5094
J. Phys. Chem. B 1997, 101, 5094-5099
Molecular Dynamics of Novel r-Cyclodextrin Adducts Studied by
13C-NMR
Relaxation
Massimo Ceccato,† Pierandrea Lo Nostro,† Claudio Rossi,‡ Claudia Bonechi,‡ Alessandro Donati,‡ and Piero Baglioni*,† Department of Chemistry, UniVersity of Florence, Via Gino Capponi, 9, 50121 Firenze, Italy, and Department of Chemistry, UniVersity of Siena, Via Pian dei Mantellini, 44, 53100 Siena, Italy ReceiVed: NoVember 18, 1996X
In the presence of poly(ethylene glycol) (PEG), R-cyclodextrin forms a molecular adduct called “molecular necklace” that belongs to the class of polyrotaxanes. The condensation of R-cyclodextrin with epichlorohydrin results in the formation of the so-called “molecular tube” (MT), a rodlike rigid molecule with an empty hydrophobic cavity that can behave as a host for ions or small organic molecules. MT has been obtained from PEG3350, considerably longer than the chain used in previous works. This paper reports an NMR investigation of the dynamic properties, via carbon spin-lattice relaxation technique, of R-cyclodextrin, the molecular necklace, and the molecular tube. The results show that the molecular tube is really formed by a linear thread of condensed R-cyclodextrin molecules and that MT possesses a faster reorientational motion than all the other compounds. UV spectroscopy shows that the molecular tube forms a host-guest system with iodine (I3-) in aqueous solution.
Introduction Since the discovery of cyclodextrins, a great number of low molecular weight inclusion compounds have been prepared and characterized.1-4 Long molecules such as poly(ethylene glycol)s (PEG), alcohols, and substituted alkanes5-9 form threaded crystalline complexes with R-cyclodextrin (R-CD) that belong to the class of “polyrotaxanes”.5,6,10 The word polyrotaxane derives from “rota” (in Latin, wheel), and “axis” (in Latin, axle). Figure 1 schematically shows the formation of a polyrotaxane: these peculiar compounds (ABn) are usually obtained from the threading of one or more cyclic molecules (B) around a linear chain (A) that penetrates their empty cavities. The structure and properties of polyrotaxanes have been the object of several works.10-16 The formation of these interesting host-guest systems is driven by the stabilization of the final product, where strong noncovalent interactions are established between molecules A and B. The formation of such noncovalent molecular assemblies has been named “synkinesis” by Fuhrhop,17 and “synkinons” are the building blocks of the assemblies. In the case of R-CD and PEG (MW ) 3350), the length of almost four oxyethylene units fits the depth of the R-CD’s hole.8 The addition of bulky stoppers at both ends of the linear component avoids the sliding away of the threaded molecules. The kinetic of the threading process significantly depends on the temperature and the nature of the solvent where the reaction takes place.19 Besides the interest in polyrotaxanes in topological chemistry,10,20 they are a great model for the study of the structure and properties of several biological systems such as membranes, ribosomes, and enzyme complexes that result from the aggregation of relatively simple subunits.8,21 Recently Harada and co-workers18,22,23 reported the formation of a threaded molecular structure, called “molecular necklace” (MN), formed by R-CD and 2,4-dinitrobenzene-PEG, and the synthesis of the so-called “molecular tube” (MT), obtained upon cross-linking of the threaded R-CD wih epichlorohydrin and * To whom correspondence should be addressed. E-mail: COLLOID@ SIRIO.CINECA.IT. † University of Florence. ‡ University of Siena. X Abstract published in AdVance ACS Abstracts, June 1, 1997.
