2842
Langmuir 1994,10, 2842-2846
Supramolecular Inclusion Complexation of Amylose with Photoreactive Dyes Oh-Kil Kim* and Ling-Siu Choi Chemistry Division, Code 6120, Naval Research Laboratory, Washington, D.C. 20375-5342 Received May 2, 1994@ A series of 4-[4-(dimethylamino)styryll-l-alkylpyridinium bromide (DASP-C,) dyes were synthesized for inclusion studies with amylose. Inclusion complexabilityof DASP dyes with amylose,and conformational transitions of amylose upon complexation with the dyes were investigated using UV-visible, fluorescence, and circular dichroism (CD) spectra in various dimethyl sulfoxide (DMSO)-H20 mixtures. Complete amylose inclusion occurs with DASP dyes ' C ~ Zwhile it decreases sharply with the dyes < C ~ ZAmylose . inclusion is sensitive to the solvent composition, showing a clear transition at a DMSO fraction, @DMSO = 0.6, above which no or negligible complexation occurs. Amylose resumes the helical conformation upon complexation (at @DMSO 4 0.6) with DASP dyes but the helical state is affected by the solvent polarity, becoming loosened with decreasing DMSO content.
Introduction Amylose is a linear polymer of D-glucose linked through an a-1,4-glucosidic bond, which is well-known to form a blue complex when iodine is included in the helical cavity. Amylose has also been known to form inclusion compounds with various low molecular organic comp~unds.l-~ The driving force for the binding of guest molecules is hydrophobic interaction that occurs in the cavity of amylose as host. The cavity has characteristics similar to those of cyclodextrins but has more flexibility in size and length of the guest binding sites. The binding sites are relatively hydrophobic so that a fluorescence probe sensitive to environmental polarity, 2-(p-toluididinyl)naphthalene-6-sulfonate,for example, exhibits a strong fluorescence in the presence of a m ~ l o s e ,while ~ , ~ it barely fluoresces in aqueous solution. A similar observation was reported with other fluorescence While the helical conformation of amylose in the solid state has been established: it still remains uncertain in solution. It has been proposed that there are three different models such as totally random coil, interrupted helix, and total helix -l~ has been studied with six residues per t ~ r n . ~ Amylose in water and dimethyl sulfoxide (DMSO) solution, and its properties are found to be affected by the composition of binary solvent, DMSO/H20 mixtures. Changes in the observed viscosity and optical rotation of amylose are dependent upon the solvent composition. Since DMSO is a good solvent for amylose, the intrinsic viscosity and radius of gyration are considerably larger in DMSO than in water.13J4 It was suggested that amylose conformation in DMSO is stiff and predominantly helical in contrast to Abstract published in Advance ACS Abstracts, June 15, 1994. (1) Nishimura, N.; Janado, M. J. Biochemistry 1976, 77, 421. (2) Nakamura, H.;Shibata, K.;Kondo, H. Biopolymers 1977,16,2363. (3) Hui, Y.; Russell, J. C.; Whitten, D. G. J . Am. Chem. SOC.1983, 105, 1374. (4) French, D.; Pulley, A. D.; Whelan, W. J. Staerke 1963,15, 349. ( 5 ) Nakatani, H.; Shibata, K.; Kondo, H.; Hiromi, K. Biopolymers 1977,16, 2367. (6)Kitamura, S.; Matsumori,S.; Kuge, T. J.InclusionPhenom. 1984, 2, 725. (7) Suddaby, B. R.; Diminey, R. N.; Hui, Y.; Whitten, D. G. Can. J . Chem. 1986, 1315. (8)Winter, W. T.; Sarko, A. Biopolymers 1974, 13, 1461. (9)Banks, W.; Greenwood, C. T. Staerke 1971,23, 300. (10) Szejtli, J.; Richter, M.; Augustat, S. Biopolymers 1967, 5 , 5. (11) Senior, M. B.; Hamori, E. Biopolymers 1973,12, 65. (12)Yamamoto, M.;Harada, S.; Sano, T.;Yasunaga, T.;Tatsumoto, N. Biopolymers 1984,23, 2082. (13) Cowie, J. M. G. Mucromol. Chem. 1983,53, 13. (14) Fujii, J. M.; Honda, K.; Fujita, H. Biopolymers 1973, 12, 1177.
