Magnetoresponse in Electrical Properties of Black LipidMembranes

Sumio Ozeki,*,† Hutoshi Kurashima,† Mamiko Miyanaga,‡ and Chie Nozawa‡

Department of Chemistry, Faculty of Science, Shinshu University, 3-1-1 Asahi, Matsumoto,Nagano 390-8621, Japan, and Department of Chemistry, Faculty of Science, Chiba University,

1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

Received September 14, 1999. In Final Form: December 6, 1999

The membrane potential and resistance of black lipid membranes (BLM), comprising didodecyl phosphiteor dipalmitoylphosphatidylcholine, markedly changed up to -50% and 4%, respectively, due to steadymagnetic fields in the region of 0.2 T. The magnetic-field effects on the electrical properties seem to occurnot via the Lorentz force on the ion flux but via the cooperative orientation of lipid molecules. Thus, theaddition of molecules having different magnetic anisotropy to a BLM modified the magnetoresponses ofthe membrane. These examples demonstrate that relatively low magnetic fields may regulate the electricalproperties of lipid membranes.

The effects of magnetic fields on living organisms havebeen investigated from many points of view, such as thestructural changes of organisms,1-8 catalytic activities,9-11

and material transport.12,13 Biological cells contain manycomponents and elementary processes for molecularsyntheses, mass transport, and metabolism. Therefore,the results reported concerning the magnetic responsesof such complex biological systems may often be obscureor contradictory. To elucidate the magnetic effects onbiosystems, those on each elementary process should beexamined. For this purpose, an artificial lipid bimolecularmembrane, the so-called black lipid membrane (BLM),may be used as a model for biomembranes, although athermodynamical BLM must be too simple. Here, wepresent the great effects of relatively low, steady magneticfields (MF, <0.5 T) on the membrane potential (Ψ) andresistance (R) of BLMs of didodecyl phosphite (DP) anddipalmitoylphosphatidylcholine (DPPC). Such significantmagnetic responses in membrane functions are notfamiliar, although it is well-known that diamagnetic lipidassemblies as well as liquid crystals and macromoleculestend to align under steady magnetic fields. 4-8,14

Figure 1 shows typical examples of changes in Ψ andR of a BLM of DP with the application of MFs (H)

perpendicular to the membrane. The BLM in aqueoussolutions was formed at 298 ( 0.1 K for DP and 311 ( 0.1K for DPPC in the hole (ca. 0.8 mm diameter) of a thinTeflon sheet (0.5 mm thickness) which divided the Tefloncell into two compartments.15 Ψ arising across a BLM bymaintaining a 10-fold difference in the NaCl concentration(1 × 10-3 and 1 × 10-2 mol/dm3) was measured with acouple of Ag-AgCl electrodes. The dilute side of thesolution was taken as being negative. R of a BLM in a 1× 10-3 mol/dm3 NaCl solution was measured by means of

* To whom all correspondence should be addressed. TEL: 81-263-37-2567.FAX: 81-263-37-2559. E-mail: sozeki@ripms.shinshu-u.ac.jp.

† Shinshu University.‡ Chiba University.(1) Ramon, C.; Ayaz, M.; Streeter, D. D., Jr. Bioelectromagnetics 1981,

2, 285.(2) Ramon, C.; Martin, J. T.; Powell, M. R. Bioelectromagnetics 1987,

8, 275.(3) Biophysical Effects of Steady Magnetic Fields; Maret, G., Boccara,

N., Keipenheuer, J., Eds.; Springer-Verlag: Berlin, 1986; pp 2-51.(4) Maret, G.; Dransfeld, K. Physica 1977, 86-88B, 1077.(5) Hong, F. T. J. Colloid Interface Sci. 1977, 58, 471.(6) Seeling, J.; Borle, F.; Cross, T. A. Biochim. Biophys. Acta 1985,

814, 195.(7) Bragonza, L. F.; Blott, B. H.; Coe, T. J.; Melville, D. Biochim.

Biophys. Acta 1984, 801, 66.(8) Tenforde, T. S.; Liburdy, R. P. J. Theor. Biol. 1988, 133, 385.(9) Haberditzl, W.; Muller, K. Z. Naturforsch. 1965, 20b, 517.(10) Haberditzl, W. Nature 1967, 213, 72.(11) Ueno, S.; Iwasaka, M. J. Appl. Phys. 1996, 79, 4705.(12) Okazaki, M.; Maeda, N.; Shiga, T. Eur. Biophys. J. 1987, 14,

139.(13) Higashi,T.;Yamagishi,A.;Takeuchi,T.;Date,M. Bioelectrochem.