S1089-5647(96)03844-8 CCC: $14.00
Figure 1. Formation of a polyrotaxane.
removal of the linear PEG chain.22 The final molecular tube is a rodlike, rigid molecule that keeps the same internal hydrophobic cavities of R-cyclodextrin molecules. The threading process does not occur if PEG is replaced with propylene glycol, because of the presence of branched methyl groups in the chain that increase the cross section of the linear polymer.23 Similarly, the formation of MN is avoided when R-CD is substituted with one of its larger homologues, β- or γ-cyclodextrin, which possess bigger cavities and cannot establish proper interactions with the linear polymer. Removing the PEG chain from the adduct produces the final molecular tube that can act as a host for reversible binding of small organic molecules and ions.8,22 In the recent past the study of molecular recognition operated by low molecular weight ligands has been carried out on crownethers,24 cryptands,25,26 cyclophanes,27 and calixarenes28,29 that selectively bind ions (such as Na+, K+, Cs+, guanidinium, and UO22+) and organic molecules (like chloroform, benzene, toluene, or fullerenes). On the other hand, very large hostguest systems are required for the study of more complicated biological complexes such as enzyme-substrate, antigenantibody, DNA, RNA, and cell adhesion systems.8 The molecular nanotube can be used as a powerful host that can entrap small ions and organic molecules by establishing strong intermolecular interactions with them. This feature is very important with the perspective of using host species as molecular cages in enantiomeric enrichment, isomer separation, molecular recognition, and removal of undesired compounds from industrial products. While the structural, dynamic, and thermodynamic properties of ligands such as crown-ethers and calixarenes have been extensively studied, as well as the formation of the inclusion compounds that they form with ions and organic molecules, much work needs to be done to determine the physicochemical © 1997 American Chemical Society
Molecular Dynamics of R-CD Adducts
J. Phys. Chem. B, Vol. 101, No. 26, 1997 5095
a
b
Figure 2. Synthesis of MN, PEG-MT, and MT.
and chemical properties of nanomaterials such as the molecular tube, especially for their potential use as strong entrapping cages for suitable guest compounds. In this paper we report a study on the dynamic properties of free R-CD, MN, PEG-MT, and MT (see Figure 2) by 13CNMR at different temperatures in DMSO-d6. Our results show that R-CD molecules are indeed aligned in the final MT structure and that the molecular tube shows a faster reorientational motion with respect to R-CD, MN, and PEG-MT. Finally, we demonstrated the formation of a host-guest inclusion complex when an aqueous solution of I3- is added to a water dispersion of the molecular tube, by recording UV spectra. Materials and Methods R-CD, 2,4-dinitrofluorobenzene (DNFB), dimethyl sulfoxide, N,N-dimethylformamide, epichlorohydrin, and PEG3350-bisamine were purchased from Sigma-Aldrich Srl (Milan, Italy). Hydrochloric acid, sodium hydroxide, diethyl ether, deuterium
oxide, and deuterated dimethyl sulfoxide were from Fluka (Milan, Italy). All chemicals were used as received. For all experiments we used bidistilled water purified with a MilliQ system (Millipore) to remove colloidal impurities. Synthesis. The synthesis of the MT is a four-step procedure.18,22,23 NMR, polarimetry, and LC chromatography were used to isolate, purify, and characterize the reaction intermediates and the final products as well. The first step of the synthesis consists in the formation of the adduct between poly(ethylene glycol)-bisamine (PEG-BA) and R-CD: a water solution of R-CD (5 g/35 mL) and an aqueous solution of PEG-BA (0.57 g/5 mL) were mixed at room temperature so that the molar ratio between the two components was about 30. After 10 min, the solution became turbid, and after 1 h a thick solid gel formed. The gel is presumably due to aggregation of several polyrotaxanes that entrap a large amount of solvent in the free cavities. The gel was then filtered under vacuum and stored in a desiccator under vacuum in the
5096 J. Phys. Chem. B, Vol. 101, No. 26, 1997
Figure 3. Elution diagram of the mixture R-CD/2,4-dinitrobenzenePEG and MN (1.7 × 70 cm, Sephadex G-50, DMSO). Absorbance was measured at λ ) 361.5 nm and the optical rotation at 589 nm.
