An Anhydrous Proton ConductorBased on Lactam±LactimTautomerism of Uracil

Masanori Yamada and Itaru Honma*[a]

Introduction

The nucleic acids DNA and RNA are among the most importantmaterials for life processes[1] and are highly specific functionalmolecules. In addition, DNA or RNA can be isolated fromsalmon milts, shellfish gonads, or waste liquor from pulp facto-ries, which are generally discarded as industrial waste. The uti-lization of DNA or RNA as functional materials such as bioma-terials,[2±6] materials for the adsorption of harmful com-pounds,[2±5, 7] and novel biomatrices,[2, 6, 7] has been reported.[8±10]

Whereas many studies[2±10] have focused on DNA and RNA asfunctional polymer materials, the nucleic acid bases adenine,guanine, cytosine, thymine, and uracil have not been consid-ered as functional materials. Nevertheless these heterocycliccompounds have high thermal stability (<250 8C) and can beexpected to act as functional materials at high temperatures.

Anhydrous proton-conducting materials are important ele-ments of polymer electrolyte membrane fuel cells (PEMFCs)operating at intermediate temperatures (100±200 8C).[11±16] Es-pecially composite materials consisting of a strong acid suchas phosphoric or sulfuric acid and heterocyclic bases such asimidazole (Im) or benzimidazole (BnIm) have been used asproton conductors under anhydrous (low-humidity) conditionsat intermediate temperatures.[17±24] If heterocyclic nucleic acidbases such as adenine and uracil were utilized for an anhy-drous, proton-conducting membrane, low-cost, environmental-ly benign, and nonhazardous PEMFCs could be constructed.

We prepared a composite salt of an RNA base and a surfac-tant by mixing uracil with the acidic surfactant monododecylphosphate (MDP). This U±MDP composite salt showed aproton conductivity of 6 î 10�4 S cm�1 at 160 8C under anhy-

[a] Dr. M. Yamada, Dr. I. HonmaEnergy Materials Group, Energy Electronic InstituteNational Institute of Advanced Industrial Science and Technology (AIST)Umezono 1-1-1, Tsukuba, Ibaraki 305±8568 (Japan)Fax: + (81) 29-8615829E-mail : i.homma@aist.go.jp

Supporting information for this article is available on the WWW underhttp://www.chemphyschem.org or from the author.

724 ¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/cphc.200301015 ChemPhysChem 2004, 5, 724 ±728

drous conditions. Moreover, this proton conductivity wasfound to be stable (for 50 h at 160 8C). The proton-conductingmechanism of the U±MDP composite salt depends on protontransport through a different conductive pathway, based onthe lactam±lactim tautomerism of uracil by the MDP dopingratios.

Results and Discussion

Characterization and Molecular Structure of U±MDP Compo-site Salts

Composite salts of an RNA base and an acidic surfactant wereprepared by doping uracil (U) with monododecyl phosphate(MDP). The molecular structures of MDP and U are shown inScheme 1. Uracil exists in the lactam form under neutral condi-

tions and in the solid state, and as a mixture of the lactam andlactim tautomers under acidic conditions.[25] Powder samplesof the composite salts were compacted into pellets. Figure 1 aand 1 b respectively show the thermogravimetry (TG) and dif-ferential thermal analysis (DTA) of pure uracil (1), U±12.6 mol %MDP (2), U±21.0 mol % MDP (3), and pure MDP (4) for a heat-ing rate of 10 8C min�1 up to 300 8C under a flow of dry nitro-gen. Pure uracil showed neither a TG weight loss nor an endo-thermic or exothermic peak up to 250 8C. In contrast, U±MDPcomposite salts exhibit a TG weight loss of a few percent at200 8C due to dehydration of phosphate groups and the evap-oration of water of hydration of MDP. Additionally, above200 8C, composite salts show large TG weight losses due to theevaporation or decomposition of MDP molecules in the materi-al. Such TG weight losses were also obtained with MDP/BnImcomposite materials.[22] These results suggested that U±MDPcomposite salts do not produce diffusible ions by melting ofthe samples and are stable at intermediate temperatures(<200 8C) under anhydrous conditions. An endothermic peakat approximately 40 8C indicates the evaporation of water mol-ecules from the composite salt.

