Review: fluorescent cyclodextrins for molecule sensing

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Supramolecular Science 3 (1996) 3 l-36 Q 1996 Elsevier Science Limited Printed in Great Britain. All rights reserved 0968-5677/96/$15.00

Review: fluorescent cyclodextrins for molecule sensing Akihiko Ueno Department of Bioengineering, Faculty of Bioscience and Biotechnology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226, Japan (Received 72 December 1995; revised 25 March 1996)

Cyclodextrins (CDs), which are spectroscopically inert, were converted into fluorescent CDs by modification with one or two fluorophores. Many fluorescent CDs changed the fluorescent intensities upon addition of guest compounds, causing the locational change of the fluorophore mostly from inside to outside of the CD cavities. On this basis, the fluorescent CDs were used as fluorescent chemosensors for molecule recognition. Modified CDs bearing two naphthalene or pyrene moieties exhibit intramolecular excimer emission and their guest-responsive excimer intensity variations were used for molecule sensing. Fluorescent CDs bearing a dansyl moiety decreased the fluorescence intensity upon guest addition, reflecting the environmental change around the fluorophore from the hydrophobic interior of the CD cavities to bulk water solution. Modified CDs bearing a p-NJ-dimethylaminobenzoyl (DMAB) moiety exhibit dual emissions from nonpolar planar (NP) and twisted intramolecular charge transfer (TICT) excited states, and the TICT emission intensity was useful for sensing molecules. A biotin-bound DMAB system was also constructed, and the presence of the protein (avidin) was found to enhance the NP fluorescence. This avidin-bound DMAB system showed higher sensitivities and stronger binding ability for guest species than the system without avidin. 0 1996 Elsevier Science Limited (Keywords:cyclodextrin; fluorescence;sensor)

INTRODUCTION Cyclodextrins (CDs) are cyclic oligosaccharides, consisting of six, seven and eight glucose units for a-, /I- and y-CD, respectively (Figure I). The internal diameter increases with bncreasing numbed of glucose units, as0 shown by 4.9A for a-CD, 6.2A for /?-CD, and 7.9A for y-CD’. They form inclusion complexes with a variety of organic compounds in aqueous solution, accommodating a guest molecule in their central cavityte3. Hydrophobic interaction between the CD framework and a guest molecule is usually considered as the major driving force in this inclusion phenomenon. CDs are spectroscopically inert, but we

Figure 1 Illustrative

sns3:113-0

representatnon

for CD structures

have shown that they can be converted into spectroscopically active hosts by modification with one or two chromophores, and found that their spectroscopic properties change upon guest accommodation. On this basis, we reported that dye-modified CDs can be used as colour-change indicators for moleculesb7. We also prepared many fluorescent CDs and have shown that they can be useful as fluorescent chemosensors for molecule recognition. This review presents guestresponsive fluorescence properties of various fluorophore-attached CDs and their abilities for sensing molecules. The idea for using fluorophore-attached CDs as sensors came from the finding that pyrene-modified yCD forms an association dimer and exhibits excimer emission around 470nms9. When we added guest species into the solution of the pyrene-modified CD, there occurred a ‘remarkable decrease in the excimer emission and an increase in the monomer emission around 397nm. The analysis of the guest-induced circular dichroism and absorption variations substantiated that this fluorescence variation occurs associated with the conversion from the association dimer to 1: 1 host-guest complexes (Figure 2). Since the guest-induced fluorescence variation is greatly affected by the size and the shape of the guest species, it was suggested that this system can be used as a sensory system of molecular recognition for detecting various organic compounds”.

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Figure 2 Guest-induced dissociation of the appended y-CD into 1:1 host-guest complexes

dimer

of pyrene-0.6 Cl

Although the above sensory system bears a pyrene moiety at C-6 of one of the glucose members of y-CD, the modified y-CDs with a pyrene moiety at C-2 or C-3 were prepared later and it was found that their association and binding behaviours were greatly affected by the position modified”, 12.

