Langmuir 1988,4,759-762
759
An Excitable Liquid Membrane Possibly Mimicking the Sensing Mechanism of Taste Kenichi Yoshikawa,* Masaru Shoji, Satoshi Nakata, and Shiro Maeda Department of Chemistry, College of General Education, Nagoya University, Nagoya 464-01, Japan
Hiroshi Kawakami Department of Electronic Engineering, Faculty of Engineering, University of Tokushima, Tokushima 770, Japan Received September 10, 1987. I n Final Form: January 6,1988 Studies have been made on electrical oscillations across a liquid membrane consisting of an oil layer, nitrobenzene, which lays between two aqueous layers, one of which contains a soap, sodium oleate or sodium stearate. The features of the oscillation, i.e., amplitude, frequency, frequency modulation, and shape of the pulses, change in a characteristic manner with the addition of various chemical species belonging to different taste categories. The change of the oscillatory pattern could be reproduced by a theoretical calculation based on a set of nonlinear kinetic equations.
Introduction In living systems, recognition of molecules, such as taste or smell, is essential for the maintenance of life. In taste or smell, the information on chemical structure and concentration of the chemical stimuli is converted into information of nerve impulses, though the detailed molecular mechanism is not well understood.1*2 Thus, if one wants to mimic the mechanism of chemical sensing in biological systems, the development of excitable or oscillatory system is necessary. Various types of artificial membranes with excitability have been reported, but to our regrets the reproducibility of the manner of electrical oscillations has been rather poor."' It is well-known that a self-amplification and self-organization of movements a t a fluid interface are caused by heat or mass transfer across the fluid interface? In general the movement of the interface due to this Marangoni instabilitys is depressed in the presence of a surfa~tant.~JOContrary to such an experimental trend, Dupeyrat and Nakache11J2found quasi-periodic variations of a relaxation type in the interface tension and the electrical potential in an oil/water system in the presence of a cationic surfactant. Recently, we found that sustained rhythmic oscillations are generated in a liquid membrane consisting of water/oil phases, where one of the aqueous phases contains a cationic surfactant.'"'' As the reproducibility of the oscillations in this liquid membrane was quite good, we could quantitatively study
the effect of various chemical species on the oscillation~.'"'~ We have reported that the frequency, amplitude, and shape of the electrical oscillations change markedly on addition of various chemical species to the aqueous phase. In the present study, we would like to report a new oscillatory system at a oil-water interface in the presence of a soap, sodium oleate or sodium stearate.
Experimental Section Experiments were performed in an apparatus with a U-shaped glass tube (12-mm i.d.). The apparatus is the same as in the A nitrobenzene solution (4 mL) of 5 mM previous 2,2'-bipyridine was placed in the base of the U cell. Aqueous solutions (10 mL each) were introduced simultaneously into the arms of the U cell above the organic phase, without stirring. All measurementswere carried out at 25 O C . The voltage across the liquid membrane was measured with a Hitachi-Horiba F-7 pH/mV meter connected by two salt bridges to two Ag/AgCl electrodes. In order to ascertain the reproducibility, at least three experimental runs were performed for each set of hed conditions. Essentially the same results were obtained for each experimental run. The interfacial tension between the organic and aqueous phases was measured by the same apparatus as described in the previous study,le with a Shimazu RMB-50 electronic balance. Sodium oleate and sodium stearate were available from Wako Pure Chemical Industries, LM. (Osaka).
