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Light-Driven Transmembrane Ion Transport by Spiropyran-Crown Ether Supramolecular Assemblies Rafail F. Khairutdinov*,† and James K. Hurst‡ Department of Chemistry & Biochemistry and Center for Nanosensor Technology, University of Alaska Fairbanks, Fairbanks, Alaska 99775-6160 and Department of Chemistry, Washington State University, Pullman, Washington 99164-4630 Received September 9, 2003. In Final Form: December 10, 2003 Spiropyrans undergo reversible photoisomerization between their ring-closed (spiro) and ring-open (merocyanine) forms when exposed alternately to ultraviolet and visible light. Rates of K+ ion leakage from phosphatidylcholine unilamellar vesicles containing a K+-selective crown ether attached to an amphiphilic spiropyran increased markedly when the vesicle assembly was illuminated with UV light. This effect was fully reversible, with K+ leak rates returning to their basal levels upon illumination with visible light. In contrast, UV-induced photoisomerization of a similar spiropyran-crown construct that did not strongly coordinate K+ was not accompanied by enhancement of K+ leakage. Transient spectrophotometry revealed that, immediately following photoisomerization, the merocyanine form of the dye was in a moderately polar environment, consistent with its location in the glycerophosphate backbone region of the vesicle. Within several milliseconds, the polarity of the environment increased, as indicated by a hypsochromic shift in the merocyanine visible absorption band. This environmental relaxation within the vesicle is similar to the behavior of simple vesicle-bound spiro compounds that lack an appended crown ether and has been attributed to relocation of the dye toward the aqueous-organic interfacial region of the membrane following its conversion to the more polar merocyanine form. The enhanced K+ leak rate attending photoisomerization is therefore suggested to be a consequence of this relocation, thereby giving the tethered crown ether proximity to K+ ions located in the aqueous core and bulk aqueous phases. Following either photoinitiated or thermal ring closing, the dye-crown construct again becomes confined to the membrane interior, terminating merocyanine-mediated K+ transport. The transport quantum yield, defined as the number of K+ ions released per photon absorbed, is ∼0.3.
* Address correspondence to this author. E-mail:
[email protected]; (907) 474-7654 (voice); (907) 474-5640 (fax). † University of Alaska Fairbanks. ‡ Washington State University.
Lipid bilayer membranes are impermeable to small ions. Ion diffusion across cell membranes is facilitated both by ion-selective carrier molecules and by ion channels.11 In carrier complexes, the ion is placed in a polar pocket surrounded by a nonpolar envelope, thereby increasing its lipophilicity. Azobenzene and its derivatives attached to ion binding molecules have been extensively studied as model photoswitching compounds.10 Photocontrol of ion transport in these systems can arise from changes in the ion-binding affinities of the carrier moieties that accompany cis-trans photoisomerization. Alternatively, structural changes associated with isomerization of the intercalated dye can lead to disruption of the normal bilayer structure, promoting electrolyte leakage by passive diffusion.12 Spiropyrans and spirooxazines attached to crown ethers have also been used for light-controlled ion transport through polymers and bulk liquid membranes.13-16 UV illumination causes the spiropyran (SP) ring of a spiropyran-crown ether conjugate to open, converting it to its more polar merocyanine (MC) form (Scheme 1), with a consequent increase in transport rates.17,18 This effect
(1) Lehn, J.-M. Supramolecular Chemistry: Concepts and Perspectives; VCH: Weinheim, 1995. (2) Ikeda, T.; Sasaki, T.; Ichimura, K. Nature 1993, 361, 428-430. (3) Sackmann, E. Science 1996, 271, 43-48. (4) Cornell, B. A.; Braach-Maksvytis, V. L. B.; King, L. G.; Osman, P. D. J.; Raguse, B.; Wieczorek, L.; Pace, R. J. Nature 1997, 387, 580583. (5) Khairutdinov, R. F.; Hurst, J. K. Nature 1999, 402, 509-511. (6) Cattrall, R. W. Chemical Sensors; Oxford University Press: Oxford, 1997. (7) Ikeda, T.; Tsutsumi, O. Science 1995, 268, 1873-1874. (8) Gobbi, L.; Seiler, P.; Diederich, F.; Grammlich, V. Helv. Chim. Acta 2000, 83, 1711-1723. (9) Borisenko, V.; Burns, D. C.; Zhang, Z.; Woolley, G. A. J. Am. Chem. Soc. 2000, 122, 6364-6370. (10) Malval, J.-P.; Gosse, I.; Morand, J.-P.; Lapouyade, R. J. Am. Chem. Soc. 2002, 124, 904-905.