that in water, where its hydrodynamic properties exhibit a random coil behavior.14J5 While the intrinsic viscosity of amylose in that solvent mixture increases with increasing DMSO content (from water to pure DMSO), a transition appears where the DMSO content is 66% and the corresponding transition is also observed with specific optical rotation of amylose in that solvent mixture.I6-l8 It is of particular interest to study how the conformational state of amylose (in a given solvent) is altered by forming inclusion complexes with guest molecules and how the complexabilityof amylose is affectedby the solvent conditions and guest structures. It is suggested that a cooperative binding is the complexation process between amylose and guest molecules,19~z0 implying the possibility of conformational change (induced by guest) in amylose due to the complexation. Some studies ofthis aspect have been made by surface tension and viscosity measurements of surfactants. 18,20 In this paper, we make a systematic assessment of complexation behavior of amylose with photoreactive dyes having a stilbazolium chromophore, based on spectroscopic studies of W-vis, fluorescence, and circular dichroism (CD) in various compositions of DMSO-H20 mixtures. Molecules with this chromophore have the largest secondorder polarizabilitiesz1such that nonlinear optical materials based on this chromophore have been studied in recent A potential benefit of inclusion formation of photoreactive dyes is to induce photo- and thermal stability to the dyes through chemical rigidization and shielding of the guest molecules by host, amy10se.Z~
@
~~
(15) Ring, S.G.;L'Anson, K. J.; Moms, V. J. Macromolecules 1986, 18, 182. (16) Dintzis, F. R.; Tobin, R. Biopolymers 1969, 7, 581. (17) Keim, H.; Klosowaki, J.; Steger, E. J . Mol. Struct. 1976,28, 1. (18)Hui, Y.; Zou, W. In Frontiers in Supramolecular Organic Chemistry and Photochemistry; Schneider, H.-J., Duerr, H., Eds.; VCH: Deerfield Beach. FL. 1991: DD 203-221. (19) Yamamoto, M.; Sano, T.; Harada, S.;Yasunaga,T. Bull. Chem. SOC.JDn. 1983. 56. 2643. ~ ~ (20j Bulpin, P. V.; Cutler, A. N.; Lips, A. Macromolecules 1987,20, 44. (21) Girling, I. R.; Cade, N. A.; Kolinsky, P. V.; Earls, J. D.; Cross, G. H.; Peterson, I. R. Thin Solid Films 1986, 132, 101. (22) Huang, J. Y.; Lewis, A.; Loew, L. J . Biopolym. SOC.1988, 53, 665. (23) Anderson, B. L.; Hoover, J. M.; Lindsay, G . A.; Higgins, B. G.; Stroeve, P.; Kowel, S. T. Thin Solid Films 1989, 179, 413. (24) Kim, 0.-K.; Choi, L. S. Unpublished results. ,.I
~
?
This article not subject to U.S.Copyright. Published 1994 by the American Chemical Society
Complexation of Amylose with Dyes
Langmuir, Vol. 10, No. 8, 1994 2843
4-[4-(dimethylamino)styryl]-l-alkylpyridinium bromide (DASP-C,)
Experimental Section Materials. Several different molecular weight amylose samples (MW = 4000 and 10 000)were obtained as reagent grade from commercial sources (Aldichand Sigma) and used as received. 1dkylpyridinium bromides Among 4-[4-(dimethylamino)-styryll(DASP-C,), some (C1and Czz homologs)were commercial products and others (c6,CS, Clo, Cl2, C14, CIS, and CIS homologs) were synthesized by modifying the known procedure.26*26They were recrystallized twice from ethanol. Reagents for the synthesis, such as 1-alkyl bromide, 4-picoline, and 4-(dimethylamino)benzaldehyde are commercial products and were used as received. Cetyltrimethylammonium bromide (CTAB) is a commercial product and was used aRer recrystallizing from ethanol. Synthesis. A synthetic scheme for DASP-C, is given in the following. The key reaction is base-catalyzed aldol condensation between N-substituted picolin and an amino-substituted benzaldehyde. The typical procedure for the synthesis is described with DASP-CI4. A mixture of 27.7 g (0.10 mol) of 1-tetradecyl
+
CH3*
toluene
WAVELENGTH (nml
Figure 1. UV-visible spectra of DASP-C~Z (3.0 x M) in the absence of amylose a t different DMSO fractions (@DMSO) in DMSO-H20 mixture: (a) 0.2; (b) 0.4; (c) 0.6; (d) 0.8.