Figure 2 shows IR spectra of pure uracil (a), U±2.1 mol %MDP (b), U±4.2 mol % MDP (c), U±8.4 mol % MDP (d), U±

21.0 mol % MDP (e), U±42.1 mol % MDP (f), and pure MDP (g).The absorption band at 1100 cm�1, attributed to the C�Ostretching vibration of a tertiary alcohol,[26] indicates the forma-tion of COH groups (lactim form)[25] when uracil is increasinglydoped with MDP. The intensity of the absorption band at1100 cm�1 as a function of MDP doping ratio is shown inFigure 3. The amount of COH groups (i.e. , uracil molecules inthe lactim form) linearly increases with increasing MDP dopingratio. In contrast, the absorption band at about 3100 cm�1 for

Scheme 1. Molecular structures of monododecyl phosphate (MDP) and thelactam and lactim forms of uracil (U).

Figure 1. TG (a) and DTA (b) curves of U±MDP composite salts at a heatingrate of 10 8Cmin�1. 1) Pure uracil, 2) U±12.6 mol% MDP composite salt, 3) U±21.0 mol% MDP composite salt, 4) pure MDP.

Figure 2. IR spectra of U±MDP composite salts with different MDP dopingratios. a) pure uracil, b) U±2.1 mol% MDP composite salt, c) U-4.2 mol% MDPcomposite salt, d) U±8.4 mol% MDP composite salt, e) U±21.0 mol% MDP com-posite salt, f) U±42.1 mol% MDP composite salt, and g) pure MDP.

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the N�H stretch of uracil[26±28] shows a constant intensity of ap-proximately 50 % (Figure 3). Additionally, the appearance of ab-sorption band at 960 cm�1 for the stretching vibration ofPO4

2�[29, 30] indicates deprotonation of the phosphate group ofMDP by interaction with uracil. These results suggest that mo-lecular structure of uracil partially changes from the lactam tothe lactim form by protic addition of MDP molecules (lactam±lactim tautomerism of uracil[25]). The POH groups of MDPstrongly interact with the unprotonated �N= groups of uracil(lactim form), and the U±MDP composite salt is thus produced.Therefore, the intensity of the N�H stretching band at3100 cm�1 remains constant in spite of the tautomerism to thelactim form. We also recorded XRD patterns of the compositesalts (Figure S1, Supporting Information), but these were foundto be superpositions of the patterns of the individual com-pounds.

Anhydrous Proton Conductivity of U±MDP Composite Salts

Proton conductivity measurements on U±MDP composite saltswere performed by the ac impedance method over the fre-quency range from 1 Hz to 1 MHz under a flow of dry nitrogen.Thus, the measured impedance response indicates an anhy-drous (water-free) proton conductivity.[22±24] Figure 4 a showsthe typical impedance response (Cole±Cole plots) of U±MDPcomposite salts at 70, 100, and 150 8C. These plots show a fea-ture similar to that of highly proton conducting membranessuch as Nafion, organic/inorganic hybrid membranes mixedwith heteropolyacids,[31, 32] MDP/BnIm,[22] 2-undecylimidazole(UI)/MDP,[23] and poly(vinylphosphonic acid) (PVPA)/heterocy-cle[24] composite materials. The resistances of the compositesalts were obtained by extrapolation to the real axis. In con-trast, these composite salts did not exhibit electronic conduc-tivity under dc conditions. In addition, diffusible ions otherthan protons do not exist in the composite salts. Therefore,the impedance responses are due to purely anhydrous protontransfer in the composite salts.

Anhydrous proton conductivities of U±MDP composite saltswith MDP doping ratios of 1.3, 4.2, 21.0, and 42.1 mol % MDPin the temperature range of 60±160 8C are shown in Figure 4 b.Pure uracil did not have a measurable proton conductivity (s<10�8 S cm�1 at 160 8C). The pure DNA bases adenine, guanine,

thymine, and cytosine also exhibited no measurable protonconductivity (<10�8 S cm�1 at 160 8C).