presence of concentrated sulfuric acid (yield ) 90%). The product was then dissolved in DMF, a large excess of 2,4dinitrofluorobenzene was added, and the solution stirred at room temperature for 24 h. Finally the addition of diethyl ether precipitated a yellow solid that was filtered under vacuum, washed with ether, and stored in a desiccator in the presence of concentrated sulfuric acid (yield ) 70%). Unreacted R-CD and PEG were removed by washing with water. Column chromatography (Sephadex G-50, DMSO) was used to further purify the product by checking the absorbance spectrum and the optical solution of each eluted portion. The elution plot showed two peaks: the first referred to the compound that had the highest molecular weight (about 24 000) and was optically active, while the second peak corresponded to R-CD that was still present in the solution. The UV/vis spectrum shows the presence of the molecular necklace in the first elution peak and of unreacted DNFB that appears in another peak. Figure 3 shows peaks A, B, and C that are related to the molecular necklace (MN), PEG-BA (that reacted with DNFB), and R-CD, respectively. After removing the solvent, the solid was dissolved in a 10% aqueous solution of NaOH and treated with an excess of epichlorohydrin that binds two adjacent molecules of R-CD by cross-linking -OH groups. The mixture was stirred at room temperature for 36 h. After neutralization of the solution with HCl and evaporation of the solvent under vacuum, a viscous yellow oil was obtained, and the hydrolysis of the dinitrobenzene residues from the PEG chain was achieved by treating the product with aqueous 25% NaOH for 24 h at room temperature and then neutralizing with HCl. The final solution was eluted through a chromatography column (Sephadex G-25, water). Figure 4 shows the elution pattern that was obtained by measuring the optical rotation of each eluted portion. For NMR experiments we used Varian XL200 and Bruker AMX600 instruments. UV absorbance spectra were recorded with a Perkin Elmer Lambda 5 spectrophotometer. The carbon spin-lattice relaxation rates of a 0.16 mol dm-3 solution of R-CD in DMSO-d6 and 7 × 10-3 mol dm-3 solutions of PEGMT and MT (to obtain the same number of carbon atoms in all cases) have been measured in the temperature range 25-70 °C. Experiments of 13C-NMR were performed to determine the rate constant of relaxation R1 with the sequence (180°-τ-90°-t)n, modified to remove the scalar coupling between 1H and 13C nuclei. The proton decoupling “broad band” induced an increment in the 13C signal, due to the nuclear Overhauser effect
Ceccato et al.
Figure 4. Elution diagram of the reaction mixture of PEG-MT with 25% NaOH (2.2 × 93 cm, Sephadex G-25, water). Absorbance (A, right axis) was measured at λ ) 361.5 nm to detect all dinitro derivatives, and optical rotation (R, left axis) was determined at λ ) 589 nm to detect the PEG-MT complex.
(NOE). The value of R1 was calculated from the following equation:
ln
A∞ - Aτ ) -τ‚R1 2A∞
(1)
where A∞ is the integral of the absorbance line before the perturbance and Aτ is the integral of the absorbance line at time τ. Experimental errors for R1 were (5%. In these conditions we observed that the only effective mechanism of relaxation was the dipolar interaction. In fact, other mechanisms can be excluded according to NOE results. The dipolar interaction in solution, modulated by Brownian motions, results in a magnetic field swinging that can produce relaxation. Therefore, from carbon spin-lattice relaxation rate measurements, we obtained a complete description of the dynamic properties of PEG-MT and MT at different temperatures. For protonated carbon nuclei (the proton-carbon distance being constant and known), the following equations hold: DD RDD 1 ) R1‚χ