(11) Van Winkle, L. J. Biomembrane Transport; Academic Press: New York, 1999. (12) See, e.g., Lei, Y.; Hurst, J. K. Langmuir 1999, 15, 3424-3429, and references therein. (13) Kimura, K.; Yamashita, T.; Yokoyama, M. J. Chem. Soc., Perkin Trans. 2 1992, 613-619. (14) Sakamoto, H.; Yokohata, T.; Yamamura, T.; Kimura, K. Anal. Chem. 2002, 74, 2522-2528. (15) Kimura, K.; Sakamoto, H.; Nakamura, M. Bull. Chem. Soc. Jpn. 2003, 76, 225-245. (16) Inouye, M.; Ueno, M.; Tsuchiya, K.; Nakayama, N.; Konishi, T.; Kitao, T. J. Org. Chem. 1992, 57, 5377-5383. (17) Durr, H.; Bouas-Lauren, T. H. Photochromism: Molecules and Systems; Elsevier: Amsterdam, 1990. (18) Proceedings of the 1st International Symposium on Organic Photochromism. Mol. Cryst. Liq. Cryst. A 1994, 246, 1.
Introduction One focal point in advanced materials research has been the use of microphase-organized integrated chemical systems to accomplish specific tasks.1-5 Many of these applications require molecular switches, that is, systems that possess two or more reversibly interconvertible states. Photoresponsive membrane-based ion gating devices are particularly interesting because they have numerous potential applications to optical sensors, information storage, energy conversion and storage, and optobioelectronic devices.1,6,7 Nonetheless, only a few fully light-driven molecular switches have been reported.8-10 Here, we describe a new mechanism for photocontrol of transmembrane ion transport involving modulation of the spatial separation of ions and ion carriers attending photoinitiated opening of a spiropyran ring.
10.1021/la035683l CCC: $27.50 © 2004 American Chemical Society Published on Web 01/24/2004
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Scheme 1. Ring Opening-Closing Isomerization Reactions of 1,4,7,10-tetraoxa13-azacyclopentadecane, 13-[(3′,3′-dimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-[2H]indol]-1′(3′H)-yl)acetyl]a
a SP-Cr5 refers to the ring-closed (spiro) form of the dye in the conjugate and MC-Cr5 refers to the corresponding ringopen (merocyanine) form of the dye.