CH3-(CH2),-Br
i
CH3CN':
288
( CH,),CH3
388
188
588
618
788
888
1 388
WAVELENGTH (nm)
1
I methanol
piperidine, c H 3 \ N 0 C H 0 CH[
-
I
bromideand9.4g(O.l1mol)of4-picolinein20mLoftoluenewas refluxed for 2 h. After cooling down the reaction mixture, the precipitate was filtered, washed throughly with toluene, and dried in vacuo, yielding 18.5 g (50%) 1-tetradecylpicolinium bromide. A methanol solution (100 mL) of 7.3 g (0.045 mol) of 4-(dimethylamino)benzaldehyde, 16.7 g (0.045 mole) of 1tetradecylpicolinium bromide, and 3 mL of piperidine as catalyst was refluxed for 2 h and then most of the methanol was distilled off. After allowing to cool down t o room temperature, the purplered precipitate, DASP-Cl4, was filtered off and washed with carbon tetrachloride, yielding 8.2 g (36%)after drying in vacuo. The product was recrystallized twice &om ethanol. The purity of the product was confirmed by UV-vis spectra. Details of all syntheses will be published elsewhere. Sample Preparation. Weighed amounts of amylose and the dye were dissolved in DMSO and diluted with water to give the desired compositionof solvent mixtures and a fixed concentration of the reactants. The molar ratios of amylose t o the dye in the solution samples were varied up to 100 (unit mole):l, where the dye concentration was fixed a t 3.0 x M in most cases. Spectroscopic Measurements. W-vis spectra of the solution samples were recorded using a Perkin-Elmer Lamda Array 540 spectrophotometer. Fluorescence spectra of the dye (25) Hassner, A.; Birnbaum, D.;Loew, L. M. J. Org. Chem. 19&4,49, 2546. (26) Bourson, J.; Valeur, B.
J.Phys. Chem. 1989,93,3871.
Figure 2. W-visible spectra of DASP-Czz (3.0 x 10+ M) in the presence of amylose (3.0 x M) at different @DM930in DMSO-H20 mixture: (a) 0.2; (b) 0.4; (c) 0.6; (d) 0.8. (serving as fluorescence probe) with or without amylose in the solution were recorded using a Spex Fluorolog-2 spectrofluorometer. Circular dichroism (CD)spectra of the amylose-DASP dye systems were recorded using a JASCO Model 600 spectropolarimeter with a 20-mm cylindrical quartz cell a t room temperature. The sample solutions for CD were stored overnight before the measurement and the dye concentration in the solution was in the range of 10-4-10-6 M.
Results and Discussion W-Visible Spectra. Conformation of the long tail dyes, particularly DASP-Cla and DASP-CZZ(which are totally insoluble in water), is sensitive to the composition of DMSO-H20 mixtures. As shown in Figure 1, at a low DMSO volume fraction such as @DMSO < 0.5, DASP-CZZ exists predominantly as dimeric aggregates (with the absorption at ca. 420 nm), whereas at @DMSO > 0.7, they stay in the fully monomeric state (with ca. ,Imm= 475 nm), showing a transition at @DMSO x 0.6. However, by adding amylose to the solution (Figure 21,the absorption at ,Ima = 420 nm (due to the dimeric aggregation) disappears, suggesting that the dye aggregates are completely dissociated into either the monomeric state in solution and/ or the inclusion state in the amylose cavity, depending on the solvent composition. When the solvent mixture is at @DMSO < 0.8, the absorption band at ,Imax = 475 nm is completely red shifted to ca. Am= = 500 nm and there is a tendency of increase in the red shift with decreasing DMSO content. Even under an extremely low DMSO content, no precipitation of the dye was observed. When the solvent has @DMSO I0.8, the monomeric band, ,Imm= 475 nm remains unchanged by the addition of amylose, suggesting that no inclusion occurs under this condition.
