Bioinspired Molecular Lantern: Tuning the Firefly Oxyluciferin Emission with Host–Guest Chemistry

Abstract

Fireflies generate flashes of visible light via luciferase-catalyzed chemiexcitation of the substrate (luciferin) to the first excited state of the emitter (oxyluciferin). Microenvironment effects are often invoked to explain the effects of the luciferase active pocket on the emission; however, the exceedingly complex spectrochemistry and synthetic burdens have precluded elucidation of the nature of these interactions. To decipher the effects of microenvironment on the light emission, here the hydrophobic interior of cucurbit[7]uril (CB7) is used to mimic the nonpolar active pocket of luciferase. The hydrophobic interior of CB7 induces shifts of the ground-state pKas by 1.9-2.5 units to higher values. Upon sequestration, the emission maxima of neutral firefly oxyluciferin and its conjugate monodeprotonated base are blue-shifted by 40 and 39 nm, respectively, resulting in visual color changes of the emitted light.

Kinetics Inside, Outside and Through Cucurbiturils

“…To this day, this represents the largest fluorescence enhancement observed upon dye encapsulation into CB[n]s. Again, timeresolved fluorescence measurements could be used to monitor the concomitant formation of these assemblies; the lifetime of the 2 : 2 aggregate is 5.7 ns, vs. only 40 ps (in methanol) [44] for free thiazole orange. Examples include the CB [7] encapsulation of oxyluciferin (15), [47] 2-aminoanthracenium (16), [48] BODIPY-benzimidazole conjugate 17, [49] coumarin dyes 18 (R = H and CH 3 ), [8,50] and anticancer drug camptothecin (19). Lifetimes of 2.0, 3.8 and 16.6 ns were assigned to free ellipticine 14, binary complex 14 · CB [8] and ternary complex 14 · CB[8] 2 , respectively.…”

“…Under acidic conditions, the side chain is cleaved as the acetal unit is hydrolyzed to acetone and 2-aminoethanol, despite the shielded environment of CB [7]; 19, 35 and 67 % of the cargo was released after 3 h at pH 6, 4 and 2, respectively. [102] Experiments were repeated with doxorubicin (47). While only 1.2 % leakage was detected at biological pH (7.4), at pH's that are typical for endosomes and lysosomes in tumor cells (6.0, 5.5 and 5.0), cargo release reached 53, 73 and 92 %, respectively, after 48 h. [102] Zhang and coworkers connected azobenzene-functionalized mesoporous silica nanoparticles filled with doxorubicin (47) to methylviologen units attached to a "self"-peptide (a sequence that inhibits clearance by macrophages), using CB [8] (see Figure 5).…”

“…The ammonium groups interact with the carbonylated portal of CB [7] and 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 maintain the integrity of the coatings around the nanoparticles. Addition of adamantylammonium (26) triggers its disintegration and the release of doxorubicin (47). Lowering the pH (down to pH 2) allows protons to compete with the ammonium cations for CB [7] binding, and the degradation of the coatings.…”

“…[104] Ji and coworkers attached doxorubicin (47) to methylviologen via a hydrazone linkage (see prodrug 49 in Figure 7), and tethered it to a polyethylene glycol chain terminated with a 2naphthyl group (guest 50) using CB [8]. Slightly poorer retention was observed with larger, 5 nm wide pores.…”

“…[104] Ji and coworkers attached doxorubicin (47) to methylviologen via a hydrazone linkage (see prodrug 49 in Figure 7), and tethered it to a polyethylene glycol chain terminated with a 2naphthyl group (guest 50) using CB [8]. When rehydration was carried out with a solution of adamantylammonium (26), the latter released micelles into (47) and subsequent coating with alternate layers of polymer 48 and CB [7]. Release was triggered at pH 5.0, as the hydrazone pro-drug is hydrolyzed to doxorubicin (47) (40 % after 72 h).…”