has been attributed to increased ion-binding affinities of the crown ethers in their MC isomeric states leading, in polymers, to increased efficiency of site-to-site ion hopping and, in bulk liquid membranes, to increased ion solubilization within the organic phase that constitutes the barrier to ion diffusion.14,15 In the present system, UVinduced SP6MC photoisomerization of the spiropyran moiety of the vesicle-bound conjugate also increases transmembrane K+ diffusion. As described herein, control studies using analogues incapable of strongly coordinating K+ are incompatible with this effect arising from simple disruption of the membrane bilayer structure. Rather, it appears that displacement of the dye-crown ether conjugate from the membrane interior to the aqueous-organic interface is the critical step that allows the conjugate to act as an ion carrier. Specifically, we propose that, in its SP form, the relatively hydrophobic dye confines the crown to the hydrocarbon microphase, restricting its approach to the aqueous-organic interface. Upon photoisomerization to the MC form, the spatial relocation that is triggered by the increased polarity of the dye allows access to the interface and subsequent capture of a potassium ion. On the basis of measured quantum efficiencies, the K+-bound conjugate, at least, must be sufficiently hydrophobic to permeate the bilayer, following which K+ is released into the opposite aqueous phase. The dye subsequently reverts to its SP form by thermal ring closing, again restricting movement of the crown out of the organic microphase and, thereby, its ability to transport K+. The proposed mechanism is illustrated stylistically in Figure 1. Experimental Section Preparation and Characterization of SP-Crown-Doped Vesicles. Monoaza-[15]crown-5 (Cr5) and monoaza -[18]crown6 (Cr6) were attached at the 1- position of 1′,3′,3′-trimethyl-6nitrospiro[2H-1-benzopyran-2,2′-indoline] following procedures described in the literature.16 Egg phosphatidylcholine (PC) was isolated by solvent extraction and chromatography on alumina;19 the purified product was stored at -10 °C as a chloroform solution. Unilamellar vesicles ([PC] = 10-2 M) with average diameters of 70 nm containing SP-Cr5 or SP-Cr6 were prepared in 20 mM Tris/Cl buffer, pH 8.0, from evaporated films by high-pressure extrusion through track-etched 50-nm pore size Poretics filters.20 The relative molar concentration of SP-crown and PC in membrane forming solutions was 1:100, as determined by the weight of materials. Vesicles containing entrapped K+ were (19) Singleton, W. S.; Gray, M. S.; Brown, M. L.; White, J. L. J. Am. Oil Chem. Soc. 1965, 2, 53-56. (20) Mayer, L. D.; Hope, M. J.; Cullis, P. R. Biochim. Biophys. Acta 1986, 858, 161-168.
Figure 1. Hypothetical scheme for transmembrane ion transport mediated by a spiropyran-crown ether conjugate. Photoisomerization (step 1) converts the thermodynamically favored SP dye moiety to its less stable MC form. Subsequent relocation of the conjugate toward the interface (step 2) allows coordination of K+ from the aqueous phase (step 3) and its translocation across the bilayer (step 4). Release of K+ at the opposite interface (step 5) is followed by thermal ring closing, regenerating the SP isomer (step 6). prepared by adding ∼0.02 M KCl to the buffer prior to extrusion, followed by removal of external potassium ions by size exclusion chromatography on Sephadex G-100. Typically, 10 mL of the vesicle suspension was applied to a 30 × 2 cm column and collected in an equal volume of eluate following passage of the void volume. The amount of entrapped K+ was determined by destroying the vesicles by ultrasonic disruption and measuring the concentration released into the medium using a K+-specific ion electrode. Physical Measurements. Continuous UV photolysis experiments were performed using 1.5-kW xenon lamp whose output was focused and passed through aqueous CuSO4 and Schott UG11 (light transmission between 260 and 390 nm) or OG590 (light transmission at λ > 560 nm) glass filters. The filtered light was then passed via an optical fiber bundle to the reaction flask. Light intensities were measured using a calibrated PowerMax 500D Laser Power Meter; with the UV filters in place, the effective value obtained was 2 × 10-9 einstein/cm2-s. The samples were stirred during illumination using a magnetic bar. K+ release was monitored using Orion 93-19 K+-specific and Orion 90-02 counter electrodes connected to an Orion 701A/digital Ionanalyzer (detection limits are 1.0 M to 10-6 M; the coefficient of variation is 3%). Optical spectra were recorded using a Hewlett-Packard 8452 diode array instrument interfaced to a ChemStation data acquisition/analysis system. Transient spectra and kinetics were measured by laser flash photolysis using the third (355 nm) harmonic output from a Continuum Surelite III Nd:YAG laser as the excitation source. Kinetic curves taken for analysis were averages of traces taken from at least 10 individual laser pulses. Explicit experimental procedures and the instrumental setup have been described in detail elsewhere.21
Results and Discussion Photoisomerization of Vesicle-Bound Spiropyran-Crown Ether Conjugates. Spiropyrans absorb light strongly only in the ultraviolet spectral region, whereas, in addition to displaying very similar UV spectra, the merocyanine analogues have intense bands extending into the visible region.17,18 This photochromic behavior arises because the spiro carbon in the ring-closed forms of these compounds effectively blocks transannular conjugation between the indoline and pyran rings. However, heterolytic cleavage of the C(spiro)-O bond generates a considerably more polar molecule whose B-conjugation now extends over the entire molecular framework (Scheme 1), giving rise to low energy allowed electronic transitions. These merocyanine visible bands are also solvatochromic, shifting to lower energies with increasing medium polari(21) Lymar, S. V.; Khairutdinov, R. F.; Soldatenkova, V. A.; Hurst, J. K. J. Phys. Chem. B 1998, 102, 2811-2819.