2844 Langmuir, Vol. 10,No. 8, 1994
WAVELENGTH
(nm)
Figure 3. W-visible spectra of DASP-Cn (3.0 x M) in M) upon addition of CTAB the presence of amylose (3.0 x (1.0 x 10-3 M) at different @DMSO in DMSO-H20 mixture: (a) 0.2; (b) 0.4; ( c ) 0.6; and (d) 0.8.
This is reminiscent of a micellar solubilization of cyanine dye aggregates in the presence of aqueous sodium dodecyl sulfate,27where the absorption bands (A = 440 nm and 465 nm) due to dye aggregation sharply diminished and the monomeric band (A = 484 nm) is red shifted to 502 nm. Corresponding changes such as band shift and enhanced intensity in the fluorescent emission are also observed in the presence of the micelles. Such a micellar effect brings about a remarkable increase in photostability of the dye. Questions can be raised as to whether the dye molecules are truly included in the amylose cavity. A control experiment was carried out to verify this. Long chain hydrocarbon surfactants such as sodium dodecyl sulfate have been known to form an inclusion complex with amylose20~28 through a cooperative mode ofbinding. When the amylose complex with DASP-CZZwas treated with cetytrimethylammonium bromide (CTAB) in a DMSOHzO mixture, a similar absorption band as in Figure 1 was observed (Figure 3). This suggests that a large portion of the included DASP-Czz is replaced by nonchromophoric CTAB, thereby being forced to stay in the bulk solution. The extent of the replacement is more pronounced at a low DMSO content under which the inclusion is facilitated. The situation after the dye replacement has a resemblance t o that where the dye is dissolved alone (without amylose) in the solvent mixture. 'The same experiment with DASPC1, for example, showed no change because no inclusion occurs with the dye, independent of the solvent composition. The aggregation behavior of the DASPs at low @DMSO = 0.2changes drastically depending on the alkyl tail length of the dyes; DASP-Cl8 forms relatively small dimeric aggregates, and DASPs with less than Clz do show negligible or no aggregation. A relatively strong hydrophilicity of these dyes is assumed due to the pyridinium cation in the molecule. Inclusion of these dyes with amylose shows a reciprocal tendency of the dye solubility; the lower the solubility, the higher the inclusion results. Inclusion of these dyes with amylose, which occurs only at @DMSO I0.6, caused a shift of the absorption band (Ama = 475 nm) to ca. 500 nm as in the case of DASP-CZZ. However, no or negligible inclusion was observed with DASPs with less than Clz. This tendency may vary depending on the amylose molecular weight,28but the present DASP dyes are not affected by amylose molecular weight. A strong hydrophobic character in the guest (27)Humphry-Baker, R.;Graetzel, M.; Steiger, R. J.Am. Chem. Soc. 1980,102,847. (28)Wulff, G.;Kublik, S. Mukromol. Chem. 1992,193,1071.
