Dielectric Constant

Because of the dipolar ion structure of the amino acids, they are able to considerably increase the dielectric constant of the solvent in which they are dissolved (23-25). This increase is on such a high an order of magnitude that it is explicable only on the basis of the possession by these compounds of a large permanent electric moment.

The electric moments of the amino acids themselves cannot be determined by Debye's procedure because of their nonpolar solvents. This fact suggests the highly polar character of the amino acids.

The moment can be calculated from the spatial considerations on the basis of a dipolar ion structure. If the center of the positive charge is located on the a-nitrogen atom, and the center of the negative charge is midway between the two oxygens of the carboxyl group, the dipole distance is 2.96 A, and the electric moment is the product of the elementary charge on the electrons

4.8 X 10-10 esu by 2.96 X 10"8 cm = 14.2 X 10"18esu

The Debye relation is P = 4nN/u2/9 kT for compounds with high permanent dipoles. P is the molar polarization and fi is the electric moment. The term P is a linear function of the dielectric constant of the medium, and the dielectric constant is related to the square of the electric moment per molecule of the compound.

The electric moment of a compound such as /i-alanine would be expected from the Debye relation to be greater, and the effect on the dielectric constant of the medium would be expected to be greater than revealed by the isomeric compound a-alanine.

All «-amino acids produce an increase in the dielectric constant of water by about 23 per mol, which is independent of concentration. In the case of glycine, where the dielectric constant of the solution is 135, the increase is 2.5 mol/L. The dielectric increment per mole of solute, 3, is shown in the following equation:

For water at 25°C, the dielectric constant D0 is 78.54; 5 = 23 for a-alanine and S = 35 for ^-alanine.

The value of 6 at a pH in the range 4.5 to 7.5 for glycine was measured. 8 falls steeply on either side of this (isoelectric) pH range as the initial dipolar ions convert into either cations or anions with simultaneous loss of dipolar ion character.

The values of d for all a-amino acids range from 22.6 to 23.2; the values for the /i-amino acids are about 35.

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