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Figure 2. Left panel: absorption spectra of SP-Cr6 in PC vesicles under continuous UV and visible (VIS) illumination. Conditions are as described in the Experimental Section. Right panel: dependence of λmax for MC-Cr6 upon the solvent dielectric constant (). The solvents used (with in parentheses)26 were cyclohexane (2.03), diethyl ether (4.33), chloroform (4.8), tetrahydrofuran (7.6), dichloromethane (9.08), pyridine (12.4), 2-propanol (18.3), ethanol (24.3), methanol (32.6), and glycerol (42.5). Upper and lower dashed horizontal lines correspond to λmax of MC-Cr6 in the PC membrane at 10-6 s after applying a 355-nm laser pulse and after 100 s continuous UV illumination, respectively.
ties.22 Optical absorption spectroscopy can therefore be used effectively to investigate both the distribution of isomeric forms and the microenvironment surrounding the merocyanine form. In PC vesicles, the spiropyran-crown ether conjugates SP-Cr5 and SP-Cr6 exhibited negligible absorption in the visible region, indicating that at most only a few percent of the dye was in its merocyanine form. Under continuous illumination with UV light a visible band absorbing maximally at -520 nm appeared, indicative of photocleavage of the C(spiro)-O bond. Using a typical value of ≈ 3 × 104 M-1cm-1 for the extinction coefficient of the merocyanine visible band,23 we estimate that under steady-state conditions the fractional conversion to the mero form was ∼3%. Illumination into the merocyanine band with visible light caused photobleaching with the spectrum reverting to that of the spiro form (Figure 2), as expected from the general photochemical behavior of this class of compounds.17,18 As we have previously found with other vesicle-bound photochromic spiro compounds,24,25 these SP-Cr conjugates could be cycled repetitively between their isomeric forms by alternate exposure to UV and visible light without observation of any irreversible changes in their optical properties. The visible band maxima of the MC-Cr5 and MC-Cr6 obtained in PC vesicles (∼520 nm) during continuous illumination with UV light were comparable to the wavelength maxima measured in polar solvents, that is, with an effective g 40, consistent with localization of the dyes within the polar headgroup region of membrane (Figure 2). The positions of the visible band maxima remained unchanged when 0.02 M K+ was added to the external solution, indicating that either the fraction of K+ bound was small or that binding to the crown moiety did not perturb the merocyanine spectra. In homogeneous acetonitrile solution, where K+ binding is extensive, addition of KCl did not change the position of the visible absorption band maxima of MC-Cr6. The response of the vesicle-bound SP-Cr conjugates to 5-ns laser flash excitation at 355 nm was biphasic, (22) Pozzo, J.-L.; Samat, A.; Guglielmetti, R.; De Kekueleire, D. J. Chem. Soc., Perkin Trans. 2 1993, 1327-1332. (23) Yagi, S.; Maeda, K.; Nakazumi, H. J. Mater. Chem. 1999, 9, 2991-2997. (24) Khairutdinov, R. F.; Giertz, K.; Hurst, J. K.; Voloshin, N. A.; Minkin, V. I. J. Am. Chem. Soc. 1998, 120, 12707-12713. (25) Khairutdinov, R. F.; Hurst, J. K. Langmuir 2001, 17, 68816886.