Kim and Choi molecule is known to be essential for the amylose inclusion. It is said that sodium alkyl sulfates, for example, require eight carbon atoms of surfactant for complexation and 12 carbon atoms of surfactant are required for the cooperative binding.lg Since the red shift of absorption bands due to amylose inclusion is not common with other dye systems, it may imply that DASP dyes have an unusually sensitive solvatochromism such that the charge-transfer states of the dyes are sensitive to the microenvironmental polarity change.29 An alternative interpretation for the red shift is that conformation of DASP molecules does cooperatively interact with the host amylose.l9~20If this is the case, the chromophore of the DASP can assume an extended chain conformation, when the amylose helix is loosened with an enlarged inner diameter under a high polar solvent condition such as at @DMSO I0.6. This will allow DASP molecules to rearrange their conformation to assume an extended and delocalized n-conjugation in the chromophore. Under such conditions, a better cooperative binding is induced between the host-guest to bring about a helical conformation. A similar interpretation is made with cyanine dyes stabilized in micelles.27Such a conformational mode might be further related to the solid-state thermochromism of the inclusion complexes of the present system.24Another feature of the present inclusion system is that the complexes are very soluble in water, regardless of the alkyl carbon length in the dye, and they never separate out from the solution. And a red-colored soft solid form of amylose-DASP complexes was obtained from the solution by dialysis and freeze-drying and they are readily redissolved in water. This is quite unusual, because amylose complexes with fatty acids and some aromatic compounds have very low solubility in water. This is reminiscent of the case when a minor modification (e.g., one substitution every 13 glucose units) is made on the glucose units with hydroxypropyl groups, a high solubility and stability in water are attained with helical inclusion complexes without retrogradation,28and that upon complexing with a bulky guest molecule, the helix of amylose inclusion must adopt a loosened conformation containing seven (or eight) glucose units (instead of six) per t ~ r n . ~ ~ ~ Fluorescence Spectra. For amylose inclusion studies, hydrophobic fluorescence dyes such as 2-(p-toluidinyl)naphthalene-6-sulfonate are used to probe the binding state and to determine binding constants and thermodynamic parameter^.^^^^^ Since dye molecules are included in the hydrophobic environment of the amylose cavity, the solubility of the inclusion complex is not affected by the guest dye but is dominated by the host amylose. For the same reason, the excited state of the included dye molecule is not directly affected by the polarity change in the bulk solvent but by a conformational change in the amylose. Since the amylose conformation (without inclusion) in HzO is a random coil in contrast to a stiff and predominantly helical state in pure DMS0,14z31it is plausible that the excited state of the included dye is affected indirectly by solvent polarity. The present inclusion systems, particularly DASP-Czz shows a strong enhancement of fluorescent intensity. Interestingly enough, in the presence of amylose, the fluorescence intensity of the dye increases with increasing DMSO content, showing a transition at @DMSO = 0.6 and decreases sharply with a further increase in DMSO content (Figure 4). In the absence of amylose, the dye fluorescence (29)Ficht, K.;Fischer, K.; Hoff, H.; Eisenbach,C. D. MukromoZ. Chem. Rapid Commun. 1993,14,515. (30)Yamashita, Y.; Monobe, K. J.Polym. Scz., Polym. Phys. Ed. 1971, 9,1471. (31)Jordan, R. C.;Brant, B. A. MucromoEecuZes 1980,13,491.
Complexation of Amylose with Dyes
440.0
Langmuir, Vol. 10, No. 8, 1994 2845
620.0
800.0
WAVELENGTH lnm)
Figure 4. Fluorescence emission spectra (at1415 nm) of DASPM) in the presence (-) and absence (- -) of C22 (3.0 x amylose (3.0 x M) at different @DMSO in DMSO-H20 mixture: (a) 0.2;(b) 0.4; (c) 0.6; (d) 0.8.
I 0.1
0.2 0.3 0.4
0.6
0.6
0.7
I
1
I
0.8
0.9
1.0
DMSO FRACTION I in DMSO/H,O Mixture
I
Figure 6. Relative fluorescence intensity of DASP-C, (n = 10-22) in the presence of amylose (3.0 x as a function of @DMSO in DMSO-H20 mixture: 22 (0);18 (8);16 (e); 14 (0); 12 (a);10 (0).
