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Langmuir 199410, 232-234
Unusually Large Induced Circular Dichroism of an Aromatic Compound Bound to Helical Superstructures of Chiral Ammonium Bilayers Naotoshi Nakashima,? Reiko Ando, Tsuyoshi Muramatsu,t and Toyoki Kunitake' Department of Chemical Science and Technology, Faculty of Engineering, Kyushu University, Fukuoka 812, Japan, Department of Applied Chemistry, Faculty of Engineering, Nagasaki University, Nagasaki 852, Japan, and Faculty of Fisheries, Nagasaki University, Nagasaki 852, Japan Received February 24,1993. I n Final Form: October 20,1999 9-Anthracenecarboxylatebound to aqueous dispersionsof the helical superstructure of bilayers of chiral double-chain ammonium amphiphiles exhibited very large induced circular dichroism ([dl = 107 depcm2-dmol-1). This arises from strong exciton coupling among the chromophores bound to the chiral bilayer surface and disappears completely upon warming to temperatures above crystal-to-liquidcrystal phase transition of the bilayer. The intensity of induced circular dichroism at the helical bilayer matrix was compared with those in nonhelical bilayer dispersions.
Introduction Unique chiroptical properties have been shown to arise from assembled organized structures such as liquid crystalline phases and aqueous chiral bilayer membranes.' We have reported that circular dichroism (CD) of chiral synthetic bilayers which contain p-phenylene? b i ~ h e n y l , ~ n a ~ h t h a l e n eand , ~ azobenzene6 moieties is remarkably enhanced due to the dipole-dipole coupling of the organized chromophores. Furthermore, large circular dichroism was induced when Methyl Orange was bound to some of these chiral bilayersae It has been found independently by Yamadaet aL7and by us8p9that chiral synthetic bilayers produce helical superstructures. Helical bilayers are formed also from phospholipid-nucleoside conjugate compounds10 and from unpolymerized and polymerized diacetylenic aldonamides."J2 These helices conceivably possess strong chiral microenvironments a t the bilayer surface; therefore it is expected that unique microenvironments of the helical superstructure provide strongly chiral binding sites. We report herein that this expectation is in fact the case. Chiral amphiphiles 1 (L-, D-)spontaneously form bilayer vesicles and fibrous bilayers in aqueous solution and these + Present address: Department of Applied Chemistry, Faculty of Engineering, Nagasaki University. t Faculty of Fieheries, Nagasaki University. Abstract published in Advance ACS Abstracts, December 1, 1993.
(1) Hatano, M. Advances in Polymer Science; Okamura, S., Ed.; Springer-Verlag: Berlin, 1986; Vol. 77, pp. 94-102. (2) Kunitake, T.; Nakashima, No;Shimomura,M.; Okahata, Y.; Kano, K.; Ogawa, T. J. Am. Chem. SOC.1980,102,6642. (3) Kunitake, K.; Nakashima, N.; Morimitau, K. Chem. Lett. 1980,
1347. (4) Nakaehima,N.;Kimizuka,N.;Kunitake,K. Chem.Lett. 1985,1817. (5) Nakashima, N.; Morimitsu, K.; Kunitake, T.Bu1l.Chem. SOC. Jpn. 1984,57,3253. (6) Nakashima, N.; Fukushima, H.; Kunitake, T. Chem. Lett. 1981, 1207. (7) Yamada, K.; Ihara, H.; Ide, T.; Fukumoto, T. Chem. Lett. 1984, 1713. (8) Nakashima,N.;Asakuma,S.;Kim, J.-M.;Kunitake,T. Chem.Lett. 1984, 1709. (9) Nakashima, N.; Asakuma, S.; Kunitake,T.J. Am. Chem. SOC.1985, 107, 509. (10) Yanagawa, H.; Ogawa, Y.; Furuta, H.; Tsuno, K. J. Am. Chem. SOC.1989,111,4567. (11)Frankl, D. A.; OBrien, D. F. J. Am. Chem. SOC.1991,113,7436. (12) Fuhrhop, J.-H.; Blumtritt, P.; Lehmann, C.; Lugar, C. J. Am. Chem. SOC.1991, 113, 7437.
