Ultracentrifugation molar mass from

For the analysis of light scattering experiments the refractive indices of the DADMAC/AAM solutions at dialysis equilibrium were determined, showing the validity of the additivity principle [67]. The additivity could also be proven for the partial specific volumes which are necessary to calculate molar masses from ultracentrifugation experiments [131]. These dependencies are summarized in Fig. 24. 

This is the Svedberg equation for the molar mass. If an independent value of D is available, the measurement of s suffices to determine M. Later we will show that the value oi D/s can be obtained from an ultracentrifugation experiment. 

Equation (9-114) corresponds to a Gaussian distribution function (see also Section 8.3.2.1). The molar mass can be calculated from the position of the inflection point of the function c = /(r — r ). For proteins in CsCl/ H2O, the lower molar mass limit giving a meaningful measurement is 10 000-50 000 g/mol molecule. The limit is essentially governed by the length of the ultracentrifuge cell ( 1.2 cm) and the optimal values of r — for this length. 

Blood serum (separated from fibrinogen and/or fibrin by ultracentrifugation) contains serum albumins. These possess a molar mass of 67 500 and an isoelectric point at pH 4.8-5.0. The serum albumins of higher mammals differ particularly in the amino acid residues at the carboxyl end of the protein chain, while the N-terminal amino acid residue is always asparagine ... 

Equation 9.8.22 is needed for sedimentation equilibrium, a method of determining the molar mass of a macromolecule. A dilute solution of the macromoleeule is placed in the cell of an analytical ultracentrifuge, and the angular velocity is selected to produce a measurable solute concentration gradient at equilibrium. The solute concentration is measured optically as a function of r. The equation predicts that a plot of In (cb/c°) versus will be linear, with a slope equal to Mb (l — E p) jlRT. The partial specific volume is found from measurements of solution density as a function of solute mass fraction (page 234). By this means, the molar mass Mb of the macromolecule is evaluated. 

Example 11.1 J The molar mass of a protein from ultracentrifugation experiments... 

The data from an equilibrium ultracentrifugation experiment performed at 300 K on an aqueous solution of a protein show that a graph of In c agcunst (r/cmy is a straight line with a slope of 0.729. The rotationcd rate of the centrifuge was 50 000 rotations per minute and b = 0.70. Calculate the molar mass of the protein. 

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