increases with increasing DMSO content, showing no transition (Figure 4). This is clearly related to the observations in the absence of amylose (Figure 1)that the absorption of DASP-Cz2 at A,,= = 475 (due to the monomeric state)increases with increasing DMSO content and that the absorption of the dye at Am= = 420 (due to the dimeric state) decreases with increasing DMSO content. In other words, the extremely low fluorescence intensities at @DMSO = 0.2 and 0.4, for example, are due to a high concentration of the dimeric state (of the dye) that has a self-quenching effect.32 Therefore, the fluorescence ofthe dye in the absence of amylose results solely from the monomeric state. As discussed earlier, in the presence of amylose, the dimeric state disappears completely and, furthermore, the monomeric state exists exclusively in the inclusion, except @DMSO = 0.8 and 1.0, where virtually no inclusion occurs with the dye (Figure 2). Consequently, under such high DMSO concentrations,there is no or a negligible difference in the fluorescence intensity between the systems with and without amylose. In Figure 5, the fluorescence intensity change of DASP dyes (in the presence of amylose) is plotted as a function of DMSO content in the solvent mixture, where the relative fluorescence intensity was (32) Pal, M. K.;Pal,P. K. Makromol. Chem.,Rapid Commun. 1988, 9, 237.
determined by taking ratios of fluorescence at the individual DMSO content relative to that at @DMSO = 1.0. The relative fluorescence intensity of the dye (DASP-C,) is higher with increasing alkyl carbon length, suggesting that the dye chromophore is more rigid (by amylose) with longer alkyl lengths. Also, it clearly indicates that the inclusion complexes with DASP dyes having alkyl carbon length >Clz have a transition at @DMSO = 0.6, but with alkyl carbon length IC~Z,this transition shifts toward the higher DMSO region. In the former, most of the dyes in the solutions (except for @DMSO 2 0.8) exist in the inclusion such that the fluorescence results mainly from the inclusion state but negligibly from the noninclusion state of the dyes. Conversely in the latter (alkyl length 5 CIZ),the fluorescence contribution comes mostly from the noninclusion state because even under a low DMSO such as @DMSO 5 0.6, the inclusion formation is low due to the insufficient hydrophobicity of the alkyl tails. Accordingly, the transition tends to shift toward a higher DMSO content. This is consistent with the earlier statement based on the visible spectra observation that no or negligible amylose inclusion occurs with the DASPs 5 ClZ. The highest fluorescence observed at @DMSO = 0.6 suggests that amylose in that inclusion state is fully helical or that, at least, the dye molecules in the inclusion are rigid. Enhancement in fluorescence due to the inclusion formation with dyes has been known for c y c l ~ d e x t r i n s ~ ~ ~ ~ ~ and a m y l ~ s e . ~Then, , ~ the question is what makes the difference in the fluorescence intensities among the inclusion complexes formed at @DMSO 5 0.6 under which the dye (e.g. DASP-CZZ) is fully included (Figure 2)? The most reasonable answer is the difference in conformational states of the inclusion complexes in such low DMSO systems. This means the degree of rigidness of the dye by amylose in the helical state under the influence of various DMSO contents. In other words, through the inclusion complexation with a strongly hydrophobic dye, amylose can resume the helical state even in a high polar condition but still undergoes the polar influence on the helical state. Aloose helical state is probably formed under 20%DMSO, for example, and the included dye molecules are conformationally somewhat flexible, leading to a relatively smaller fluorescence. In this sense, the dye conformation in the inclusion complex is the least flexible at @DMSO = 0.6, a relatively tight helical state of the complex. Circular Dichroism Spectra. Circular dichroism (CD) spectroscopy has proven to be extremely useful for conformational studies of polypeptide and polysaccharide complexes in which an achiral molecule with a suitable chromophore is conformationally associated with a chiral ~ ~ ~ ~ ~an achiral chrocenter of m a c r o m ~ l e c u l e . When mophore is introduced into the chiral cavity such as cyclodextrin, for example, an induced Cotton effect, induced CD (ICD), is usually o b ~ e r v e d . ~ Few ~ , ~studies ~ on ICD have been done with amylose inclusion complexes because of low solubility; even in solution, they often precipitate out in a while.28 As shown in Figure 6, ICD spectra of DASP-CZZhave an absorption near the absorption band of the dye chromophore, which is split into a positive and a negative (33) Kinoshita, T.;Iimura, F.; Touji, A. Biochim. Biophys. Res. Commun. 1973,51, 66. (34) Hoshino, M.;Imamura, M.;Ikehara, K.; Hama,Y.J.Phys. Chem. 1981,85, 1820. (35) Ritcey, A. M.; Gray, D. G. Biopolymers 1988,27, 479. (36) Ogawa, K.;Hatano, M. Carbohyr. Res. 1978, 67, 527. (37) Kobayashi, N.;Minato, S.;Osa, T. Makromol. Chem. 1983,184, 2123. (38) Bortolus, P.; Monti, S. J . Phys. Chem. 1987,91, 5046.