Gl

G2

G3

62

G4

GS

G3

G6

67 Guest

G4

G9

RI ;

CCOH

G9: Rl=OH. Glo:

HOY’ CIP

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Rz=H. R3=H

R,=H, R+i,

RpOH

Gil: R,=H, R*=OH, R,=H

‘**,,RQ H

GlO

G6

GI Gs: R,=H, &=H,

CDs bearing two naphthalene or pyrene moieties

We prepared y-CD derivatives bearing two pyrene units as an improved series of sensors, because two fluorophore-appended CDs change their fluorescence spectra without any need for association dimer formation. The interesting point of this series is that there are four geometrical isomers AB, AC, AD and AE di-substituted-CD derivatives, where the glucose units of y-CD are named A, B, C, D, etc. in the order along the CD circle. Since the two pyrene moieties are located at different positions on the CD rim, they may have different guest-responsive properties. Figure 3 shows fluorescence spectra of AB, AC, AD and AE isomers of bispyrene-modified CDs (1) in 30% DMSO aqueous solution. All these fluorescent CDs exhibit predominant excimer emission around 520nm, and the intensities increased upon addition of (-)-borne01 whereas, in the case of AD and AE isomers, the intensities decreased upon addition of lithocholic acid (LCA), and consequently it is clear that the variation of the fluorescence spectra depends on both the kinds of host and guest species. On this basis, we showed

G8

Number

3

612:

R,=OH. &=H,

R,=OH

Gll

612

G7 U-W

(DCA) (CDCA) UJDW (CA)

Figure 4 Guest-induced fluorescence variations of excimer emission intensities of AB, AC, AD and AE isomers of bispyrene-modified yCDs (1) in 30% DMSO aqueous solution (7.5 PM). The guest concentration is l.OmM for GlG7, and 0.1 mM for G&G12

rco

A8

AC

K

AD

Scheme 1

(1)

AB

1

Wavelength (nm)

l

guest

:, I:. ‘._; p-\

(4)

Figure 3 Fluorescence spectra of AB, AC, AD and AE isomers of bispyrene-modified y-CDs (l), alone (7.5 PM, _) or in the presence of (-)-borne01 (l.OmM, -- - -) or lithocholic acid (0.1 mM, . .) in 30% DMSO aqueous solution

Figure 5 Induced-tit type of guest binding of y-CD and B-CD derivatives bearing one or two fluorophore moieties

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variations in the excimer emission intensity as bars for the four isomers to each guest (Figure 4)13. These bars form a pattern to each guest and we observed different patterns for various organic compounds (Gl-G12), The same approach was used with two naphthalenemodified (2-naphthylsulphonyl-modified) /I- and y-CDs (2,3) for detecting molecules14-‘6. In the case of P-CD bearing two substituents, there are three geometrical isomers AB, AC and AD. Figure 5 shows guestinduced conformational changes of the ?-CD derivatives (eqn 1) and /?-CD derivatives (eqn 2). It is noted that, in such /?-CD derivatives, the excimer formation is not possible without accommodation of guest species in the CD cavity because one naphthyl moiety is included in the cavity and another is located outside the CD cavity in the absence of a guest. In this system, we observed predominant excimer emission around 410nm except for the AB isomer upon guest addition, as we had expected. We examined the guest-responsive properties of these seven fluorescent CDs using the excimer emission intensity and observed remarkable differences between the two series of j?- and y-CD derivatives16. Large guest molecules, such as ursodeoxycholic acid (UDCA) (Figure 4) and chenodeoxycholic acid (CDCA) (Figure 4), exhibit high responsitivity for y-CD derivatives whereas tricyclic and bicyclic ball-like guests, such as adamantanol and (-)-bomeol, exhibit high sensitivities for P-CD derivatives, and a smaller guest, such as cyclohexanol, exhibits a negligible response. However, it is noted that our initial study on 2-naphthylacetylmodified y-CD exhibits a predominant excimer emission, which is hardly changed by guest addition. Obviously, the kind of linking part is important. The above results, therefore, are consistent with the fact that naphthylacetyl units are flexible and form excimers both inside and outside the CD cavity, resulting in no change in the fluorescence spectra17. In the naphthylsulphonyl units are rigidly contrast, oriented and exhibit both normal and excimer fluorescence, and the ratio of the two fluorescence intensities changes depending on the guest species accommodated in the cavities. So, it is interesting to see what happens if the linkage between CD and the chromophore is made more rigid. From this view point, we prepared 2naphthylsulphenyl-modified y-CDs (4), in which the linking part is the sulphur atom. However, we observed only normal fluorescence for these y-CD derivatives and increases in their intensities upon guest addition”. Some other naphthalene-appended CDs also showed guest-responsive variations in the fluorescence spectra’9-22. We also prepared anthracene-modified CDs22-27, but these CDs are not suitable for sensor purposes because they undergo inter- or intramolecular photodimerization. The structural features of the above fluorescence CDs were also substantiated by circular dichroism spectra. Most of them exhibit exciton-coupling patterns based on the electronic interactions between the two chromophores in the CD cavities, but they change the

