Complexation of β-cyclodextrin with dual molecular probes bearing fluorescent and paramagnetic moieties linked by short polyether chains

Abstract

Electron paramagnetic resonance (EPR) and fluorescence spectroscopies provide molecular-level insights on the interaction of paramagnetic and fluorescent species with the microenvironment. A series of dual molecular probes bearing fluorescent and paramagnetic moieties linked by flexible short polyether chains have been synthesized. These new molecular probes open the possibility to investigate various multi-component systems such as host-guest systems, polymeric micelles, gels and protein solutions by using EPR and fluorescence spectroscopies concertedly. The EPR and fluorescence spectra of these compounds show that the dependence of the rotational correlation time and fluorescence quantum yield on the chain length of the linker is not linear, due to the flexibility of the polyether linker. The quenching effect of the nitroxide moiety on the fluorescence intensity of the pyrene group varies with the linker length and flexibility. The interaction of these dual molecular probes with β-cyclodextrin, in solution and in polymeric gels, was evaluated and demonstrated by analysis of EPR and fluorescence spectra.

A Comparison of the Behavior of Monomolecular and Dual Molecular Probes in F127/Cyclodextrin Systems

“…Moreover, the changes in the fluorescence intensity are less for system F127‐3 that contains HPB in higher concentration. In a previous study on the behavior of Py(EG) n and Py(EG) n T probes, we have shown that complexation with cyclodextrin does not involve inclusion of the pyrene moiety into the cavity . In the presence of HPB, it is thus possible that the complexation involves the inclusion of ethylene glycol chains.…”

“…Previously, the interaction with cyclodextrin of the molecular probes involved in this study was investigated by either fluorescence or EPR spectroscopy. We have found that both fluorescence and EPR parameters are sensitive to this interaction and we evidenced the role played by the linker in the geometry of the complex . The F127 systems investigated in this study were previously characterized macroscopically by dynamic rheometry and the tube inversion method (Figure S1, Supporting Information), in order to obtain the temperature corresponding to the transition from micelle to gel ( Table 1 ).…”

“…In aqueous solution, their fluorescence spectra do not show the vibronic shape that is characteristic to pyrene emission, and their fluorescence intensity decreases slowly and linearly with temperature. In the presence of β‐cyclodextrin or in the case of probes trapped in hydrogels, the fluorescence spectra show a vibronic‐like structure with maxima arising at lower wavelengths as compared to aqueous solution . Similarly to these cases, in all F127 systems considered in the present study, the Py(EG) n and Py(EG) n T probes show two emission peaks at 390 and 410 nm.…”

“…For instance, in the F127‐1 system, the a N value of Py(EG) 1 T is 16.33 G, while the a N value of Py(EG) 6 T is 16.57 G ( Table 2 ). Due to the conformational particularities of the oligoethylene oxide chain, the dual probes do not show a linear correlation between the EPR spectral parameters and the chain length . Therefore, the conformation adopted by the oligoethylene oxide chains can also influence the accessibility of a certain probe to a specific region .…”

“…In this study, we analyzed the spectral changes of molecular probes with fluorescence and/or paramagnetic sensing groups ( Figure ) in aqueous systems containing F127 at concentration of 16.6%, in the presence and in the absence of 2‐hydroxypropyl‐β‐cyclodextrin (HPB). Our choice for these probes to investigate the phase transition in F127 stemmed from the structural similarity between the linker (oligoethylene glycol chains) connecting the sensing groups and the units of the block copolymer, which can ensure targeting of different regions in the F127 systems.…”