Highly Sensitive Fluorescence Response to Inclusion Complex Formation of Berberine Alkaloid with Cucurbit[7]uril

Abstract

The effect of inclusion complex formation on the fluorescence properties of berberine, a clinically important natural alkaloid, was studied using cucurbit [7]uril as macrocyclic host compound. The formation of a very stable 1:1 inclusion complex led to about 500-fold fluorescence intensity enhancement, which facilitated the detection of berberine even below nanomolar concentration. Addition of NaCl caused a significant change in the association constant and the fluorescence characteristics of the complex, whereas the variation of the anion had a small effect. 1-Alkyl-3-methylimidazolium type ionic liquids altered the fluorescent properties of the berberine-cucurbit[7]uril complex much more efficiently than did NaCl. Time-resolved fluorescence studies showed ternary complex formation. Because berberine fluorescence is insensitive to pH and increases substantially upon inclusion in cucurbit[7]uril, the reversible self-assembly of this host-guest pair may find analytical application in enzyme assays.

Kinetics and Thermodynamics of Berberine Inclusion in Cucurbit[7]uril

“…25 Therefore, the fluorescence intensity change directly reflects the alteration of the 1:1 B + −CB7 complex concentration. The singletexcited B + is not released from the host because its lifetime in CB7 (11.6 ns) 23 is much shorter than the rate of egression.…”

“…22-24 About 500-fold fluorescence intensity enhancement was observed upon its inclusion in CB7. 25 In the present work, we exploit the favorable properties of B + to detect the complexation dynamics with CB7 after the fast mixing of the components with stopped-flow method. The formulas of the investigated substances are given in Scheme 1.…”

“…25 Although hydrophobic and charge-dipole host-guest interactions contribute to the stabilization of the inclusion complex, the strongly negative binding enthalpy (Table 1) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 hydrophobic core of the macrocycle. 30,31 CB7 contains on average 7.9 water molecules, which do not form energetically stable hydrogen bond network and their electrostatic interactions are weaker in the interior of the nonpolar, extremely nonpolarizable host than in the bulk solution.…”

Threading the Needle: Fluorescent Poly-pseudo-rotaxanes for Size-Exclusion Sensing

“…31,41,42 Thus, steady-state emission measurements were carried out below the saturation concentrations Page 4 of 9 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 (Fig. S7).…”

“…Previously, the enhancement of the PL signal observed on binding of a guest species within a CB[n] host has been attributed to increasing the guest solubility and changing the polarity of the microenvironment. 42,44,45 Titration of CB [5] into PPyV gives rise to little or no change to the PL spectrum.…”