2846 Langmuir, Vol. 10, No. 8, 1994
> t Y
t G:
5
Figure 6. Induced circular dichrosim (ICD) spectra (using a 20-mm quartz cell) of DASP-Czz (3.0 x M) at different @DMSO in DMSO-H20 mixture: (a) 0.2; (b)0.4; (c) 0.6; (d) 0.8; (e) 1.0;(f) 0.7.
component. The shape of the ICD band splitting indicates that the optical activity of the dye arises from a helical arrangement of the chromophore (a chain conformational effect) and is not a simple association of the dye molecules at chiral centers. Such optical activity of helical macro,~~ molecules was first treated theoretically by M ~ f f i t tin dealing with a macromolecule in which identical chromophores are arranged in a regular array. This allows for the transfer of the excitation from one chromophore to another, developing a delocalized excitation (excitons) over several residues of the macromolecule. His results predict the splitting of the CD absorption band into two components of equal intensity but opposite sign. The exciton splitting of the CD bands is predicted, but the resulting spectra are not symmetrical and the band shape depends on the helix geometry. The exciton-type Cotton effect in the longer wavelength is observable exclusively a t @DMSO 5 0.6, although there exists a weak negative CD signal at @DMSO = 0.7 in the same region and no signals of absorption at @DMSO =- 0.7. (39)Moffitt, W.J. Chem. Phys. 1966,25, 467.
Kim and Choi This is not surprising because no inclusion occurs under @DMSO > 0.7. This indicates that a true transition occurs at @DMSO = 0.6. Quite surprisingly, there is another exciton band around 420 nm which corresponds to the absorption of a dimeric aggregate of the dye. This band appears only at @DMSO = 0.7 and there are no other chromophores present in the solution. However, there exist negligible amounts of dimeric aggregates and of dye inclusion in the solution a t @DMSO = 0.7 and instead, most of the dye molecules exist in the noninclusion monomolecular state. It seems to imply therefore that the optical activity in the dye molecules (at @DMSO = 0.7) is produced outside the amylose under the chiral influence of amylose. However, this possibility is still difficult to understand. In separate experiments from this, we have observed a very unusual case, in that DASP-CZ~ exhibited a similar optical activity in the same absorption region in the absence of amylose at @DMSO 5 0.6,*O where dimeric aggregates are in equilibrium with the monomeric state. Therefore, it is unclear whether the CD band around 420 nm at @DMSO = 0.7 is chirally induced by amylose in solution. In conclusion, inclusion complexation of amylose with DASP dyes is controlled by alkyl tail length of the dye and the solvent composition of DMSO-H20 mixtures. A full amylose complexation requires the alkyl length of the dye to be >Clz and @DMSO to be 50.6. The chromophore rigidness in amylose helix is facilitated with increasing alkyl length of the dye and is maximized a t @DMSO = 0.6. The @DMSO = 0.6 has therefore a special meaning in amylose inclusion as transitions for inclusion formation as well as for helical conformation. The helical state of amylose upon complexation seems to become loosened with decreasing DMSO content below @DMSO = 0.6.
Acknowledgment. The authors gratefully acknowledge partial funding support from the Office of Naval Research. They are thankful Dr. James Ferretti, Laboratory of Biophysical Chemistry, National Heart, Lung and Blood Institute, National Institutes of Health, for allowing the use of his spectropolarimeter. (40) Kim, 0.-K.; Choi, L. S. Unpublished results.