χDD )
RDD 1
(2)
NOEexp NOEtheor
[
2 2 2 3τc 6τc 1 h γHγC ) + + 10 r6 1 + ω2Cτ2c 1 + (ωH + ωC)2 τ2c C-H τc
1 + (ωH - ωC)2 τ2c
(3)
]
(4)
These equations calculate the effective correlation time τc. R1DD is the dipolar contribution to the experimental spin-lattice relaxation rate; χDD is the dipolar fraction term obtained from the ratio between the experimental and theoretical broad-band proton-carbon NOE; γH, γC, ωH, and ωC are the proton and carbon magnetogyric ratio and Larmor frequencies, respectively; rC-H is the proton-carbon internuclear distance, and τc is the effective rotational correlation time. Results and Discussion In our experimental conditions the 1H-13C dipolar interaction is the only effective relaxation mechanism that is able to
Molecular Dynamics of R-CD Adducts
J. Phys. Chem. B, Vol. 101, No. 26, 1997 5097
TABLE 1: Chemical Shift of r-CD, MN, PEG-MT, and MT in DMSO-d6 (600 MHz) shift (ppm) of R-CD
MN
PEG-MT
MT
1
C1-H C2-H C3-H C4-H C5-H C6-H C2-OH C3-OH C6-OH C8-OH C(PEG)-H C7-H C8-H CH3 (ethanol) CH2 (ethanol) O-H (ethanol) H2O epichlorohydrin epichlorohydrin epichlorohydrin C1 C2 C3 C4 C5 C6 (free) C6 (bound) C (PEG) C (bridge 1) C (bridge 2)
4.88 3.37 3.86 3.47 3.66 3.73 5.60 5.54 4.58
H-NMR 4.90 3.39 3.88 3.48 3.70 3.76 5.65 5.57 4.64 2.66
102.0 72.2 72.2 82.2 73.4 60.0
4.69 3.63 3.91 3.54 3.68 3.42-3.50 4.83 5.00 4.60 5.22 2.66 3.60 3.72 1.16 3.56 4.54 3.76 3.65-3.67 3.82-3.88 3.23-3.27
13C-NMR 102.0 102.4 72.3 73.0 72.3 74.0 73.5 72.6 82.2 70.9 60.0 61.5 63.0 40.5 40.5 60.1 75.8
4.62 3.43 3.81 3.50 3.68 3.42 4.79 4.90 4.53 5.09 2.64 3.56 3.73 1.20 3.52 4.47 3.76 3.65-3.67 3.82-3.88 3.23-3.27 102.0 72.8 74.3 72.5 70.5 61.5 63.0 59.9 75.8
TABLE 2: Relaxation Time of r-CD, MN, PEG-MT, and MT as a Function of Temperature at ω ) 200 MHz, Calculated as Average Value over All Signals R1 (average ) at
R-CD MN PEG-MT MT
25 °C
40 °C
55 °C
70 °C
10.30 10.00 1.90 2.00
9.70 9.30 1.50 1.30
8.60 7.70 1.25 1.00
7.50 7.00 1.00 0.75
contribute to the experimental value of R1 (χDD g 0.95). 1H13C relaxation rate measurements on R-CD, MN, PEG-MT, and MT were carried out at different temperatures (T) in order to analyze the dynamic properties of these molecules. In fact the temperature dependence of R1 reveals whether the molecular reorientational dynamic is fast (ω0τc , 1) or slow (ω0τc . 1): in the case of fast motion (ω0τc , 1), R1 decreases with T due to the decrement of J(ω) at ω ) ω0. In all the studied cases, R1 lowers as T increases (see Table 1), and therefore all the chemicals show fast motions. This first experimental result is particularly interesting, because it shows that R-CD, MN, PEG-MT, and MT possess fast dynamics in solution, in spite of their high molecular weight, as Table 2 shows. The average correlation times over all carbons (τc), calculated from the relaxation rate R1, are 7.5 × 10-10, 6.5 × 10-10, 1.5 × 10-10, and 1.8 × 10-10 s for R-CD, MN, PEG-MT, and MT, respectively (see Table 3). From these τc data it is evident that the dynamics of R-CD and MN in solution are much slower than those of PEG-MT and MT, in spite of their lower molecular weight. The reason of such discrepancy is to be ascribed to the presence of a main internal axis in PEG-MT and MT molecules. In the case of MN, the PEG chain is not able to force R-CD molecules to rotate simultaneously around the preferential symmetry axis; therefore its dynamic is slow.