cyclodextrins:

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patterns to simple positive or negative bands or decrease the absolute dichroism intensities upon guest addition. Dansyl-modified CDs Dansylglycine- and dansylleucine-mod$ed CDs. As the next step, we prepared dansyl-modified CDs2s3’, such as dansylglycine+CD (S), dansyl-L-leucine+CD (6) dansyl-L-leucine-y-CD, dansyl-D-leucine-/?-CD (7) and dansyl-D-leucine-y-CD. Dansyl is known as a probe which exhibits a strong fluorescence in a hydrophobic environment but a weak one in bulk water solution. We found that the /I-CD derivatives of these series form self-inclusion complexes in which the appended dansyl chromophore is included in its own cavity and exhibits strong emission. The B-CD derivatives decrease their fluorescence intensities around 535nm upon guest addition, excluding the dansyl unit from the cavity into bulk water solution associated with guest accommodation (Figure 5, eqn 3)28,29.In the case of y-CD derivatives, they also decrease fluorescence intensity upon addition of larger guest species, but increase the intensity upon addition of the smaller species due to co-inclusion of the fluorophore and a guest molecule in the large y-CD cavity (Figure 5, eqn 4)3’.

The binding constants of the dansyl-modified CDs were obtained from the guest-induced fluorescence intensity variations on the basis of 1:l stoichiometry. In the case of B-CD derivatives, dansyl-L-leucine+CD gives larger binding constants than dansyl-D-leucine-/ICD and both show improved binding abilities over dansylglycine-modified /I-CD. For example, the binding constants of dansyl-L-leucine+CD, dansyl-Dleucine-B-CD and dansylglycine-/?-CD for 1-adamantano1 are 54200, 19900 and 8910 M-‘, respectively. On the other hand, the binding constants of dansyl+ leucine-y-CD, dansyl-D-leucine-y-CD and dansylglytine-y-CD are 116, 370 M-’ and negligible, respectively3’. The values are much smaller than those of the /I-CD derivatives and the chirality preference was reversed. The location and orientation of the dansyl moiety of dansyl-D-leucine#-CD and dansyl-L-leucine+CD were estimated by the NOE correlations between the dansyl protons and the CD protons in the NMR spectra. The result indicates that the dansyl unit in dansyl-D-leucine-/?-CD is more deeply included in the CD cavity than dansyl-r_-leucine+CD. Since deep selfinclusion usually tends to inhibit the guest accommodation, the result is consistent with the data of binding constants. Dunbar et al.32 reported two fluorescence lifetimes (5.6 and 16.9ns) for dansylglycine P-CD (5) and suggested that in-and-out location of the dansyl fluorophore is reflected in the result. This fluorescence behaviour is consistent with the results reported previously by Wang et a1.33 for other dansyl-modified P-CDs.