0743-7463f94/24l0-0232$04.50/0
Morphology at long aglng
Morphology at short aging long aglng at T d c
bilayer heating ( T a l c ) vesicles
*
Figure 1. Schematic illustration of morphological conversion between spherical and fibrous bilayers and helical euperetructures.
aggregates grow to helical superstructures at temperatures below phase transition of the bilayer.e4 The helices change their shape into vesicles and fibrous bilayers again upon heating at temperatures above T,. These processes are schematically shown in Figure 1. On the contrary, chiral amphiphile 2 does not give,helices for the entire temperature rangess In this study, the CD behavior of the helical bilayer and induced CD of chromophore, 9-anthracenecarboxylate, 3 bound to the helical bilayers of 1 (L-,D-, long aging) are compared with those of the nonhelical bilayers of 1 (short aging) and 2. The influence of strong chiral microenvironments created at the surface of the helical bilayers on the induction of CD is emphasized for the first time.
Experimental Section Preparations of amphiphiles 1 (L-,D-,DL-)and 2 (L-)were described else~here.~,~ Sodium 9-anthracenecarboxylatewas prepared by neutralization of 9- anthracenecarboxylic acid (Aldrich)with NaOH followed by recrystallization from ethanolHsO. Amphiphiles 1 and 2 were dispersed in Millipore-treated water by shaking at 50-60 OC for 30 min to give transparent dispersions (1.0 x 109 M), which were then aged at 15-20 O C (below T,)for 2-3 weeks (long aging bilayers) or cooled immediately in ice water (rapid cooling bilayers). Circular dichroism and linear dichroism (LD) spectra were recorded with a JASCO J-40AS spectropolarimeterand JASCO J-40A equipped with a LD attachment,respectively. The optical path length of a quartz cuvette used in the study was 1 mm. Differential scanning 0 1994 American Chemical Society
Langmuir, Vol. 10, No. 1, 1994 233
Aromatic Compounds Bound to Helical Superstructures
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calorimetry (DSC) was conducted with a SSC-560Uinstrument (Seiko-Denshi Co., Ltd.).
Results and Discussion Transparent aqueous dispersions of amphiphiles 1 (Land D-)give giant helices upon long aging a t temperatures below the phase transition of the bilayers. Their T,values (34 "C) are higher than those of the rapidly cooled nonhelical bilayers by 1 "C. Therefore, the molecular arrangements of these two kinds of bilayers must be subtly different. Surprisingly large CD enhancements are induced for aromatic chromophores bound to the helical bilayer. For example, when 5.0 X 106 M 9-anthracenecarboxylate is added to 1.0 X 10-4 M aqueous helical bilayers of 1 (L-) (long aging), very sharp exciton coupling is observed for lBb transition a t 261.5 nm ([e] = 4.2 X lo6)and at 265 nm ([e] = -6.0 X 106 deg-cm2.dmol-1) at 15 "C (Figure 2). Following temperature rise from 15 to 25 "C, the positive peaks are enhanced by 30-50% at the expense of the negative peak. The CD intensity decreases drastically at 27 "C and disappears completely at temperatures above 29 "C (Figure 3). The T,(peak top) of the helical bilayer is located at 34 "C (transition region, 32-38 OC). Therefore, the CD disappearance at 27-29 OC corresponds to the starting temperature of the phase transition process. A mirror-image induced CD spectrum of 3 (5.0 X le6 M)was obtained with the helical bilayer of 1 (D-1: at 15 OC, [el = +6.3 x lo6 (265 nm) and [el = -4.0 x lo6 (261.5 nm) and the ICD showed the same T,dependence. No such circular dichroism was induced for 'La transition of the anthracene, suggesting that the orientational fixation of this transition at the bilayer surface is weak. The extent of orientational fixation of guest molecules is estimated by the dissymmetry factor g. Mason13made a theoretical prediction that g is 1O4-10-6 for the random mutual orientation of a chiral chromophore and a chiral matrix and 10-"1@ for the fixed mutual orientation. Theg value which is one obtained for 1 (L-,D-) at 15 "C is 2.6 X le2, of the highest values ever observed for guest molecules embedded in chiral microenvironments. The absorption spectrum of the helix-bound 9-anthracenecarboxylate exhibits temperature dependence (see Figure 2A); ,A of the 'Bb band is located at 262 nm at 15-25 "C (below T,)and shifts to 257 nm at temperatures above 27 "C. The 'La band at 350-400 nm is not affected by temperature. The red shift of the band at T < T,, though small, suggests the formation of J-like aggregates14 from the bound chromophore, as already discussed in the case of membrane-bound cyanine and merocyanine dyes.16 (13) Mason, S. F. Chem. Phys. Lett. 1975,32,201. (14) Jelly, E.E.Nature 1936,138,1009.
X n
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Wavelength 1 nm Figure 2. Temperature dependence of absorption (A) and induced circular dichroism (B)spectraof9-anthracenecarboxylata (5 X 1od M)bound to the aqueous bilayer (long aging) of 1 (L-) (1 X lo-' M).