Table 3 reports the values of τc for the C5 and C6 signals and the calculated τj at 25, 44, 55, and 70 °C for R-CD, MN, PEGMT, and MT. The fact that τc values for C5 and C6 atoms are different is due to the jump motion of the -CH2OH group around the C5-C6 bond that affects the dynamic of C6; τj represents the correlation time related to such additional motion. τj is calculated from the following equation:31-33 -1 -1 τ-1 c (C6) ) τc (C5) + τj
(5)
r-CD. The data reported in Table 3a show that ln(τc) is proportional to T-1, suggesting that the cyclodextrin correlation time can actually be described in terms of activation energy in an Arrhenius expression. The dynamic analysis shows a limited distribution of τc along the sugar chain; in particular, τc for C1 and C4 is always larger than that for C2, C3, and C5 at all temperatures. This behavior is related to the fact that the ring formed by C1, C4, and the anomeric oxygens that link adjacent molecules of R-CD is rigid. On the other hand C2, C3, and C5 are subject to a bending with respect to the C1-C4 axis, and therefore their reorientation is faster, since they have one more component of motion. Nevertheless, since the ω0τc , 1 condition holds a less efficient longitudinal relaxation, the -CH2OH group contributes with another rotation to the motions that are typical for cyclopyranose. In fact, for the primary alcoholic residue there is a rotation around the C5-C6 bond that allows this group to have faster dynamics. According to eq 5 we calculated τc(C5) ) 7.715 × 10-10 s and τj ) 19.8 × 10-10 s for R-CD; these values show that the reorientational rotation around the C5-C6 bond contributes to the relaxation only in a little way. Molecular Necklace. This system is very similar to cyclodextrin; however, the differences in τc of the different carbon atoms are smaller. It is interesting to note that in this case the calculation of the correlation times for the relaxation of the -CH2OH group gives τc(C5) ) 6 × 10-10 s and τj ) 22.6 × 10-10 s. PEG-Molecular Tube. The calculation of the correlation times for the relaxation of the primary alcoholic group gives τc(C5) ) 4.37 × 10-11 s and τj ) 22.5 × 10-11 s. Molecular Tube. In the case of MT we observed a new signal corresponding to the C6 atom, at 60.8 ppm, that links adjacent cyclodextrins, while the signal for the free C6 appears at 63.04 ppm. This information shows that R-CDs are really condensed through -OH groups. The analysis of the temperature dependence of τc is important in order to determine the energy barrier for the reorientation of the system in solution. As previously suggested, τc can be written in the following form:30
( )
τc ) τ0c exp
Ea RT
(6)
where Ea is the activation energy for the reorientational motion. If no phase transition occurs in the solvent in the studied temperature range, the Arrhenius plot ln(τc) vs 1/T must be linear, and eq 6 can be used to study the factors that affect the dynamics of R-CD/MN/PEG-MT/MT. In fact any deviation of the experimental data from eq 6 can be ascribed to an internal molecular rearrangement due to the temperature effect on the molecular motions. A large discrepancy between the slope of the fitting curve and the expected theoretical value indicates the presence of impurities in the sample during the synthesis. Plotting the values listed in Table 3 and calculating the activation energy for each case, we obtained the results for Ea listed in Table 4. The data show that no conformation changes occur due to the temperature increment, and therefore that as T
5098 J. Phys. Chem. B, Vol. 101, No. 26, 1997
Ceccato et al.
TABLE 3: Correlation Times (ln τc) a. For R-CD T (K)
C1
C2
C3
C4
C5
C6
τj
298.16 313.16 328.16 343.16
-21.010 -21.115 -21.296 -21.561
-20.983 -21.179 -21.426 -21.665
-20.983 -21.179 -21.426 -21.665
-21.033 -21.115 -21.364 -21.585
-20.991 -21.179 -21.364 -21.643
-21.312 -21.539 -21.699 -21.929
-20.0417 -20.3421 -20.2665 -20.4686
T (K)
C1
C2
C3
C4
C5
C6
τj
298.16 313.16 328.16 343.16
-21.272 -20.828 -20.885 -20.985
-21.240 -21.240 -21.561 -21.780
-21.240 -21.240 -21.561 -21.780
-21.091 -21.115 -21.536 -21.756
-21.038 -21.330 -21.585 -21.676
-21.474 -21.533 -21.834 -21.996
-20.4343 -19.8391 -20.3225 -20.6995
b. For MN
c. For PEG-MT T (K)
C5
C6
τj
298.16 313.16 328.16
-23.8537 -24.1653 -24.4740
-24.0310 -24.3024 -24.5865
-22.2061 -22.4500 -22.6400
d. For MT T (K)
C5
C6
τj
298.16 313.16 328.16 343.16
-22.465 -22.847 -23.142 -23.416
-23.131 -23.545 -23.877 -24.159
-22.411 -22.857 -23.222 -23.514
Figure 6. Absorbance spectra of I3- and the MT/I3- system. Figure 5. Numbering of carbon atoms in a glucose ring and location of bridges between linked cyclodextrins.
