Photophysics and Release Kinetics of Enzyme-Activatable Optical Probes Based on H-Dimerized Fluorophores on Self-Immolative Linkers

Enzyme assays with supramolecular chemosensors - the label-free approach

Enzyme activity measurements are essential for many research areas, e.g., for the identification of inhibitors in drug discovery, in bioengineering of enzyme mutants for biotechnological applications, or in bioanalytical chemistry as parts of biosensors. In particular in high-throughput screening (HTS), sensitive optical detection is most preferred and numerous absorption and fluorescence spectroscopy-based enzyme assays have been developed, which most frequently require time-consuming fluorescent labelling that may interfere with biological recognition. The use of supramolecular chemosensors, which can specifically signal analytes with fluorescence-based read-out methods, affords an attractive and label-free alternative to more established enzyme assays. We provide herein a comprehensive review that summarizes the current state-of-the-art of supramolecular enzyme assays ranging from early examples with covalent chemosensors to the most recent applications of supramolecular tandem enzyme assays, which utilize common and often commercially available combinations of macrocyclic host molecules (e.g. cyclodextrins, calixarenes, and cucurbiturils) and fluorescent dyes as self-assembled reporter pairs for assaying enzyme activity.

Recent advances in self-immolative linkers and their applications in polymeric reporting systems

Interest in self-immolative chemistry has grown over the past decade with more research groups harnessing the versatility to control the release of a compound from a larger chemical entity, given...

Photochemical Amplifier Based on Self-Immolative Dendritic Spacers

A self-immolative dendritic structure was synthesized. It is based on phenol derivatives with three hydroxymethyl arms at both ortho and para positions of the core unit, potentially releasing up to 27 leaving groups in a third-generation dendrimer. The triggering event is the photolysis of a photosentive ortho-nitrobenzyl group. In doing so, we expected to transform a weak chemical or photochemical input into a large chemical output, which fulfills the definition of a molecular amplifier. Such dendrimers could find application as an indicator, a drug-delivery vector, or a solubilizing agent. The prepared dendrimer indeed released up to 27 leaving groups upon photolysis at 360 nm.

A highly selective and light-up red emissive fluorescent probe for imaging of penicillin G amidase inBacillus cereus

A highly selective and red-emissive fluorescent probe (HCyNB) for penicillin G amidase (PGA) has been prepared and used for imaging of endogenousPGAin penicillinase-producingBacillus subtilis.

Aggregation-induced emission spectral shift as a measure of local concentration of a pH-activatable rhodamine-based smart probe

Generating activatable probes that report about molecular vicinity through contact-based mechanisms such as aggregation can be very convenient. Specifically, such probes change a particular spectral property only at the intended biologically relevant target. Xanthene derivatives, for example rhodamines, are able to form aggregates. It is typical to examine aggregation by absorption spectroscopy but for microscopy applications utilizing fluorescent probes it is very important to perform characterization by measuring fluorescence spectra. First we show that excitation spectra of aqueous solutions of rhodamine 6G can be very informative about the aggregation features. Next we establish the dependence of the fluorescence emission spectral maximum shift on the dimer concentration. The obtained information helped us confirm the possibility of aggregation of a recently designed and synthesized rhodamine 6G-based pH-activatable fluorescent probe and to study its pH and concentration dependence. The size of the aggregation-induced emission spectral shift at specific position on the sample can be measured by fluorescence microspectroscopy, which at particular pH allows estimation of the local concentration of the observed probe at microscopic level. Therefore, we show that besides aggregation-caused quenching and aggregation-induced emission also aggregation-induced emission spectral shift can be a useful photophysical phenomenon.

Self-immolative spacers: kinetic aspects, structure-property relationships, and applications

Self-immolative spacers are covalent assemblies tailored to correlate the cleavage of two chemical bonds after activation of a protective part in a precursor: Upon stimulation, the protective moiety is removed, which generates a cascade of disassembling reactions leading to the temporally sequential release of smaller molecules. Originally introduced to overcome limitations for drug delivery, self-immolative spacers have gained wide interest in medicinal chemistry, analytical chemistry, and material science. For most applications, the kinetics of the disassembly of the activated self-immolative spacer governs functional properties. This Review addresses kinetic aspects of self-immolation. It provides information for selecting a particular self-immolative motif for a specific demand. Moreover, it should help researchers design kinetic experiments and fully exploit the rich perspectives of self-immolative spacers.

Evaluation of ¹¹¹in-labelled exendin-4 derivatives containing different meprin β-specific cleavable linkers

BACKGROUND

Cleavable linkers, which are specifically cleaved by defined conditions or enzymes, are powerful tools that can be used for various purposes. Amongst other things, they have been successfully used to deliver toxic payloads as prodrugs into target tissues. In this work novel linker sequences targeting meprin β, a metalloprotease expressed in the kidney brush-border membrane, were designed and included in the sequence of three radiolabelled exendin-4 derivatives. As radiolabelled exendin-4 derivatives strongly accumulate in the kidneys, we hypothesised that specific cleavage of the radiolabelled moiety at the kidney brush-border membrane would allow easier excretion of the activity into the urine and therefore improve the pharmacological properties of the peptide.

RESULTS

The insertion of a cleavable linker did not negatively influence the in vitro properties of the peptides. They showed a good affinity to the GLP-1 receptor expressed in CHL cells, a high internalisation and sufficiently high stability in fresh human blood plasma. In vitro digestion with recombinant meprin β rapidly metabolised the corresponding linker sequences. After 60 min the majority of the corresponding peptides were digested and at the same time the anticipated fragments were formed. The peptides were also quickly metabolised in CD1 nu/nu mouse kidney homogenates. Immunofluorescence staining of meprin β in kidney sections confirmed the expression of the protease in the kidney brush-border membrane. Biodistribution in GLP-1 receptor positive tumour-xenograft bearing mice revealed high specific uptake of the 111In-labelled tracers in receptor positive tissue. Accumulation in the kidneys, however, was still high and comparable to the lead compound 111In-Ex4NOD40.

CONCLUSION

In conclusion, we show that the concept of cleavable linkers specific for meprin β is feasible, as the peptides are rapidly cleaved by the enzyme while retaining their biological properties.

Dual enzyme-responsive “turn-on” fluorescence sensing systems based on in situ formation of 7-hydroxy-2-iminocoumarin scaffolds

We herein report a novel class of dual enzyme-responsive fluorogenic probes based on two orthogonal deprotection reactions via the “covalent assembly” principle. Sensing of two different enzymes (hydrolase and nitroreductase) through domino reactions, producing the push–pull backbone of a fluorescent 3-substituted 7-hydroxy-2-iminocoumarin dye, is reported.

Disassembly kinetics of quinone-methide-based self-immolative spacers that contain aromatic nitrogen heterocycles

We prepared several pyridine- and pyrimidine-based self-immolative spacer groups to evaluate the significance of the resonance energy of the spacer aromatic ring on the kinetics of 1,4- and 1,6-elimination reactions, which govern spacer disassembly. Subsequently, we relied on a photoactivation procedure to accurately analyze the disassembly kinetics. Beyond providing new results that are relevant for deriving quantitative structure-property relationships, herein, we demonstrate that pH value can be used as an efficient parameter to finely control the disassembly time of a self-immolative spacer after an initial activation.