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R 5:

H -CY-CH

6,7:

FH3 b

Scheme 2

Dansyl-monesin+CD triad system. We prepared oZ dansyllysine+CD and released the protecting Z

moiety so as to produce free amino unit34. To the amino group was reacted monensin carboxyl group to produce dansyl-monensin+CD (8), in which monensin was covalently bonded with o-amino group of lysine unit by amide bond. Monensin is known as an antibacterial compound capable of binding Na+ ions and performs transport of the ion through biomembranes. On this basis, we expected that monensin part acts as a Na+-ion responsive hydrophobic environment. When we compared it with the binding ability of dansylglycine-modified B-CD, we found that the monensin system enhances binding abilities for various guests and the abilities are further enhanced by the presence of a Na+ ion34. The remarkable results were found for nerol and geraniol as guests, as shown by 64- and 92-fold improved binding abilities, respectively, which are further improved by the Na+ ion.

are due to the co-operation effect, and the structural study is needed to clarify this point. p-N,N-dimethylaminobenzoyl-modified

CDs

There are several reports which present the effects of CDs on the TICT emission of p-iV,N-dimethylaminobenzonitrile in aqueous solution3c39. The TICT emission is the one that arises from twisted intramolecular charge transfer excited state, and fluorophores of this kind exhibit dual emissions from nonpolar planar (NP) and TICT excited states. We prepared p-N& dimethylaminobenzoyl (DMAB)-modified CDs (10) as sensors for molecules using the TICT emission40-42. /3CD derivative exhibits strong TICT emissions around 495nm with weak NP fluorescence around 370nm. In contrast, c+ and y-CD derivatives exhibit weak TICT emission around 540 nm40,4’ The intensity of the TICT emission of the /&CD derivative decreases with increasing concentration of a guest compound in aqueous solution. However, the emission intensity of the a-CD derivative increases when the guests are linear alcohols or acids such as pentanol or pentanoic acid42. Branched compounds are less effective in changing the fluorescence intensity, indicating that a-CD cavity prefers the linear-shaped molecules.

Dansyl-modified CD dimer. We prepared dansylmodified /I-CD dimer as the fluorescent CD that can bind a large molecule by co-operation of two CD units35. When we ‘used steroidal compounds, such as UDCA, CDCA, deoxycholic acid (DCA) and cholic acid (CA) (Figure 4), we found remarkable decreases in the fluorescence intensity for UDCA and CDCA, more remarkably for the former, while we found a slight increase and negligible change in the fluorescence intensity for DCA and CA, respectively. It is remarkable molecular recognition because UDCA, CDCA and DCA are geometrical isomers only with the difference of the position or stereochemistry of one hydroxyl group and CA has one more hydroxyl group than others. It is still a problem as to whether these results

We have prepared a new type of fluorescent CD (11) that has the DMAB unit as a fluorophore and biotin as a protein-binding site43. The CD derivative exhibits predominant NP fluorescence with an indication of the presence of TICT emission around 500nm. We used avidin as the protein that tightly binds biotin and observed remarkable enhancement (8.4-fold) in NP

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type molecular cleft for barbiturate4’. It is noted that these sensing systems are useful for particular chemical substances, in contrast to the CD systems which may be used for detecting a variety of organic substances.

CONCLUSION In conclusion, we have shown that fluorescent CDs are useful for detecting molecules with molecular recognition. Incorporation of other functional units, such as an amino acid with a hydrophobic side chain, monensin and protein, improves the sensing and binding abilities of the fluorescent CDs. We have also succeeded in preparing colour-change indicators for molecules, which operate on a similar basis to ‘that of fluorescent CDs. All these results indicate that induced-fit type of conformational changes are essential for these sensors. In future, new types of fluorescent CDs with much larger binding and remarkable molecular recognition abilities will be designed on this basis. The molecule-sensing devices of these fluorescent CDs may be useful, particularly when we apply the pattern analysis method for plural fluorescent outputs from series of fluorescent CDs, comparing the patterns of many organic substances pre-memorized in computers.