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Figure 3. Temperature dependence of induced circular dichroism of 9-anthracenecarboxylate (5x lod M)bound to the aqueous chiral bilayer of 1 (L-)(1X lo-' M)prepared by long aging (0) and rapid cooling (0).
The formation of the chromophore aggregate at the helical bilayer surface is supported by the molar ratio dependence given in Table 1. The A, value shifts from 262 to 258 nm with decreasing molar ratios of chromophore is typical of that of the (3)/bilayer (1(L-)). The latter A, isolated species such as those dissolved in ethanol. The magnitude of the CD spectrum decreasedremarkablywith the decrease in the molar ratio. These data strongly suggest that the large induced CD value is produced by coupling of the transition dipoles of the J-aggregated" (head-to-tail orientation) chromophores bound to the helical superstructure. In the case of a bilayer dispersion of 1 (L-)which is rapidly cooled and shows no helix formation, the bound (15) Nakashima,N.; Ando, R.; Fukuehima, H.; Kunitake, T. J. Chem. SOC.,Chem. Commun. 1982,707.
234 Langmuir, Vol. 10, No. 1, 1994 Table 1. Molar Ratio (3/1(~-))Dependence of Absorption and Induced CD Spectra at 15 uv ICD [31/ [ 1(L-)l A, nm) [@I, 112 262 -650 (265nm) 114 261 -350 (264nm) 1/10 258 -19 (263nm) a The bilayer of 1 (L-) was subjected to long aging at 15-20 O C . [3] = 1.0 X 10-4 M = constant.
Nakashima et al.
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9-anthracenecarboxylate possesses A, at 257 nm at all temperatures, irrespective of the phase transition temperature. The corresponding CD spectrum at 15 "C (below T,)gives sharp maxima at 261.5 nm ([el = +2.4 X 104) and at 265 nm ([el = -3.3 X lo5). The absorption maximum (1Bb transition) of 9-anthracenecarboxylate (5.0 X 10" M) bound to an aqueous bilayer of 2 (L-) (1.0 X 10-4 M), which is not capable of helix formation, is located at 252 nm upon rapid cooling value suggests and the subsequent long aging. This A, that the binding site is highly polar, since A, of 9-anthracenecarboxylate in water (without bilayer) is located at 252 nm. Induced CD spectra (positive Cotton effect) are observed at 254 nm with [81 of +5.2 X 104 (long aging) and +3.0 X lo4 (rapid cooling). Both absorption and induced CD spectra are not affected by temperatures at 15-40 "C (T,of the bilayer, 28 "C). The spacer length in this chiral bilayer is short and, therefore, 9-anthracenecarboxylate would be bound to the very surface of the bilayer. The temperature-independent CD data observed for this system indicate that the chiral microenvironment of this binding site is not influenced by melting of the alkyl chain at T,. This is contrasting with the CD data obtained for the bilayers of 1(L-, D-),which strongly depend on T,. It should be important to confirm that the extraordinarily large induced CD is derived from the authentic CD contribution. As shown in Figure 4, very small linear dichroism was detected for chromophore 3 adsorbed to
2so
300
Wavelength I nm
Figure 4. Linear dichroism spectra of 9-anthracenecarboxylate (5 X 1od M) bound to the aqueous bilayer of 1 (L-)(1 X lo-' M): (a) long aging sample at 15 "C;(b) long aging sample at 35 O C ; (c) rapid cooling samples at 15 "C;(d) rapid cooling sample at 35 OC.
the long-aging helical bilayers a t temperatures below T,; however at temperatures above T,,observed small LD disappeared. Chromophore 3 bound to the rapid cooling bilayer of 1 (L-)gave no LD at temperatures below and above Tc. We can conclude from these results that the contribution of linear d e c h r o i ~ m ' ~on J ~the induced CD spectra is very small or negligible. In conclusion, the helical superstructure of chiral ammonium bilayers was shown to provide chiral binding sites where specificassembly of chromophoresis promoted. The importance of the superstructure itself was demonstrated by examination of the CD behavior of the chromophores bound to the bilayers in which helix formation was suppressed by insufficient aging, phase transition, or modification of the component structure.
Acknowledgment. We thank Professors H. Ihara and C. Hirayama for the use of a spectropolarimeter equipped with a LD attachment. ~~
(16) Shindo, Y.:Ohmi,Y. J. Am. Chem. SOC.1985,107,91.
(17)Johansean, L.B.-A.; Davidseon, A.; Lindblom, G.; Nordh, B. J. Phys. Chem. 1978,82,2604.
