pH-Triggered Assembling System Using Cooperative Binding between Cyclodextrin-Conjugated Poly(E-lysine)s and Anionic Guest in Aqueous Media Hak Soo Choi, Kang Moo Huh, Tooru Ooya, and Nobuhiko Yui* School of Materials Science, Japan AdVanced Institute of Science and Technology, 1-1 Asahidai, Tatsunokuchi, Ishikawa 923-1292, Japan ReceiVed: NoVember 7, 2003; In Final Form: April 6, 2004 Various types of cyclodextrin-conjugated poly(ǫ-lysine)s (CDPLs) were prepared as polymeric hosts, and the inclusion properties with 6-(p-toluidino)-2-naphthalenesulfonate (TNS) were investigated at a low concentration under various environmental stimuli. Fluorescence studies revealed that the ability of CDPLs for inclusion complexation was much stronger than that of the corresponding CDs due to their cooperative binding. This property can be controlled by the chemical composition of polymeric hosts such as changing the ring size and varying the degree of substitution of CDs. In addition, the inclusion property of the polymeric host can be modulated by environmental stimuli such as temperature, pH, and ionic strength of aqueous media. Such an interesting functionality results from the combination of the cooperative host-guest interactions and the ionic interaction between the negatively charged TNS moieties and the positive amino groups of poly(ǫ- lysine). Introduction Cyclodextrins (CDs), cyclic oligomers of R-1,4-linked gluco- pyranose units, are well-known for their abilities to include various kinds of guest molecules into their hydrophobic cavities. 1 A variety of low or high molecular weight molecules were found to be included partially or completely into their corresponding CD molecules by complex driving forces involving hydrophobic interaction, hydrogen bonding, and the release of high-energy water molecules from the CD cavity. 2 Recently, to improve the molecular inclusion abilities of the native CDs, a great deal of effort has been concentrated on the design and synthesis of novel CD derivatives. 3-5 One way to improve or modulate the binding property of CDs is to introduce CDs to polymeric structures. The introduction of such polymeric structures makes it possible to gather CD molecules closer so that neighboring CD molecules participate in the process of inclusion complexation cooperatively and simultaneously. As a result, introducing CD molecules to polymeric systems with various structures, such as side/end groups, backbone compo- nents, or cross-linked type, can improve the ability for com- plexation by providing a high local concentration at binding sites. 6 On the basis of these studies, in the course of our systematic studies, a novel polymeric host was prepared by conjugating CD molecules to biodegradable and cationic poly(ǫ-lysine) (PL). 7,8 The CD-conjugated PL (CDPL) was found to have many promising properties such as high water solubility, biodegrad- ability, and functionality that would be useful for a broad range of applications. We have investigated the inclusion property of R- or -CDPL with a model guest 3-(trimethylsilyl)propionic acid (TPA) at relatively high concentrations (over 1 wt %). An interesting complexation-induced phase separation was observed as a result of the dual complexation phenomena: cooperative interactions of TPA/CD inclusion and ionic interaction between the PL backbone and TPA. On the basis of these dual complex interactions, the supramolecular assembly could exhibit rapid responses to any small change of pH or temperature. However, we could not observe any significant change in both pH- sensitivity and thermosensitivity at relatively low concentrations (below 0.5 wt %). In this study, R- and -CDPLs with different degrees of substitution (DS) were synthesized and used as cationic polymeric hosts. The inclusion properties of R- or -CDPLs were investigated with 6-(p-toluidino)-2-naphthalenesulfonate (TNS) to understand the mechanism of intermolecular inter- actions at a low concentration (CDPL, 1 × 10 -3 M). TNS was selected as a fluorescence probe due to the hydrophobic and ionic end-groups such as TPA, which would play a dominant role in inducing dual complexation phenomena. The fluores- cence intensity of TNS was observed in the presence of the polymeric hosts under various environmental stimuli such as temperature, pH, and ionic strength. Results and Discussion TNS is a well-known fluorescence probe, which shows a strongly increased intensity with shift of the emission maximum in apolar solvents or in hydrophobic cavities of CD. Kondo et al. suggested that the fluorescence enhancement of TNS in the presence of CDs resulted from the inclusion complexation of TNS. 9 This property of TNS was exploited to evaluate the complexation abilities of R- or -CDPL in comparison with those of R- or -CD. In addition, the effect of chemical structures of host, the degree of substitution (DS) of CDs and ring size, and solution conditions such as ionic strength, surfactant effect, temperature, and pH on the fluorescence intensity of TNS changes in various CDPL solutions. Effect of Conjugating CDs on the Fluorescence Intensity. As shown in Figure 2, the relative fluorescence intensity (I/I 0 ) of TNS at 445 nm was markedly enhanced over 100-fold by inclusion complexation with various CDPLs, while R- or -CD/ * To whom correspondence should be addressed. Phone: +81-761-51- 1640. Fax: +81-761-51-1645. E-mail: yui@jaist.ac.jp. 7646 J. Phys. Chem. B 2004, 108, 7646-7650 10.1021/jp037388f CCC: $27.50 © 2004 American Chemical Society Published on Web 05/14/2004