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fluorescence by the presence of avidin. It is surprising that in the absence of avidin, the NR fluorescence intensity decreases with increasing guest concentration for most guests but it increases with increasing guest concentration in the presence of avidin. One possible explanation is the insertion of the fluorophore from inside the CD cavity to the pocket of avidin where biotin is included. The presence of avidin results in the markedly improved binding of the fluorescent CD as shown by 3370 and 30100M-i for UDCA in the solutions without and with avidin, respectively.

3 4 5 6 7 8 9 10 11 12 13

Anthranic acid-modified CDs

14

As other fluorophore modified CDs, Hamada et al.44Y45reported the sensor abilities of fluorescent CDs with one or two anthranic acid moieties. The basis for the molecule-response behaviour of these fluorescent CDs is the same as that of other CDs mentioned above, although the recognition features are slightly different. Fluorescent CDs and other molecule-sensing systems

Several molecule- or ion-sensing systems other than CD ones have been reported*, for example, fluorescent boronic acid for saccharide47, chelation-enhanced chemosensor for phosphate anion4*, and bispyrene-

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Wenz, G. Angew. Chem. Int. Ed. Engl. 1994,33,803 Bender, M.L. and Komiyama, M. ‘Cyclodextrin Chemistry’, Springer, Berlin, 1978 Szejtli, J. ‘Cyclodextrin Technology’, Kluwer, Dordrecht, 1988 Ueno, A., Kuwabara, T., Nakamura, A. and Toda, F. Nature 1992,356, 13 Ueno, A. Adv. Mater. 1993, 5, 132 Kuwabara, T., Matsushita, A., Nakamura, A., Ueno, A. and Toda, F. Chem. Let?.1993,208l Kuwabara, T., Nakamura, A., Ueno, A. and Toda, F. J. Phys. Chem. 1994,98,6297 Ueno, A., Suzuki, I. and Osa, T. Chem. Lett. 1989, 1059 Ueno, A., Suzuki, I. and Osa, T. J. Am. Chem. Sot. 1989, 111, 6391 Ueno, A., Suzuki, I. and Osa, T. Anal. Chem. 1990,62,2461 Ueno, A., Suzuki, I. and Osa, T. Chem. Lett. 1989,2013 Suzuki, I., Sakurai, Y., Ohkubo, M., Ueno, A. and Osa, T. Chem. Lett. 1992,2005 Suzuki, I., Ohkubo, M., Ueno, A. and Osa, T. Chem. Lett. 1992,269 Minato, S., Osa, T. and Ueno, A. J. Chem. Sot. Chem. Commun. 1991, 107 Moriwaki, F., Kaneko, H., Ueno, A., Osa, T., Hamada, F. and Murai, K. Bull. Chem. Sot. Jpn. 1987,60,3619 Minato, S., Osa, T., Morita, M., Nakamura, A., Ikeda, H., Toda, F. and Ueno, A. Photochem. Photobiol. 1991,54,593 Ueno, A., Moriwaki, F., Osa, T., Hamada, F. and Murai, K. Bull. Chem. Sot. Jpn. 1986. 59,465 Ueno, A., Minato, S. and Osa, T. Anal. Chem. 1992,64, 1154 Ueno, A., Tomita, Y. and Osa, T. J. Chem. Commun. 1983,976 Ueno, A., Moriwaki, F., Osa, T., Hamada, F. and Murai, K. Tetrahedron L&t. 1985,26, 3339 Ueno, A., Suzuki, I. and Osa, T. Makromol. Chem. 1987,8, 131 Ueno, A., Moriwaki, F., Osa, T., Hamada, F. and Murai, K. Tetrahedron 1987,43, 1571 Ueno, A., Moriwaki, F., Osa, T., Hamada, F. and Murai, K. J. Am. Chem. Sot. 1988,110,4323 Ueno, A., Moriwaki, F., Azuma, A. and Osa, T. J. Chem. Sot. Chem. Commun. 1988, 1373