Results and Discussion Effect of Alcohols on Oscillations. Figure 1shows the oscillations of voltage across a liquid membrane containing sodium oleate, Figure la-c, and sodium stearate, (1) Fundamentala of Seneory Phyeiology; Schmidt, R. F., Ed., Figure ld,e. The liquid membrane consisted of an oil layer, Springer-Verb Berlin, 1986. (2) Neurobiology of Taate and Smell;Finger, T. E.; Silver, W. L., W.; nitrobenzene-containing 5 mM 2,2'-bipyridine, imposed Wiley: New York, 1987. between two aqueous phases, 0.1 mM sodium oleate con(3) Teorell, T.3. Gen. Phy8iOl. 1969, 42, 831-845. (4) Shaahoua, V. E. Nature (London) 1967,215,846-847. (6) Mueller, P.;Rudm, D. 0. Nature (London) 1968, 217, 713-719. (6)Pant, H.C.; Roeenberg, B. Biochim. Biophys. Acta 1971, 226, 379-381. (7) Kobatake, Y. Adu. Cbm. Phys. 1976,29,319-340. (8) Linde, H.;Schwarte, P.; Wilke, H. Dynamics and Instability of Fluid Interfaces;Smnsen, T. S., Ed.;Springer-Verlag: Berlin, 1979; pp 76-119. (9) Lewie,J. B.;Pratt, H. €2. C. Nature (London) 19SS,171,11sCrll66. (10) Gamer, F.H.:Nutt, C. W.; Moutadi, M. F. Natwe (London) 1965, 175,603-606. (11) Duwyrat, M.: Nakache, E. Bioelectrochem. Bioenera. 1978, 6, - 134-141. (12) Dupeyrat, M.;Nakache, E. Synergetic8 Far from Equilibrium; Pocault, A.; Vidal, C., Ede.; Springer-Verb Berlin, 1979 pp 166-160.
0743-7463/SS/2404-0759$01.50/0
(13) Yoehikawa, K.;Mataubara, Y. J. Am. Chem. SOC. 1983, 105, 5967-5969. (14) Yoshikawa, K.;Mateubara, Y. Biophys. Chem. 1983,17,183-186. (16) Yoehikawa, K.;Mataubara, Y. J. Am. Chem. SOC. 1984, 106, 4423-4427. (16) Yoahikawa, K.;Omochi, T.; Matsubma, Y. Biophys. - . Chem. 1986, 23,211-214. (17) Yoehikawa, K.;Omochi, T.; Mataubara, Y.; Kourai, H. Biophys. Chem. 1986,24,111-119. (18) Yoshikawa,. K.:. Nakata.. 5.:. Omochi,. T.;. Collacicco, G. Lanamuir 1986,2,715-717. (19) Nakata, S.;Yoshikawa, K.; Iehii, T. Nippon Kagaku Kaishi (J. Chem. Soc. Jpn., Chem. Ind. Chem.) 1987,496-501.
0 1988 American Chemical Society
760 Langmuir, Vol. 4,No. 3, 1988
Yoshikawa et al.
5
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.-0
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.+
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30
60
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1
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1
I
30
60
90
120
150
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Figure 1. Oscillations of electrical potential with (a) 5 v/v % ethanol, (b, d) 5 v/v % 1-propanol, and (c, e) 5 v/v % 1-butanol in
the presence of (a, b, c) 0.1 m M sodium oleate and (d, e) 0.1 m M sodium stearate in the left aqueous phase.
