T H E DIELECTRIC ABSORPTION AND DIELECTRIC CONSTANT OF SOLUTIOKS OF ALIPHATIC AMINO ACIDS HUGO FRICKE
AND
ADOLF PARTS
Walter R . J a m e s Laboratory f o r Biophysics, T h e Biological Laboratory, COId Spring Harbor, Long Island, N e w Y o r k Received J u l y 20, 1398
Lintiart (15) has reported a region of anomalous dispersion in solutions of glycine and certain other amino acids at wave lengths around 1 meter. The principal subject of this paper is a study of the dielectric absorption of solutions of some of the aliphatic amino acids on the long wave length side of this region. In the range of wave lengths used the dielectric constant is still practically constant. The values of dielectric absorption obtained by us are considerably smaller than would be expected from Linhart’s measurements, and his conclusion that the relaxation time is that of a structure many ti1nr.s greater than the amino acid molecule is not confirmed by our work. EXPERIXENTAL PROCEDURE
The method consists in balancing, in a resonance circuit, two identical microelectrolytic cells, one of which contains the amino acid solution and the other a 5olution of potassium chloride. The cells are balanced by changing the concentration of potassium chloride and by varying a parallel condenser. ’The experimental arrangement is shown in figure 1. The apparatus contain3 tn-o interchangeable push-pull valve generators, G, one of which can be operated bctween 2.05 and 16.4 megacycles, and the other between 16.4 and 65.6 megacycles. The rcsonance circuit d contains the interchangeable inductance L, the variable condenser C, the niicro condenser Ct and the electrolytic cell E. The electrolytic cell is inserted into the circuit at PI and PZby banana plugs and jacks. Part of the resonance circuit is surroundcd by the tight grounded copper shield S,but C, and E are placed outsidc thc shield in order to makc these units easily accessible. The axis of C is extcndPd with a hard rubber rod M hich passes through the shield so that it is possible to vary C without opening the shield. Ct contains two platrs, 2.5 cm. in diameter. The capacity is varied by varying the distance betneen the plates, for which purpose one of the plates is mounted on a micrometer screw. In the position where Ct is usually used, its capacity can be read to an accuracy ol 0.001ppf. 1171
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HUGO FRICKE AKD ADOLF PARTS
A is tuned to resonance by varying C , which is actuated at a distance by a pulley and thread arrangement. The point of resonance and the intensity of the oscillating current are recorded by the tube volti-neter T. By an arrangement not shown in figure 1, the cell and the microcondenser Ct can be introduced into one arm of a resistance-capacity bridge, allowing measurements to be made between 0.25 and 2048 kilocycles. The generators were calibrated by means of a cathode ray oscillograph. An auxiliary oscillator was adjusted to a frequency of 2.05 megacycles by means of a wavemeter. Through an inductive coupling, this oscillator impressed an oscillating potential on one of the pairs of plates of the oscillograph, while the other pair of plates was connected to the resonance circuit at PI and Pz. With the resonance circuit kept tuned, frequencies of 2.05,
- - - --.-IVALVE GENERATORS
G
. ..
FIG.1. Diagram of resonance apparatus FIG.2. Electrolytic cells
4.10, 8.19, and 16.4 megacycles were now established on one of the generators by the well-known method of producing certain types of stationary patterns on the oscillograph. This generator was thereafter used (at 16.4 megacycles) instead of the 2.05-megacycle oscillator for calibrating the other generator, which was used at 16.4, 32.8, and 65.6 megacycles. The construction and mounting of the cells (No. 3) chiefly used in the present work are shown in figure 2. The cell has the form of a test tube 1.4 cm. in diameter. The electrodes are of platinum, 4.5 mm. in diameter and approximately 4 mm. apart. Connection through the glass is made by platinum wire 0.75 mm. in diambter and 6 mm. long. These wires connect to the brass rods (a) which carry the banana plugs (b). The whole unit is kept rigid by means of two narrow strips of S'ictron (c). This material has excellent dielectric properties, and tests showed that the presence of the strips introduced no appreciable clamping in the circuit.