TABLE 4: Values of Ea for r-CD, MN, PEG-MT, and MT for C6, C5, and CH2OH Groups Ea (kJ/mol) molecule
C6
C5
CH2OH
R-CD MN PEG-MT MT
11.38 13.83 15.05 19.41
12.08 10.37 16.81 17.90
7.73 25.65 23.80 20.89
increases the motion becomes faster and faster, but its kind remains the same. MT is characterized by an Ea value that is larger than that for the other compounds, particularly with respect to R-CD (see the data for C6 and C5); such behavior can be explained by considering that the mass of the MT is at least 20 times larger than that of cyclodextrin, and thus the inertia for the reorientation of the molecular tube is larger. This
means that for MT the initial reorientation is more difficult, but the correlation time is shorter because of the higher activation energy. Moreover, the Ea for the internal orientation of the -CH2OH group is larger for MN than for cyclodextrin. This evidence can be explained by considering that many close threaded cyclodextrins produce a steric hindrance that results in a slower motion with respect to free cyclodextrin molecules in solution. Host-Guest Interactions. In order to study the capability of MT to include ions or small molecules in its empty cavity, we investigated the inclusion compound formed from MT and an aqueous solution of I3- by recording UV spectra. Figure 6 reports the absorbance spectra of I3- and a MT/I3- mixture and shows a significant shift of the λmax value. This change is comparable to the wavelength shift recorded for the MT/I3system found by Harada,22 using a shorter molecular tube, and that has been related to the formation of a host-guest inclusion complex. Our finding confirms the formation of the MT/iodine inclusion complex even for a much longer nanotube, with an
Molecular Dynamics of R-CD Adducts estimated length of 16-23 nm. Further studies are being carried out at this moment with the purpose of testing the host properties of MT toward organic molecules, such as caffeine and aldehydes, for their potential industrial applications. Conclusions R-, β-, and γ-cyclodextrins form crystalline complexes when treated with a proper long-chain polymer. In this paper we report the synthesis of a supramolecular compound called “molecular tube” (MT) made up of R-cyclodextrin (R-CD) molecules threaded on a linear chain of poly(ethylene glycol)bisamine (MW ) 3350) and condensed with epichlorohydrin. 1H-NMR studies confirm the rodlike structure of MT, whereas 13C-NMR experiments have been performed in order to investigate the dynamics of R-CD, R-CD/PEG complexes, and the final MT. The study of the dynamics of R-CD derivatives is of great importance in order to clarify the structure stability and the selective uptake properties of these compounds. Our results indicate that, in spite of its large molecular weight, MT possesses a faster reorientational motion than R-CD and its precursors, due to the presence of a main rotation axis in the MT molecule. The minor influence of local motions observed in the case of MT with respect to R-CD and its derivatives suggests a conservative almost linear arrangement of the R-CD moieties in the nanotube. The effect of this peculiar behavior on the uptake inclusion properties is an important issue that needs to be further studied. In fact, depending on the dynamics within the molecular tube, ions or organic molecules with an appropriate size can be either transported and stored in the tube’s cavity or absorbed at the end of the rod structure, or they can remain confined on its external surface. By recording UV spectroscopy, we also showed the formation of a host-guest inclusion compound between MT and I3- ions in water solution; this finding is particularly relevant for further studies on the inclusion properties of the molecular tube toward ions and organic molecules for scientific and industrial applications. Acknowledgment. The authors acknowledge the Italian Ministry of University (MURST), the Consortium for the Study of Large Interface Systems (CSGI), and the European Union (Grant No. ERB PECO 940005) for partial financial support. References and Notes (1) Szejtli, J. Industrial Application of Cyclodextrins. In Inclusion Compounds; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Eds.; Academic Press: London, 1984; Vol. 3, pp 331-90.