taining (a) 5 v/v ?% ethanol, (b) 5 v/v ?% 1-propanol, or (c) 5 v/v % 1-butanol (on the left-hand side) and 0.5 M NaCl (on the right-hand side). We used 2,2’-bipyridine to reduce the impedance in the organic phase, which contributes to diminish the external noise. When the concentration of sodium oleate was decreased below 0.1 mM, the amplitude of the oscillations tended to decrease. When the concentration was increased above 0.1 mM, oscillations became irregular and were accompanied by a decrease in amplitude. It is interesting that the amplitude of the oscillations increased markedly with an increase in the hydrophobicity of the alcohols. Additionally, each modulation of the oscillations has its own characteristic pattern. Parts d and e of Figure 1show the oscillation of the electrical potential when sodium oleate is replaced by sodium stearate. It is interesting that the amplitude of the oscillations is decreased in the liquid membrane which contains sodium stearate, compared with that which contains sodium oleate. This result suggests that the stability of the monolayer of the surfactant between the organic and aqueous phases has a large influence on the manner of oscillations. The molecules of stearate can form a more rigid monolayer than the molecules of oleate, and they may be unsuitable for inducing the repeated construction and destruction of the monolayer, which is the key step in the oscillation, as will be discussed below. Mechanism of Oscillation. In order to obtain further insight into the mechanism of oscillation, the interfacial tension has been monitored. Into a cylindrical glass cell, with an inner diameter of 40 mm, was placed 10 mL of organic solution, and 10 mL of aqueous solution was added gently. Then, a platinum plate (9.8 X 20 X 0.03 mm), previously polished with 250-mesh emery, was placed through the interface of the organic and aqueous phases. Figure 2 exemplifies the periodic change of the interfacial
20 m i n
Figure 2. Rhythmic change of the apparent interfacial tension. Aqueous phase, 0.1 m M sodium oleate and 5 v/v % 1-butanol; organic phase, nitrobenzene-containing 5 m M 2,2’-bipyridine. tension, obtained by the Wilhelmy method, between the organic phase and the aqueous phase with oleate and butanol. As the interfacial tension is directly related to the concentration of surfactant at the interface,2othe rhythmic change of the interfacial tension clearly indicates that the concentration of oleate at the interface changes repeatedly between high and low values. We have also measured the rate of transfer of alcohol from the aqueous phase to the organic phase (data are not shown). We found that the concentration of 1-butanol in the organic phase increased at a relatively rapid rate during the initial 1&20 min (which corresponds to the induction period of the oscillation), during which time there was a rapid decrease of the potential. Then the rate of transfer gradually decreased throughout the oscillatory period. Taste Profile of the Liquid Membrane. We have examined the effect of chemical substances belonging to the four basic taste categories1i2on the manner of the electrical oscillation. We used the liquid membrane of nitrobenzene between two aqueous phases, one of which contained 0.1 mM sodium oleate and 10 v/v 9% 1-propanol. (20) Adamson, A. W. Physical Chemistry of Surfaces; Interscience: New York, 1960.
Langmuir, Vol. 4,No. 3, 1988 761
An Excitable Liquid Membrane
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Figure 3. Oscillations of electrical potential by the addition of 0.5 mL of (a) 0.1 M sodium chloride, (b) 1M sucroae, (c) 0.001 M quinine chloride, and (d) 0.03 M hydrochloricacid to the left aqueous phase. Other experimental conditions were the same as for Figure Ib, except that the concentration of 1-propanolwas 10 vfv %.
After about 1h from the construction of the liquid membrane, when the oscillations became stable, various chemical stimuli were added to the aqueous phase containing 1-propanol. Sodium chloride, sucrose, quinine chloride, and hydrochloric acid were examined as the typical substances of taste:'s2 salty, sweet, bitter, and sour, respectively. In the presence of sodium chloride, small rhythmic oscillations were generated together with giant oscillations of longer periods, see Figure 3a. When sucrose was added to the aqueous phase, the amplitude of the oscillations became irregular or chaotic, as shown in Figure 3b. Downward pulses were generated in the presence of quinine chloride, see Figure 3c, and small upward pulses were generated in the presence of hydrochloric acid, see Figure 3d. It is quite interesting to note that the oscillatory characteristics, such as the amplitude, frequency, modulation of amplitude, and/or frequency, and the shape of the pulses, change remarkably, depending on the nature of chemical stimulant. Theoretical Analysis of Oscillations. On the basis of the experimental results, together with the results of previous s t u d i e ~ , l ~the - l ~mechanism ~~~ for the sustained Oscillations in the liquid membrane with sodium oleate can be explained in the following kinetic model, considering the repetitive construction and destruction of the surfactant monolayer at the interface. Though we have previously proposed model equations for the oscillation,2' limit-cycle oscillations were obtained only for a quite narrow range of the change of parameters; that is, the oscillations were numerically rather unstable. In addition to this, "irregular oscillations" could not be reproduced in the simulation by using the old theoretical equations. Thus, we have developed new equations. Let X,Y, and Z be the concentrations of the key chemicals, Xi the concentration (21) Toko, K.; Yoahikawa, K.; Tsukiji, M.; Nosaka, M.; Yamafuji, K. Biophys. Chem. 1985,22,151-158.