Proton conductivity of the composite salts at 160 8C underanhydrous condition as a function of MDP doping ratio isshown in Figure 5. The conductivity increased with increasing

MDP content and reached a constant value at doping ratiosgreater than 21.0 mol % MDP. Maximum anhydrous protonconductivity is 6 î 10�4 S cm�1 for the U±42.1 mol% MDP com-posite salt. These relations between the proton conductivityand the MDP doping ratio coincided with the amount of uracilmolecules in the lactim form in U±MDP composite salts (seeFigure 2). The anhydrous proton conductivity of U±MDP com-posite salts was found to be stable (for 50 h at 160 8C).

Figure 3. Intensity of IR absorption bands at 1100 cm�1 (*) and 3100 cm�1 (*)as a function of MDP doping ratio from 0 to 63.1 mol%. The intensity corre-sponds to the transmittance in Figure 2.

Figure 4. a) Typical impedance responses (Cole±Cole plots) of U±MDP compo-site salts at 70 (&), 100 (~), and 150 8C (*) in the frequency range 1 Hz to1 MHz. b) Proton conductivities of U±MDP composite salts with different MDPdoping ratios under anhydrous conditions (dry nitrogen) ; ! 1.3, & 4.2, ~ 21.0,and * 42.1 mol% MDP.

Figure 5. Anhydrous proton conductivities under dry nitrogen at 160 8C as afunction of MDP doping ratio from 0 to 63.1 mol% for U±MDP composite salts(*) and pure MDP (&).

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Finally, we determined the activation energy Ea of protonconduction for U±MDP composite salts. Figure 6 shows the Ar-rhenius plot of proton conductivity for composite salts with1.3, 4.2, 21.0, and 42.1 mol % MDP. The solid lines in Figure 6

are the results of least-squares fitting. The activation energiesestimated from the slopes are 0.27±0.28 eV at the low MDPdoping ratios (1.3 and 4.2 mol%). This value is almost the sameas those of other materials reported as anhydrous proton con-ductors, such as solid electrolytes,[33±35] fullerene deriva-tives,[36, 37] UI/MDP,[23] and PVPA/heterocycle composite materi-als.[24] These results indicate that the proton-transport mecha-nism in U±MDP composite salt is not a vehicular process but aproton-hopping mechanism of the Grotthuss type.[38] In con-trast, activation energies of the materials with 21.0 and42.1 mol % MDP are approximately 0.1 eV. These different acti-vation energies of approximately 0.3 and 0.1 eV suggest thatthe proton-conduction mechanism depends on the concentra-tion of protic carriers in the composite salt and thus differs be-tween low and high MDP doping ratios.

Proton-Conducting Mechanism of U±MDP Composite Salt

Previously, we reported anhydrous proton-conducting mem-branes of acid±base composite materials with heterocyclessuch as Im, pyrazole, 1-methylimidazole, or BnIm.[24] In thiscase, a Grotthuss-type diffusion mechanism was proposed, inwhich the transport of the proton in basic heterocycle mole-cules can occur from protonated molecules (proton donors) tounprotonated neighbor molecules (proton acceptors) with anactivation energy of approximately 0.25 eV.[22±24] In addition,acidic molecules, such as phosphoric acid or sulfuric acid, arealso described as proton donors and acceptors with formationof protonic defects and are considered to undergo intermolec-ular proton-transfer reactions.[39] However, we previouslyshowed[22±24] that proton transfer between acidic molecules hasa high activation energy of approximately 1 eV. Here, we ob-tained activation energies of approximately 0.3 and 0.1 eV atlow and high MDP doping ratios, respectively. These two acti-vation energies are indicative of proton transfer via different

proton-conducting pathways in U±MDP composite salts. Thelow activation energies (<0.3 eV) are due to proton transferbetween heterocyclic molecules,[22±24] in this case uracil mole-cules. Table 1 shows how the activation energy of anhydrousproton conduction and the molecular form of uracil depend

on the MDP doping ratio. At low MDP doping ratios, uracilmolecules exist as the lactam form according to IR measure-ments (Figure 2 b and c). Therefore, proton transfer in U±MDPcomposite salts with low MDP doping ratios occurs via uracilmolecules in the lactam form. In contrast, at high MDP dopingratios, uracil molecules partially exist in the lactim form (Fig-ure 2 e and f, Table 1). Uracil molecules in the lactim form areprotonated by proton addition from neighboring MDP mole-cules. Proton transport in close-packed uracil molecules canoccur from protonated uracil molecules to neighboring unpro-tonated uracil molecules. Thus, U±MDP composite salts exhibittwo types of proton-conducting mechanism owing to lactam±lactim tautomerism. These anhydrous proton conducting ma-terials are extremely novel because they consist of cheap andnonhazardous biomaterials. The conducting properties arepromising for electrolyte membranes for PEMFCs operatingabove the boiling point of water.