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Ueno, A., Moriwaki, F., Azuma, A. and Osa, T. J. Org. Chem. 1989,54,295 Ueno, A., Moriwaki, F., Azuma, A. and Osa, T. Curbohydr. Res. 1989, 192, 173 Ueno, A., Moriwaki, F., Suzuki, I., Osa, T., Ohta, T. and Nozoe, S. J. Am. Chem. Sot. 1991, 113,7034 Ueno, A., Minato, S., Suzuki, I., Fukushima, M., Ohkubo, M., Osa, T., Hamada, F. and Murai, K. Chem. Lett. 1990,605 Wang, Y., Ikeda, T., Ueno, A. and Toda, F. Chem. Lett. 1992, 863 Hamada, F., Kondo, Y., Ishikawa, K., Ito, H., Suzuki, I., Osa, T. and Ueno, A. J. Inclusion Phenom 1993,15,273 Ueno, A. et al. J. Am. Chem. Sot. (submitted) Dunbar, R.A. and Bright, F.V. Supramolec. Chem. 1994,3,93 Wang, Y., Ikeda, T., Ikeda, H., Ueno, A. and Toda, F. Bull. Chem. Sot. Jpn. 1994,67, 1598 Nakamura, M., Ikeda, A., Ise, N., Ikeda, T., Ikeda, H., Toda, F. and Ueno, A. J. Chem. Sot. Chem. Commun. 1955,721 Nakamura, M., Ikeda, T., Nakamura, A., Ikeda, H., Ueno, A. and Toda, F. Chem. Lett. 1995,343 Cox, G.S., Haupman, P.J. and Turro, N.J. Photochem. Photobiol. 1984,39,597 Nag, A. and Bhuttacharyya, K. Chem. Phys. Lett. 1988,151,474 Nag, A., Dutta, R., Chattopadhyay, N. and Bhuttacharyya, K. Chem. Phys. Lett. 1989, 157,83

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Nag, A. and Bhuttacharyya, K. J. Chem. Sot. Faraday Trans. 1990,86,53 Hamasaki, K., Ikeda, H., Nakamura, A., Ueno, A., Toda, F., Suzuki, I. and Osa, T. J. Am. Chem. Sot. 1993, 115, 5035 Hamasaki, K., Ueno, A., Toda. F., Suzuki, I. and Osa, T. Bull. Chem. Sot. Jpn. 1994,67,516 Hamasaki, K., Ueno, A. and Toda, F. J. Chem. Sot. Chem. Commun. 1993,331 Wang, J., Nakamura, A., Hamasaki, K., Ikeda, H., Ikeda, T. and Ueno, A. Chem. Lett. 1996,303 Hamada, F., Kondo, Y., Ishikawa, K., Ito, H., Suzuki, I., Osa, T. and Ueno, A. J. Inclusion Phenom. 1994, 17,267 Hamada, F., Ishikawa, K., Ito, R., Shibuya, H., Hamai, S., Suzuki, I., Osa, T. and Ueno, A. J. Inclusion Phenom. 1995, 20, 43 Czarnik, A. (Ed.) ‘Fluorescent Chemosensors for Ion and Molecule Recognition’, ACS Symposium Series 538, American Chemical Society, 1992 James, T.D., Sandanayake, K.R.A.S. and Shinkai, S. Supramolec. Chem. 1995,6, 141 Vance, D.H. and Czarnik, A.W. J. Am. Chem. Sot. 1994, 116, 9397 Aoki, I., Kawahara, Y., Sakaki, T., Harada, T. and Shinkai, S. Bull. Chem. Sot. Jpn. 1993, 66,927

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