1
t
Figure 4. Computer simulation of the electrical oscillation. Parameters: a2 = 0.14, bl = 0.2, b2 = 0.05, c1 = 0.4, c2 = 0.1, d = 5, e = 5; (a) al = 0.08, (b) al = 0.062, (c) al = 0.04, (d) a1 = -1.16.
of oleate near the interface, Yi the concentration of alcohol near the interface, and Zi the concentration of the aggregate or complex of oleate and alcohol at the interface. The following scheme can be considered as the mechanism to be correlated with the oscillation in the liquid membrane, where Xb and Yb mean the concentrations of oleate and alcohol in the bulk aqueous phase, respectively. r'
xb
,----.
7 xi
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't
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bulk organic phase
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The system kinetics may be considered under the following assumptions: (a) The concentrations of oleate and alcohol in the bulk aqueous phase remain constant. (b) The rates of diffusion of oleate and alcohol from the bulk aqueous phase to the interface are expressed as al - b1Xi - clZi and a2 - b2Yi - c2Zi,respectively. The terms al - blXi and a2 - bzYi denote the transfer of surfactant and alcohol due to the concentration gradient between the bulk aqueous phase and interface of X and Y, respectively, including the rate of change from Xi and Yi into Zi. The third term represents the effects of the negative feedback induced by the increase of Zi. (c) The rate of formation of Zi from Xi and Yi is expressed as a function F(Xi,Yi). (d) The rate of escape of Zi into the bulk organic phase is expressed as a function G(Zi). Under these assumptions, the kinetics of the migrations of oleate and alcohol are written by the differential equations dXi/dt = al - blXi - clZi (la) dYi/dt = a2 - bzYi - c2Zi dZi/dt = F(Xi,Yi) - dG(Zi)
Ob)
(14 The term F(X,Yi) is ordinary considered to be a complicated function of Xi and YP In order to avoid intro-
762
Langmuir 1988,4,762-765
ducing too many parameters and to make clear the intrinsic nature of oscillations, F(Xi,Yi)is given by e(Xi+Yi), a simple form for the synergetic effect of Xi and Yi in the present paper. Coefficients ai, bi, ci, d , and e are parameters. Self-oscillatory states can be obtained if G(Zi) has “N”-shape nonlinearity. To express the N-shape nonlinearity, we adopt G(Zi) as a simple third-order function
G(ZJ = (-Zi
+ Z:)/3
(2)
According to the two-dimensional van der Waals equation for the surfactant molecules at the interface,20an N-shape relationship for the surface pressure, n, vs concentration of the surfactant molecules is expected. Thus,we estimate that the rate G(Zi! possesses N-shape characteristics. Figure 4 exemphfies the numerical results of the equations, where only the parameter al is changed successively, from part a to d, while the other parameters remain constant. It is quite interesting to note that the numerical results in Figure 4a-d correspond well to those of the experimental trends shown in parts c, b, d, and c of Figure 1, respectively. This means that various patterns of the
oscillations shown in Figures 1-3 are, for the most part, attributable to the change of the r a t e of diffusion of oleate and/or alcohol. The change of the diffusion rates should be induced by the chemical substances added to the aqueous phase. Though the experimental basis of the parameters is not yet satisfactorily resolved, it may be expected that the intrinsic nature of the oscillationsin the liquid membrane can be interpreted within the context of the above calculations. The observed change of the electrical potential may also be explained, taking into account the dependence of the potential on the concentration of the surfactant molecules at the interface.15
Acknowledgment. We thank Dr.T. Ishii and Prof. C. O’Connor for helpful suggestions. This work was partly supported by a Grant-in-Aid for Scientific Research to K.Y.from the Ministry of Education, Science and Culture of Japan, the Shimadzu Science Foundation, and the Inamori Science Foundation. Registry No. NaCI, 7647-14-5; HCl, 7647-01-0;nitrobenzene, 98-95-3; sodium oleate, 143-19-1;sodium stearate, 822-16-2; ethanol, 64-17-5; 1-propanol, 71-23-8; 1-butanol, 71-36-3; sucrose, 57-50-1; quinine chloride, 60-93-5.