J. Phys. Chem. B, Vol. 101, No. 26, 1997 5099 (2) Garcia, O.; Quintela, P. A.; Schuette, J. M.; Varga, R.; Yoon, H. R.; Kaifer, A. E. The Interactions of Vesicle-forming Surfactants with Cyclodextrins. In Inclusion Phenomena and Molecular Recognition; Atwood, J. L., Ed.; Plenum Press: London, 1990; pp 251-9. (3) Cooper, A. J. Am. Chem. Soc. 1992, 114, 9208. (4) Uekama, K.; Fujinaga, T.; Hirayama, F.; Utagiri, M.; Yamasaki, M. Int. J. Pharm. 1982, 1. (5) Saenger, W. Structural Aspects of Cyclodextrins and their Inclusion Complexes. In Inclusion Compounds; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Eds.; Academic Press: London, 1984; Vol. 2, pp 23159. (6) Mc Mullan, R. K.; Saenger, W.; Fayos, J.; Mootz, D. Carbohydr. Res. 1973, 31, 37. (7) Born, M.; Ritter, H. AdV. Mater. 1996, 8 (2), 149. (8) Harada, A. Coord. Chem. ReV. 1996, 148, 115. (9) Wenz, G.; Keller, B. Macromol. Symp. 1994, 87, 11. (10) Gibson, H. W.; Liu, S.; Lecavalier, P.; Wu, C.; Shen, Y. X. J. Am. Chem. Soc. 1995, 117, 852. (11) Agam, G.; Gravier, D.; Zilkha, A. J. Am. Chem. Soc. 1976, 98, 5206. (12) Frish, H. L.; Wassenmann, E. J. Am. Chem. Soc. 1961, 83, 3789. (13) Billmeyer, F. W. Textbook of Polymer Science; Interscience: New York, 1965; p 27. (14) Kitaigorodskin, A. J. Organic Chemical Crystallography; Consultants Bureau: New York, 1961; Chapter 1. (15) Schillg, G. Justus Liebig Ann. Chem. 1969, 721, 53. (16) Ishin, R.; Angel, E. K. J. Am. Chem. Soc. 1991, 113, 8188. (17) Fuhrhop, J. H.; Ko¨ning, J. Synkinesis and Synkinons of Supramolecular Assemblies. Membranes and Molecular Assemblies: The Synkinetic Approach; Monograph in Supramolecular Chemistry No. 5; The Royal Society of Chemistry: Cambridge, 1994; p 4. (18) Harada, A.; Kamachi, M. Macromolecules 1990, 23, 2821-3. (19) Ceccato, M.; Lo Nostro, P.; Baglioni, P. Langmuir 1997, 13, 243639. (20) Gibson, H. W.; Bheda, M.; Engen, P. T.; Shen, Y. X.; Sze, J.; Wu, C.; Joardar, S.; Ward, T. C.; Lecavalier, P. R. Makromol. Chem., Macromol. Symp. 1991, 42/43, 395-407. (21) Stryer, L. Biochemistry, 3rd ed.; W. H. Freeman & Co.: New York, 1988; Chapters 12 and 30. (22) Harada, A.; Li, J.; Kamachi, M. Nature 1993, 364, 516-8. (23) Harada, A.; Li, J.; Kamachi, M. Nature 1992, 356, 325-7. (24) Pedersen, C. J. J. Am. Chem. Soc. 1967, 89, 2495. (25) Lehn, J. M. Angew. Chem., Int. Ed. Engl. 1988, 27, 89. (26) Baglioni, P.; Bencini, A.; Dei, L.; Gambi, C. M. C.; Lo Nostro, P.; Chen, S. H.; Liu, Y. C.; Teixeira, J.; Kevan, L. Colloids Surf. A 1994, 88, 59. (27) Cram, D. J. Nature 1992, 356, 29. (28) Lo Nostro, P.; Casnati, A.; Bossoletti, L.; Dei, L.; Baglioni, P. Colloids Surf. A 1996, 116, 203. (29) Dei, L.; Casnati, A.; Lo Nostro, P.; Pochini, A.; Ungaro, R.; Baglioni, P. Langmuir 1996, 12, 1589. (30) Dwek, K. A. NMR in Biochemistry; Clarendon Press: Oxford, 1973; p 178. (31) Wittbort, R. J.; Szabo, A. J. Chem. Phys. 1978, 69, 1722. (32) London, R. E.; Avitabile, J. J. Am. Chem. Soc. 1977, 99, 7765. (33) Hertz, H. G. Prog. Nucl. Magn. Reson. Spectr. 1982, 26, 115.