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Figure 3. Left panel: normalized visible absorption spectra of UV illuminated PC vesicles containing MC-Cr6. Solid circles are absorbance changes measured at 10-6 s after applying a 355-nm laser pulse; open circles are the absorbance changes at 8 × 10-4 s after the pulse; solid lines are normalized absorption spectra obtained after 100 s continuous UV illumination of the same suspension. Right panel: normalized decay kinetics at 480 and 580 nm of the transient absorption of SP-Cr6 doped PC vesicles. The solid lines are the exponential best fits decay curves obtained with τ ) 0.8 ms.
exhibiting a very rapid increase in absorbance corresponding to formation of the analogous MC-Cr compounds, followed by wavelength-dependent changes on the millisecond time scale (Figure 3, right panel). Specifically, the visible absorption bands of the transient spectra recorded immediately following UV laser flash excitation were red-shifted by ∼30 nm relative to their equilibrium positions (Figure 3, left panel). These changes in transient spectra were manifested in temporal increases in absorbance at wavelengths longer than the equilibrium maximum with symmetric decreases at shorter wavelengths which could be fitted at all wavelengths to a simple firstorder decay profile whose relaxation time was τ ∼ 0.8 ms (Figure 3, right panel). This slow relaxation step was not observed when photoisomerization of the SP-Cr conjugates was carried out in homogeneous solution. Instead, only the initial increase in absorbance following UV flash excitation was observed, which was too fast to temporally resolve with our instrument. This behavior is very similar to that found for simple photochromic spiro compounds, whose photoisomerization reactions in solution occur on subpicosecond time scales27-29 and for which the slow secondary relaxation is observed only when the compounds are incorporated within vesicles.24,25 Thus, it appears that rapid SP-Cr f MC-Cr photoisomerization elicits a structural reorganization within the PC vesicle, causing the environment of the dye to become significantly more polar.30 Photoregulated Potassium Ion Release. The K+ leak rate of unilluminated PC vesicles containing 0.02 M (26) Handbook of Chemistry and Physics, 71st ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, Fl, 1990; pp 9-10, 11. (27) Wilkinson, F.; Worral, D. R.; Hobley, J.; Jansen, L.; Williams, S. L.; Langley, A. J.; Matousek, P. J. Chem. Soc., Faraday Trans. 1996, 92, 1331-1336. (28) Zhang, J. Z.; Schwartz, B. J.; King, J. C. J. Am. Chem. Soc. 1992, 114, 10921-10927. (29) Tamai, N.; Masuhara, H. Chem. Phys. Lett. 1992, 191, 189194. (30) Spiropyran derivatives, when isomerized to their corresponding merocyanine forms, tend to form J- or H-aggregates,31,32 which are redshifted or blue-shifted relative to the monomeric forms. Hence, a potential alternative explanation for the observed spectral shifts is that the SP-Cr molecules are monomeric within the PC liposomes and undergo slow H-aggregation following rapid photoinitiated cleavage of their C(spiro)-O bond. H-aggregates with large (∼70 nm) hypsochromic shifts in their visible absorption band have been documented for spiropyran molecules having two long alkyl chains in a bilayer vesicular membrane, whereas aggregation was not detected with spiropyrans containing fewer than two chains.32 Aggregate formation was also not detected in PC membranes containing related spirooxazine molecules.24 Therefore, the more plausible explanation for the flash-induced spectral shifts are changes in medium polarity caused by relocation of the MPCr within the membrane.