DIELECTRIC ABSORPTION OF AMINO ACIDS
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X small condenser, C,, is placed across the cell, rigidly attached to the cell mounting. This condenser consists of a stationary plate and a movable plate, mounted on a small micrometer screw. The plates are 8 mm. in diameter, and the leads are 3 mm. in diameter. The condenser cannot be read directly with sufficient accuracy, but each time it is reset the change in its capacity is recorded by measurement a t low frequency. At the frequencies and conductances used in the present work, the influence of electrode polarization is negligible, even when blank electrodes are used. However, the electrodes of the cells were platinized in order to make it possible to use them a t low frequencies also. For the purpose of control, certain of the measurements were also made with two other types of cell (cells No. 1 and No. 2), shown in figure 2. The dimensions of these cells will be clear from the figure. The measurements were made in a room kept at a constant temperature, which was 21.0"C. when not otherwise indicated. In order to make a measurement on an amino acid solution the procedure is as follows: The condenser, C,, on the cell with the potassium chloride solution, referred to as the K cell, is adjusted so that the capacities of the filled cells are nearly alike. C, on the cell containing the amino acid solution (the A cell) is set a t zero. The A cell is placed in the resonance circuit and the reading on the galvanometer of the tube voltmeter, at resonance, is recorded. The condition of resonance is established by varying Ct. The A cell is now replaced by the K cell, and the concentration of potassium chlcride is changed until the reading on the tube voltmeter at resonance is the same as before. The low-frequency conductance of the potassium chloride solution is thereafter measured with the bridge. At the same time, the lowfrequency conductance of the amino acid solution is also recorded, to make it possible to correct for slight changes in the conductance of this solution, caused by temperature fluctuation, etc. In actual practice the potassium chloride is adjusted twice, to give readings on the tube voltmeter which are slightly lower and slightly higher than the readings obtained for the amino acid solution. The low-frequency conductance of each of these two potassium chloride solutions is recorded, and by interpolation the lowfrequency conductance of the potassium chloride solution which exactly balances the amino acid solution is obtained. The difference in settings on Ct, with the amino acid solution and the potassium chloride solution, respectively, in the circuit, must not exceed a certain value, known from preliminary measurements. If the difference is too high, C, is reset and the comparison repeated. In the range of frequencies used, the dielectric constant of the aniino acid solution changes so little with frequency that we can use the same setting of C, at all the different frequencies. A typical resonance curve, uiz., readings on the tube voltmeter against
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readings on Ct obtained a t 65.6 megacycles, is shown in figure 3. The curve was obtained at a Conductance of 33 X lo-'' ohm-'. The resonance curves are perfectly symmetrical, showing that there is no appreciable interference of the resonance circuit with the generating circuit. The dielectric constant ea(w) and the conductivity K J W ) of the amino acid solution are obtained from the equations:
eo(@) and K,(w) are the dielectric constant and conductivity, respectively, of the potassium chloride solution a t the frequency w; k is the cell constant, defined on the basis of electric conductance measurements; AC is the differ-
FIG.3
FIG.4 FIG.3. Hesonance curve FIG.4. Diagrummatical representation of celi assemblage
ence In condenser readings (in ppf) when the K cell and the A cell are in the circuit. Under the experimental conditions used, the dielectric constant of the potassium chloride solution can be taken as equal to that of water. The electric conductance of the potassium chloride solution is derived from the ionic conduction and from the dielectric absorption of the solvent. At the concentrations used, the ionic conductance can be taken as independent of the frequency. We can therefore write
where K: is tjhe specific electric conductance of water due to its dielectric absorption. K.(o) is the low-frequency specific electric conductance of the potassium chloride solution, obtained directly in the measurement. We introduce K:
=
K=(W)
- Ka(0)
DIELECTRIC ABSORPTION OF AMINO ACIDS
1175
as the difference between the specific electric conductance of the amino acid solution at high and at low frequency, and we obtain:
DISCUSSIOK OF METHOD
The validity of equations 1 and 2 is based on the following assumptions: (1) The consumption of electric energy at thc surface of the electrodes (electrode polarization) is negligible. (2) The inductance of the cell assembly can be neglected. (9) Electric energy flows to and from the fluid by way of the stray capacities which connect the different elements of area of the enveloping surface of the fluid with each other and with the leads t o the cell. This flow of energy can be neglected in the comparison of the two cells. That these requirements are met to a sufficient extent under the experimental conditions used is shown by various kinds of control measurements described below. The influence of the stray capacities is comparatively small because of the high dielectric constant of the fluids investigated. The following considerations are presented for the purpose of elucidating the influence of this source of error. Although the cell assembly cannot be strictly represented by any simple network of conductances and capacities (inductance being neglected), for the present purpose we can use the representation shown in figure 4. In this diagram (C, U ) represents the admittance elements which solely involve the fluid.
C1 represents the admittance elements (capacities between the leads to the cell) which solely involve the dielectric (mainly air) surrounding the fluid and (CmCtuJrepresents the admittance elements which involve partly the fluid and partly the dielectric. Cmis independerlt of the fluid and we can write
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where k' is a constant;dependelit on the geometry of the cell assemblage. If we represent the complex admittance of the network (Cn,C,uL)by
l/z =
ut
-+ jC'W
then we obtain
When the frequency is so high that
lo-'5d>i >> 1
=K.$3(
we obtain
Because of the influelice of (CmC,uz),the values of the specific electric conductance and the dielectric constant of the amino acid solution calculated from equations 1 and 2 are too high by the amounts A K and ~ As, respectively: Ah, =
- Gb.,)k
( G : = ~ ~
Axa and ilea are zero when the frequeiicy is so low (or the electric conductance
of the fluid so high) that
(2x
??x lo-'zw)z
