A Fluorescence Perspective on the Differential Interaction of Riboflavin and Flavin Adenine Dinucleotide with Cucurbit[7]uril

Abstract

The interaction of the macrocyclic host, cucurbit[7]uril (CB7), with riboflavin (RF) and its derivative, flavin adenine dinucleotide (FAD), has been investigated using absorption and steady-state and time-resolved fluorescence measurements. Interestingly, in the presence of CB7, the fluorescence intensity of RF is quenched, whereas the fluorescence intensity of FAD is enhanced. It is proposed that the fluorescence quenching of RF results from the tautomerization of its isoalloxazine moiety from the lactam to the lactim forms, upon binding to CB7. Such a tautomerization can be brought about since the two lactim forms have higher dipole moments than the lactam form of RF, and thus experience much stronger dipole-dipole interactions (and hence greater binding affinities) with CB7 in the former cases than in the latter. This tautomerization in the presence of CB7 leads to a significant reduction in the observed radiative decay rate and hence a reduction in the fluorescence intensity of RF. Binding of CB7 with RF is confirmed by an increase in the rotational correlation time of RF in the presence of CB7. Geometry optimization studies indicate the formation of an exclusion complex between CB7 and RF, possibly stabilized by H-bonding interactions, as also suggested by the characteristic red shift in the absorption spectra of the CB7-RF system. In the case of FAD, both the isoalloxazine ring and the adenine moiety can interact with the CB7 host. In aqueous solutions, a good fraction of FAD molecules exists in a "closed" conformation with the adenine and isoalloxazine rings stacked together, thus leading to very efficient fluorescence quenching due to the ultrafast intramolecular electron transfer from adenine to the isoalloxazine moiety. Binding of the adenine and/or the isoalloxazine moiety of FAD with CB7 inhibits the stacking interaction and changes the "closed" conformation to the "open" conformation, wherein the adenine and isoalloxazine moieties are widely separated, thus prohibiting the electron transfer process. This reduces the inherent fluorescence quenching of FAD molecule and results in the observed fluorescence enhancement. As observed for RF, the interaction of CB7 with the isoalloxazine ring of FAD should cause fluorescence quenching due to the lactam to lactim tautomerization process. However, in the interplay between the above two opposing effects, the fluorescence enhancement due to the modulation in the conformational dynamics of FAD by the CB7 host predominates. The conformational change is in fact supported by the observation of a long lifetime component in the fluorescence decay of FAD in the presence of CB7. Moreover, at acidic pH, when FAD is already present mainly in the "open" form, the conformational dynamics no longer plays any major role and the fluorescence of FAD is quenched by CB7, as expected, due to the tautomerization at the isoalloxazine moiety.

Potential Applications of Cucurbit[n]urils Inclusion Complexes in Photodynamic Therapy

“…[37] It is interesting to note that RF is not included within the cavity of CB [7], but rather an exclusion complex is formed with its different tautomeric forms. [35] The photoactive molecule norharmane (NMH, b-carboline), which has been shown to produce singlet oxygen [38] and induce DNA damage, [39] has also been reported to form an inclusion complex with CB [7] with a binding constant of (9.0 AE 0.5) 3 10 4 M À1 . [36] An interesting case is the report of the binding of tetra(1methyl)pyridinium porphyrins as a free base or bound to Zn 2 + , Fe 3 + and Mn 3 + to CB [10] in conjunction with auxiliary pyridinium guests with binding constants for the ternary complexes in the range of 10 4 -10 5 M À1 .…”

“…[58,76] This enhancement in the fluorescence can also be seen in other photosensitizers such as TMPyP 4 + . [35] This result is not expected, because encapsulation into CB [7] should restrict the vibrational and rotational motions of the guest, resulting in a reduced non-radiative decay rate of the excited guest. [35] This result is not expected, because encapsulation into CB [7] should restrict the vibrational and rotational motions of the guest, resulting in a reduced non-radiative decay rate of the excited guest.…”

Functionalization reactions of calixarenes

“…The subsequent treatment of 252 4 •B 5 with two B equivalents yields the 3D 252 4 •B 7 assembly, (i.e. 252 4 •B 5 in which two added B equivalents are situated below and above the central core), whereas the treatment of 252 4 •B 5 with twelve 252 equivalents leads to the 2D assembly 252 16 •B 5 . It is noteworthy that in experiments the different porphyrins may be used.…”

“…[14][15][16][17] An advantageous feature of calixarenes is their easy availability. Functionalization of calixarenes at their wide and at their narrow rims, permits the synthesis of a variety of derivatives having desired properties and is the topic of the present review.…”

Complexation (cucurbit[6]uril-pyrene): Thermodynamic and spectroscopic properties

“…The homolog cucurbit [7]uril also induces the fluorescence enhancement of the natural isoquinoline alkaloids berberine [6] and coptisine [7] and the dicationic 2, 7-dimethyldiazapyrenium [8]. The formation of the exclusion complexes between thioflavin T and CB5 [9] and between riboflavin and CB7 [10] modifies the photophysical parameters of these fluorescent compounds.…”