Adsorption of Transfer Ribonucleic Acid on a Stationary Mercury Electrode M. K. Kaisheva* Department of Physical Chemistry, The University of Sofia, Sofia 1126, Bulgaria
M. Mataumoto, Y . Kita, and T. Takenaka Institute for Chemical Research, Kyoto University, Uji, Kyoto 611, Japan Received August 24, 1987. In Final Form: November 30, 1987 The influence of the double layer on the adsorption of transfer ribonucleic acid, phenylalanine-specific (tRNApb”),from yeast has been investigated by measuring the differential capacity at the stationary mercury electrode-aqueous solution interface. The concentration of tRNAPhevaried from 2 X lo* to 1.5 X lo4 mol ~lm-~. The supporting electrolyte was a 0.05 mol dm-3aqueous solution of Na@Ok Equilibrium capacity values were obtained in the interval of electrode potentials from 4 . 5 to 0.6 V vs the potential of zero charge in supporting electrolyte. Adsorption parameters of tRNAPb”were estimated on the basis of an ex rimental sample of equilibrium capacity values for concentrationsof tRNAPhehigher than lo4 mol dm- For that purpose an analytical model of differential capacity based on the Flory-Huggine isotherm was applied, which proved to be adequate for the experimental sample.
P.
Introduction Many biochemical reactions in which nucleic acids take part occur a t the charged interfaces in a living cell. The mercury electrode has been used by different authors‘-5 as a model for studying the interaction of nucleic acids with charged surfacea. It is convenient for that purpose because of the methods developed for control of its electrical parameters and for studying the properties of the double layer, such as differential capacity, etc., sensitive to the presence of nucleic acids. The only nucleic acid, for which the crystal structure has become recently known is tRNAPh”from brewer’s yeast.H The length of the molecule is about 6 nm, and it has 76 nucleotides and L-shape in crystals. An important problem is the investigation of the adsorption of tRNApb from solution, where it is biologically active and ita molecule is
* Author to whom correspondence should be addressed. 0743-7463/88/2~04-0762$01.5OI O
dynamic. There is evidencelo that a t certain conditions in solution the two ends of the L-shaped molecule come together and it becomes more compact. However, the (1) Hinnen, C.; Rousseau, A.; Parsons, R. J. Electroanal. Chem. 1981, 125, 193. (2) Miller, I. R. J. Mol. B i d . 1961, 3, 229. (3) Berg, H.; Horn, G.; Flemming, J.; Glezers, V. Bioelectrochem. Bioenerg. 1977, 4, 464. (4) Malfoy, B.; Reynaud, J. A. J. Electroanal. Chem. 1976, 67, 359. (5) Reynaud, J. A.; Malfoy, B.; Sequaris, J. M.; Sicard, P. J. Bioelectrochem. Bioenerg. 1977,4, 380. (6) Kim, S. H.;Suddath, F. L.; Quigley, G. J.; McPherson, A.; Suss-
man,J. L.; Wang, A.
D.C.)1974,185,435.
H.J.; Seeman, N. C.;Rich, A. Science (Washington,
(7) Robertus, J. D.; Ladner, J. E.; Finch, J. T.; Rhodes, D.; Brown, R. S.; Clark, B. F. C.; Klug, A. Nature (London) 1974,250, 546. (8) Rich, A.; RajBhandary, U. L. Annu. Rev. Biochem. 1976,45,806. (9) Sussman, J. L.; Holbrook, S. R.; Warrant, R. W.; Church, G. M.; Sung-Hou, Kim J. Mol. B i d . 1978, 123, 607. (IO) Olson, T.; Fournier, M. J.; Langley, K. H.;Ford, N. C. J.Mol. Biol. 1976, 102,193.
0 1988 American Chemical Society
