where M1 and M2 are the molar masses of the solvent and solute and p1 is the density of the solvent. It can be seen then that we only need to know the limiting slopes of the k and p versus mole fraction slopes to determine Pm2 and these we can obtain by measuring a number of solutions of increasing dilution.
When dealing with a protein it is important to realize that modeling it as a dipolar rigid rotator is only justified in very specific cases. In general, proteins have many rigid dipoles, polar substituents such as backbone amides, polar side chains and C-termini. Although constrained to be part of the protein, these have significant freedom and can rotate and translate at low incident field frequencies to give very large dielectric constant to proteins. The generalized Kirkwood-Frohlich theory gives a way to combine the high-frequency dielectric constant with the complicated dipolar contributions to obtain our desired static or dc dielectric constant of the protein in a polar solution. In this approach the sample of dielectric constant e is approximated to a collection of permanent rigid dipoles embedded in surroundings of dielectric constant e^ that represents the sample's high-frequency dielectric constant (which can be easily determined from a measurement of the refractive index). Focusing on a spherical region of volume V, of the order of the size of a molecule of sample and using classical continuum theory the effective aligning field is calculated (Eeff) as a function of the average field in the medium E that results in the following relation [59, 60]:
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