Conclusion

We have investigated anhydrous proton-conducting propertiesof U±MDP composite salts, which showed a proton conductivi-ty of 6 î 10�4 S cm�1 at 160 8C and an MDP content of42.1 mol %. The temperature dependence of the proton con-ductivity indicated Arrhenius-type behavior with two differentactivation energies of approximately 0.3 and 0.1 eV dependingon the proton concentration in the materials. These activationenergies suggest a Grotthuss-type proton-hopping mechanismwithout the presence of vehicle molecules such as water.Proton transfer in U±MDP composite salt is suggested to bebased on the lactam±lactim tautomerism of uracil. The utiliza-tion of biomolecules such as RNA bases for PEMFC technologyis novel and challenging; these biological products are cheap,available in large quantities from waste materials, nonhazar-dous, and environmentally benign. The biomolecule-basedcomposite material may have potential not only because of its

Figure 6. Arrhenius plots for U±MDP composite salts. Solid lines are results ofleast-squares fitting. Activation energies Ea of the proton transfer under anhy-drous condition were estimated from the slope. Doping ratios of MDP are 1.3(!), 4.2 (&), 21.0 (~), and 42.1 mol% (*).

Table 1. Dependence of activation energy of anhydrous proton conductionand molecular form of uracil in U±MDP composite salts on MDP dopingratio.

MDP doping ratio

Low HighActivation energy 0.3 eV 0.1 eV

Molecular form of uracil

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superior ion-conducting properties, especially under anhydrousconditions or at very low humidity, but also for biocompatibleelectrochemical devices such as implantable batteries and bio-sensors.

Experimental Section

Materials : Monododecylphosphate (MDP) was obtained from AlfaAesar (MA, USA), Adenine, guanine, thymine, cytosine, and uracil(U) was purchased from Wako Pure Chemical Industries Ltd. ,Osaka, Japan. U±MDP composite salts were prepared by dopinguracil with MDP. Powder samples of U±MDP composite salts werecompacted into pellets with a diameter of 12 mm in a hand pressat a pressure of approximately 600 MPa.

Characterization of U±MDP composite salts: The thermal stabilityof U±MDP composite salts was analyzed by thermogravimetry dif-ferential thermal analysis (TG-DTA) on a TG-DTA 2000S (Mac Scien-ces Co., Ltd. , Yokohama, Japan). TG-DTA measurements were per-formed at a heating rate of 10 8C min�1 under a flow of dry nitro-gen. IR spectra were recorded with a resolution of 4 cm�1 on an IRspectrophotometer FTS-60 (Bio-Rad Laboratories, Inc. , PA) with adiamond attenuated total reflection (ATR) prism (Golden Gate Dia-mond ATR System, Specac Ltd. , GA).

Proton conductivity measurements on composite salts: Protonconductivities of U±MDP composite salts were measured by the acimpedance method in the frequency range from 1 Hz to 1 MHzusing an impedance analyzer SI-1260 (Solartron Co., Hampshire,UK) in a stainless steel vessel from room temperature to 160 8C. Apellet of U±MDP composite salt (diameter: 12 mm) was sand-wiched between two gold-coated electrodes (diameter: 5 mm).The measurement direction was perpendicular to the pellet. Con-ductivities of composite salts were determined from typical impe-dance responses (Cole±Cole plots). Since all measurements weremade under a flow of dry nitrogen, the measured impedance re-sponses of U±MDP composite salts indicate anhydrous proton con-ductivities.[22±24]

Acknowledgements

This work was supported by R&D program of PEFC by NewEnergy and Industrial Technology Development Organization(NEDO), Japan.

Keywords: conducting materials ¥ materials science ¥nucleobases ¥ proton transport ¥ tautomerism

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Received: October 20, 2003 [Z 1015]

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