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Figure 4. Release of K+ from PC vesicles accompanying reversible SP-Cr a MC-Cr photocycling by exposure to UV or visible (VIS) light under continuous illumination. Conditions are as indicated in the Experimental Section. Closed and open circles correspond to SP-Cr6 and SP-Cr5, respectively.
entrapped K+ and either SP-Cr6 or SP-Cr5 incorporated within the membrane was on the order of nM/s for the conditions of these experiments. Illumination of the vesicle suspensions with UV light caused the ion leak rates to increase ∼4 times for membranes containing SP-Cr6 (Figure 4). Illumination of the system with visible light resulted in ring-closing photoisomerization (MC-Cr6 f SP-Cr6) and returned the ion leak rate to its initial value. This behavior of SP-Cr6 doped membranes is opposite to that observed for PC membranes with spiropyrans lacking crown substituents.25 In those assemblies, UV illumination caused a ∼3-fold decrease in K+ permeation rates. The effect of light on K+ transmembrane permeation for PC vesicles containing SP-Cr5 was negligible (Figure 4), presumably because the binding affinity of K+ for the Cr5 moiety is low.33 The overall quantum yield for transmembrane K+ transport, estimated as an average number of ions transported through the membrane per each light absorbed, is φ ≈ 0.3; this value is about 2-fold lower than the quantum yield of the SP 6 MC transformation.34 The pattern of variation in K+ leak rates provides insight into the nature of the structural reorganization following photoisomerization of the vesicle-bound SP-Cr. In principle, the observed increased polarity of the dye microenvironment might arise from disruption of the normal bilayer structure caused by its isomerization to the larger35,36 and more polar MC form, thereby allowing greater solvent access to the membrane interior. If this were the nature of the structural perturbation, then electrolyte leakage should increase with conversion to the MC form. However, as noted above, just the opposite behavior was observed for an amphiphilic spiropyran,25 that is, the K+ leak rate decreased upon conversion of the dye to the MC form. On the basis of these observations, we concluded that a more plausible explanation of the change in polarity was that, following photoisomerization, the MC form moved from the interior site favored by the SP form toward the aqueous-organic interface. Presumably, this displacement also improved surfactant alkyl chain packing within the membrane hydrocarbon bilayer, reducing the intrinsic permeability of K+. This same issue exists in considering the relaxational response of the (31) Krongauz, V. A.; Goldburst, E. S. Nature 1978, 271, 43-46. (32) Seki, T.; Ichimura, I. J. Phys. Chem. 1990, 94, 3769-3775. (33) Christensen, J. J.; Hill, J. O.; Izatt, R. M. Science 1971, 174, 459-467. (34) Guglielmetti, R. Stud. Org. Chem. 1990, 40, 855-878. (35) Holden, D. A.; Ringsdorf, H.; Deblauwe, V.; Smets, G. J. Phys. Chem. 1984, 88, 716-720. (36) Gruler, H.; Vilanove, R.; Rondelez, F. Phys. Rev. Lett. 1980, 44, 590-592. Polymeropoulos, E. E.; Mo¨bius, D. Ber. Bunsen-Ges. Phys. Chem. 1979, 83, 1215-1222.