Bioenerg. 1995, 36, 101.(14) Helfrich, W. Phys. Lett. 1773, 43A, 409; Z. Naturforsch. 1973,

28C, 693. (15) Khan, H. R.; Ozeki, S. J. Colloid Interface Sci. 1996, 177, 628.

Figure 1. Typical examples of the time course of changesin (A) membrane potential Ψ and (B) resistance R of a blacklipid membrane of didodecyl phosphite with the applicationof various magnetic fields perpendicular to the membrane at298 ( 0.1 K. The values at H ) 0 in these charts were -24.5mV and 526 MΩ.

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two platinized Pt electrodes. Ψ and R signals from aKeithley multimeter 2001 (input impedance >1010 Ω) werestored in a personal computer. BLM-forming solutionswere prepared by mixing 20 ppm of DP or DPPC with (3+ 2) octane-dodecane. The general features in themagnetic response (∆f ≡ ∆fH - f0, where f is |Ψ| or R, andthe subscripts 0 and H mean H ) 0 and H, respectively)are its reversibility, the maximum response (∆fmax) ataround 0.2 T (H ) Hc), and the reverse response at H >0.4 T, as shown in Figure 1. Ψ for DPPC membraneschanged by ca. 2 mV, even at 0.025 T. The reproducibilityof the trends in the magnetic response was fairly good(e.g., Figure 1B), besides some differences in their absolutevalues from run to run. The changing rate (100∆f/f0) inΨ and R reached more than -50 and 4% (13 mV and 20MΩ) at 0.2 T and 20 and -2% at 0.45 T. These magneticresponses seem to be notably large from the viewpointthat, for example, the magnetic field theoretically requiredto produce a 10% reduction in the conduction of a nerveimpulse was roughly 24 T.16

We assume that MFs should modify the apparent fixedcharge density (σ) of the membrane, besides how magneticfields affect σ, e.g., a modification of the distribution ofsmall ions around the headgroups and/or in the membraneand inherent charge-bearing ability of headgroups. Todiscuss qualitatively from the viewpoint of σ, a theory17

for a thick membrane was applied to our systems because,in our knowledge, no theory exists for a BLM. The featuresin the experimental magnetic responses of Ψ and R forDP membranes seem to be consistent with the theory (eqs11 and 13 in ref 17): For σ (10-3 mol/dm3) ) 6.4 at H )0, 4.3 at H ) Hc, and 7.4 at H ) 0.45 T, Ψ (mV), R (MΩ)are estimated to be -20, 520; -10, 546; and -24, 503,respectively. The changing rates in Ψ and R from theestimated values are -50 and 5% at Hc and 20 and -1.9%at 0.45 T, respectively, which are consistent with theexperimental rates of -50 and 4% at Hc and 20 and -2%at 0.45 T. Thus, the σ values may be used as a measurefor an effective fixed charge density of the BLM. Figure2A shows that the estimated σ changed with MF througha minimum (σmin) at around 0.2 T.

When a magnetic field was parallel to the lipidmembrane, i.e., perpendicular to the direction of ion flow,little magnetic effect was observed, suggesting that anion flow would be not directly affected by the Lorentz force.Thus, the change in σ due to magnetic fields may bebrought about by the molecular orientation, which leadsto changes in the molecular density (area per molecule)at the membrane surface. Since the magnetic orientationalenergy of a diamagnetic lipid domain containing Nmolecules (volume Nv), whose long molecular axis is atan angle φ to H, is given by -(H2/2)(ø⊥ + ∆ø cos2 φ)Nv,lipid molecules in a domain may cooperatively align inthe direction of the averaged φ according to the thermalenergy.7 If H is large enough, the long molecular axis tendsto align in a direction perpendicular to H (φ ) π/2) becauseof the negative diamagnetic anisotropy (∆ø ) ∆ø| - ∆ø⊥,where ∆ø| and ∆ø⊥ are the magnetic susceptibility paralleland perpendicular to H). Under 1 T and 298 K, significantorientational effects are expected when N is on the orderof 107 for ∆ø ∼ -1 × 10-6 (for lipids), at which point themagnetic orientational energy is comparable to thethermal energy if other energies associated with orien-tational changes, such as a surface energy, an intermo-lecular interaction energy, and an elastic energy for

