by the crystals to give a relative vapor pressure of 0.83 detd. The water content is also detd. by comparing the thickness of the crystals with the thickness of the dried protein substance in the dissolved condition. According to the Wrinch model, the protein molecule consists of a network of amino acids spread over the surface of a polyhedron, the interior of which is filled with the bound water. Detn. of the water content of egg albumin, hemoglobin, CO-hemoglobin, serum albumin and globulin and myogen by the above methods, shows that the amt. of bound water found is in the order of that postulated by the Wrinch model. The amt. of water found is 0.17-1.8 g. bound water/g. dry protein. The bound water is not to be confused with the hydration shell which surrounds a molecule. The hydration shell is more dependent upon pH than is the bound water. The protein constituents seem to be packed more closely in the protein molecule when it is in the crystalline form than when it is in the dissolved condition. The very high mol. wt. globular proteins, such as the viruses, differ little from the low mol. wt. proteins in respect to their bound water content. The diffusion coefficients vary from 10.6 for the low mol. wt. proteins to 1.17 for the virus proteins, and fulfill the requirements of the Einstein equation.

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