Fluorescent Chemosensors for Ion and Molecule Recognition

Chapter 6

Fluorescent Cyclodextrins for Detecting Organic Compounds with Molecular Recognition

Downloaded by UNIV LAVAL on July 10, 2014 | http://pubs.acs.org Publication Date: October 20, 1993 | doi: 10.1021/bk-1993-0538.ch006

Akihiko Ueno Department of Bioengineering, Faculty of Bioscience and Biotechnology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan

Cyclodextrin derivatives with one or two chromophores were used as fluorescent sensors for detecting organic compounds in aqueous solution. They undergo induced-fit type of conformational changes associated with guest binding, changing the location of the chromophores mostly from inside to outside of the cavities. The guest binding abilities of the hosts are usually reflected in the sensitivities of the sensors, and various compounds including steroids were detected with remarkable molecular recognition.

There are many hosts with which metal cations can be detected by spectral changes. However, almost no effort has been invested to construction of the hosts that exhibit spectral variations upon complexation with guest molecules. Under this situation, we have attempted to construct host compounds that are spectroscopically responsive to molecules, and recently succeeded in constructing molecule-responsive sensors or indicators. This review gives a brief survey of our work. Cyclodextrins (CDs) are doughnut shaped cyclic compounds consisting of six or more glucose units and form complexes of inclusion type with various organic compounds in aqueous solution ( 1 ) . CDs are spectroscopically inert, but they can be converted into spectroscopically active compounds by modification with chromophores. We have prepared several chromophore-modified CDs in order to construct sensors or indicators for detecting molecules in aqueous solution, and found that they change fluorescence or absorption intensities on guest binding with different sensitivities depending on the shape, bulkiness, and polarity of the guest molecules.

0097-6156/93/0538-0074$06.00/0 © 1993 American Chemical Society In Fluorescent Chemosensors for Ion and Molecule Recognition; Czarnik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

6. UENO

Fluorescent Cyclodextrins for Detecting Organic Compounds


Excimer Emission of Pyrene-modified γ - C D CDs are named as α-, β-, and γ-CD for six, seven, and eight glucose members, aand β-CD usually form 1:1 host-guest complexes (A) while γ-CD, which has a much larger cavity size, forms 1:2 host-guest complexes (B) in which two guest molecules are included in the γ-CD cavity (2,3). However, in the case of pyrene as a guest, γCD forms a 2:2 complex (C) because of the large size of pyrene (4). In this complex, two pyrene units are facing each other and thus form an excimer when one of pyrene units is excited. The excimer is an excited-state complex formed between singlet excited state and ground-state chromophores, and exhibits a broad fluorescence band in the longer wavelength region than that of normal fluorescence. In connection with the 2:2 complex between γ-CD and pyrene, we prepared pyrene-modified γ-CD (D). This modified γ-CD forms a stable association dimer and exhibits a strong pyrene excimer emission around 470 nm (5). The association dimer was converted into a 1:1 host-guest complex upon guest addition (eq. 1 in Figure 1), and consequentiy the excimer emission decreased with increasing guest concentration (Figure 2). The extent of the decrease of the excimer emission intensity to the original one (Δ1/Ι ) was used as a sensitivity parameter, and its values for various guests such as monoterpenes, steroids, alkaloids etc. were obtained. The results indicated that the sensitivity value is remarkably affected by size, shape, and polarity of the guest molecules. Thus, the pyrene-modified γ-CD was shown to act as a sensor for detecting organic compounds with molecular recognition (6). Among many guest compounds, the system was particularly sensitive to some steroids such as chenodeoxycholic acid and ursodeoxycholic acid. There was a good correlation between the fluorescence variations and the binding constants; therefore the strength of guest binding of the system is reflected in the fluorescence variation for each guest 0

Sensors for Detecting Organic Compounds by Dansyl Fluorescence Dansyl is known as a hydrophobic probe which exhibits fluorescence more strongly in a hydrophobic environment than in polar water milieue. Dansyl is also known to show fluorescence enhancement when its internal motions are restricted (7). We prepared dansylglycine-modified β-CD (β-DG, Chart 1) as a sensor, and observed that the fluorescence intensity around 560 nm decreased upon guest addition (8). This result suggests that the host undergoes a conformational change associated with inclusion of a guest molecule (eq. 2 in Figure 3), changing the location of the dansyl moiety from inside to outside of the cavity. This implies that the dansyl moiety is excluded from the hydrophobic environment of the CD cavity to water environment when the guest molecule is accommodated in the cavity. Various guest species (Chart 2) caused different decreases of the dansyl fluorescence (Figure 4). Among steroidal compounds, the host is not sensitive to ketosteroids such as progesterone, corticosterone, cortisone, prednisolone, and hydrocortisone while remarkably sensitive to chenodeoxycholic acid (CDCA) and ursodeoxycholic acid (UDCA). It is interesting that deoxycholic acid (DCA) was not effectively detected by the host in spite of the fact that it is also regioisomeric with C D C A and U D C A with one hydroxyl group at C-12 of the steroidal framework in place of C-7 of C D C A and U D C A . /Borneol was detected with a comparable sensitivity to that of C D C A , while other compounds such as menthol, fenchone, nerol, and cyclohexanol were detected with moderate sensitivities. The result demonstrates that β-DG has remarkable molecular recognition ability and is useful as a fluorescence sensor for detecting organic compounds in aqueous solution. We also prepared dansyl-L-leucine-modified β-CD (β-DL), and found that its molecular recognition abilities are similar to those of β-DG although its sensitivities are roughly twice those of β-DG (Figure 4).

In Fluorescent Chemosensors for Ion and Molecule Recognition; Czarnik, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

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FLUORESCENT CHEMOSENSORS FOR ION AND MOLECULE RECOGNITION


/ **\ / ' N \

PS/

guest

(D

Figure 1. Complexes of CDs with different stoichiometrics (A, B , C) and guestinduced dissociation of the dimer of pyrene-appended γ-CD (eq. 1).

[1-borneol]

360 400

450

500

550

600

Wavelength (nm)

Figure 2. Fluorescence spectra of pyrene-appended γ-CD (6-deoxy-6-amino-[4(pyrenyl)butanoyl]^CD) at different /-borneol concentrations in 10% DMSO aqueous solution (Adapted from réf. 5).


Fluorescent Cyclodextrins for Detecting Organic Compounds

77


6. UENO

Figure 3. Induced-fit guest binding of mono-substituted (eq. 2) and di-substituted (eqs. 3 and 4) CDs.


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FLUORESCENT CHEMOSENSORS FOR ION AND MOLECULE RECOGNITION

progesterone

corticosterone

COOH

"OH H O ^ v I A ;

Fluorescent Chemosensors for Ion and Molecule Recognition

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