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vesicle-bound MC-Cr conjugates to photoisomerization. In this case, one might argue that the increased K+ leakage induced by MC-Cr6 is a consequence of disruption of the bilayer. However, SP f MC photoisomerization in SPCr5 doped membranes caused no perceptible change in K+ efflux rates (Figure 4) despite the appearance of very similar changes in the polarity of the medium surrounding the dye. Thus, it also appears that the origin of the effect with the spiropyran-crown conjugates is displacement of the dye toward the more polar25,37-39 membrane aqueous-organic interface. UV illumination of PC liposomes in aerated solutions could also cause photooxidation of lipid molecules with a subsequent decrease in membrane integrity.40 This effect should result in a progressive increase in ionic permeabilities with illumination time, contrary to the experimental observations (Figure 4). General Comments. The proposed mechanism, illustrated in Scheme 1, provides a self-consistent view of the photodynamic response of K+-loaded vesicles containing spiropyrans,25 spirooxazines,24 and spiropyran-crown ether conjugates. In each system, control of the spatial location of the photoresponsive dye within the membrane bilayer has been used to modulate transmembrane diffusion of K+, with SP f MC photoisomerization either decreasing its permeability when simple amphiphilic dyes are used or increasing its permeability when the dye contains an appended ionophore. The origin of these effects is the different micropolarities of the dyes in their two isomeric forms. A somewhat similar approach to using membrane microphase location to regulate photodynamic activity was reported by Armitage and O’Brien, who demonstrated that photoinduced electron transfer between a hydrophobic tetraarylborate and an interfacially bound hydrophilic electron acceptor increased below the phase transition of the vesicle, an apparent consequence of expulsion of the donor from the membrane interior toward the interface resulting from the tighter alkyl-chain packing in the gel phase.41 Although the quantum yield for K+ transport in the carrier-mediated pathway is substantial, it is probably less than that for the initial SP f MC photoisomerization process34 and is certainly less than unity. Thus, there exists no evidence that, once formed, the MC-Cr6 transporter can undergo repetitive cycling across the membrane, that is, transport more than a single K+ prior to thermal ring closing (Scheme 1, step 6). The thermal MC-Cr6 f SPCr6 transformation in PC vesicles is a very slow firstorder reaction, with t1/2 ≈ 15 min at 23 °C. This suggests that the photochemical yields for transport may be limited by a very low membrane permeability of the subpopulation of MC-Cr6 that does not contain bound K+. In this circumstance, approximately one-half of the initially formed MC would be expected to diffuse to the interface whose aqueous phase is depleted in K+ and be effectively trapped at that site until SP-Cr6 was reformed. Similarly, binding, transmembrane transport, and release of K+ from MC-Cr6 from the opposite side of the membrane (Scheme 1, steps 3-5) would result in entrapment of the dye at this interface until ring closing provided a pathway for its (37) Mazeres, S.; Schram, V.; Tocanne, J.; Lopez, A. Biophys. J. 1996, 71, 327-335. (38) Wiener, M. C.; White, S. H. Biophys. J. 1992, 61, 434-447. (39) Sanders, C. R., II; Schwonek, J. P. Biophys. J. 1993, 65, 12071218. (40) Chowdhary, R. K.; Green, C. A.; Morgan, C. G. Photochem. Photobiol. 1993, 58, 362-366. (41) Armitage, B.; O’Brien, D. F. J. Am. Chem. Soc. 1992, 114, 79367403.
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redistribution. This overall mechanism would thereby account for the apparent 2-fold difference in photoisomerization and transport quantum yields. Although the present demonstration of this phenomenon has involved K+ transport, it is clear that other potential applications exist. For example, if a chemical catalyst or initiator were attached to the photoisomerizable dye, chemical reactivity could be controlled in an analogous manner by light. With a suitable binding agent attached to the spiropyran, the system could be used to control transport of therapeutical agents or other water-soluble organic and biological molecules across cell membranes. For bulk nonpolar liquid membranes separating two aqueous solutions,42 where one side of the membrane could be illuminated by light while the other kept in the dark,
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similar compounds could provide transmembrane transport against a concentration gradient. Numerous practical applications of photopumping could be envisioned, including desalinization of seawater and removal of radioactive ions from liquid wastes. Acknowledgment. This work was supported by the Center for Nanosensor Technology UAF under the contract DMEA90-02-C-0226 and grants BES-0322455 from the National Science Foundation (to R.F.K.) and DE-FG0399ER14943 from the Office of Basic Energy Sciences, U.S. Department of Energy (to J.K.H.). LA035683L (42) Goyette, M. L.; Longin, T. L.; Noble, R. D.; Koval, C. A. J. Membr. Sci. 2003, 212, 225-235.