membrane deformation, are neglected. It is possible thatsuch a domain exists in a used BLM comprising 1012

molecules, as is known in insoluble monolayers.18

When a lipid molecule tilts under a MF, the occupiedmolecular area increases monotonically with φ, and thusσ would also decrease monotonically with MF. On theother hand, with increasing φ, the hydrocarbon/waterinterface at the membrane surface should increase anddestabilize the tilted structure: The critical tilt angle (φc)must exist. Then, one possible way to increase σ or |Ψ| athigher MF than Hc would be to introduce membranedeformation in and out of a plain surface, which wouldlead to a reduction of the hydrocarbon/water interfacialenergy and to relax the orientational defects (amongdomains having different orientation at φ), respectively.The former arises from a small displacement of theheadgroups in the direction perpendicular to the surface,like a nematic liquid crystal; the latter, similar to theripple structure, arises from undulation of a membranedue to high magnetic fields (probably larger than a fewtesla, referring to the liposome case8). We found that DPPCliposomes, which were cooled from 318 to 303 K under ahigh magnetic field (11.7 T), showed a H NMR (500 MHz)pattern very similar to its ripple phase. From theseconsiderations, a possible model for the magnetic re-sponses under low magnetic fields is depicted schemati-cally in Figure 2B.

According to the model, a certain modification in themagnetic anisotropy of a membrane or domain would leadto changes in the magnetic responses of the electricalproperties. The addition of cholesterol to a DP membrane

(16) Wisco, J. P. Jr.; J. P. Barach, J. P. IEEE Trans. Biomed. Eng.1980, BME-27, 722.

(17) Ueda, T.; Kamo, N.; Ishida, N.; Kobatake, Y. J. Phys. Chem.1972, 76, 2447.

(18) Kajiyama, T.; Zhang, L.; Uchida, M.; Oishi, Y.; Takahara, A.Langmuir 1993, 9, 760.

Figure 2. (A) Apparent fixed charge density σ of the BLMestimated from the Ψ values (Figure 1A) as a function of themagnetic field (H). σ changes with the magnetic field througha minimum (σmin) at Hc. (B) A possible model for the magneticresponses in the membrane potential and resistance is depictedschematically. Because the long axis of a lipid molecule tilts byan angle of φ to a magnetic field, the occupied molecular areaat the membrane surface increases monotonically with φ, butthe increase in the hydrocarbon/water interface leads to thecritical tilt angle (φc) at Hc. A further increase in H induces acertain surface roughness to lead to an increase in σ.

Letters Langmuir, Vol. 16, No. 4, 2000 1479

shifted Hc to a lower MF along with an increase in thecholesterol content, as shown in Figure 3; on the otherhand, undecyl calix arene (UCA) did not affect Hc. Thelarge magnetic anisotropy of cholesterol assists the

magnetic orientation of DP molecules even under lowmagnetic fields; however, undecyl chains of UCA buriedin a membrane (a headgroup having eight hydroxyl groupstend to be at the membrane surfaces) should have noinfluence on the magnetic orientation of the dodecyl chainsof DP. Also, when aramethicin, a helical oligopeptide (∆ø> 0) which will align in parallel to a MF, was added to anaqueous phase of the cell during Ψ measurements, Ψchanged significantly only under a MF: ∆Ψ at 0.2 T was20 times larger than that at no MF. These examplessuggest that molecules having various magnetic anisotro-pies can regulate the magnetic response of the electricalprocesses in membrane systems.

The great magnetic responses described here in theelectrical properties are a remarkable example and seemto be the result of a cooperative molecular behavior. Theseresults demonstrate that relatively low MFs might affectat least some elementary electrical processes in biosys-tems, such as synapse and receptor potentials havingseveral tens of millivolts. Also, as is illustrated in the lastexamples, the magnetic regulation of the functions ofartificial organized membranes, such as LB membranes,may be related to molecular devices and sensors.

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Figure 3. Magnetoresponses in the membrane resistance ofDP membranes containing cholesterol at 298 K. Cholesterolcontent (mol %): O, 0; 2, 25; 0,65.

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