Recent Progress in Fluorescent Chemosensors for Protein Kinases

Protein kinases are involved in almost all biological activities. The activities of different kinases reflect the normal or abnormal status of the human body. Therefore, detecting the activities of different kinases is important for disease diagnosis and drug discovery. Fluorescent probes offer opportunities for studying kinase behaviors at different times and spatial locations. In this review, we summarize different kinds of fluorescent chemosensors that have been used to detect the activities of many different kinases.

Optical chemosensors for the detection of proximally phosphorylated peptides and proteins

Proximal multi-site phosphorylation is a critical post-translational modification in protein biology. The additive effects of multiple phosphosite clusters in close spatial proximity triggers integrative and cooperative effects on protein conformation and activity. Proximal phosphorylation has been shown to modulate signal transduction pathways and gene expression, and as a result, is implicated in a broad range of disease states through altered protein function and/or localization including enzyme overactivation or protein aggregation. The role of proximal multi-phosphorylation events is becoming increasingly recognized as mechanistically important, although breakthroughs are limited due to a lack of detection technologies. To date, there is a limited selection of facile and robust sensing tools for proximal phosphorylation. Nonetheless, there have been considerable efforts in developing optical chemosensors for the detection of proximal phosphorylation motifs on peptides and proteins in recent years. This review provides a comprehensive overview of optical chemosensors for proximal phosphorylation, with the majority of work being reported in the past two decades. Optical sensors, in the form of fluorescent and luminescent chemosensors, hybrid biosensors, and inorganic nanoparticles, are described. Emphasis is placed on the rationale behind sensor scaffolds, relevant protein motifs, and applications in protein biology.

Tuning the anion binding properties of lanthanide receptors to discriminate nucleoside phosphates in a sensing array

The development of synthetic receptors for the selective binding and discrimination of anions in water requires an understanding of how anions interact with these synthetic receptors. Molecules designed to differentiate nucleoside phosphate anions (e.g. ATP, ADP, GTP, GDP, UDP) under physiological conditions could underpin exciting new sensing tools for biomedical research and drug discovery, but it is very challenging due to the similarities in anion structure, size and charge. We present a series of lanthanide-based anion receptors and establish key structural elements that impact on nucleoside phosphate anion binding and sensing. Structural evidence of anion binding using X-ray crystallographic and NMR data, supported by DFT calculations indicate the binding modes between the lanthanide complexes and certain phosphoanions, revealing a bidentate (α-, γ-) binding mode to ATP. We further use four of the receptors to allow discrimination of eight nucleoside phosphate anions in the first array-based assay using lanthanide complexes, taking advantage of the multiple emission bands and long emission lifetimes associated with luminescent lanthanide complexes.

Zinc(ii) and copper(ii) complexes as tools to monitor/inhibit protein phosphorylation events

A perspective on the advance of copper(ii) and zinc(ii) complexes of varied ligand architectures as binders of phosphorylated peptides/proteins and as sensors of phosphorylation reactions is presented.

Structure-activity relationship study of ProxyPhos chemosensors for the detection of proximal phosphorylation and other phosphate species

Chemosensors for the detection of phosphate-containing biological species are in high need. Detection of proximally phosphorylated sites of PP_i and those found in peptides and proteins has been demonstrated using chemosensors containing pyrene, as a fluorescent reporter, and a Zn²⁺-chelate, as a phosphate-binding group. Using these sensors, detection of proximal phosphate groups is afforded by binding of at least two of the sensor molecules to the adjacent phosphates, via the Zn²⁺ centres, leading to excimer formation between the pyrene groups and the corresponding shift in emission from 376 to 476 nm. Although several reports of this chemosensor class have been made, no detailed studies of selectivity of these sensors among major phosphate targets have been reported. In this study, a library of this class of chemosensors, termed ProxyPhos, which contained various linkers and Zn²⁺-chelating groups (i.e. DPA, cyclen and cyclam), was prepared and the effects of structural variation on the sensing efficiency and selectivity were evaluated among proximally phosphorylated peptides, proteins, nucleotides, P_i and PP_i. As a result of this study, we have identified ProxyPhos library members that are most suitable for the detection of proximally phosphorylated peptides, PP_i, UTP, and a DpYD peptide motif, and have generally provided a foundation for the selection of ProxyPhos chemosensors for further development of specific biologically relevant assays. The broad utility of ProxyPhos is further supported by the demonstrated lack of these sensors' cytotoxicity, ability to rapidly permeate into live and fixed cells and compatibility with gel staining methods.

Recognition of proximally phosphorylated tyrosine residues and continuous analysis of phosphatase activity using a stable europium complex

The recognition of proteins and their post-translational modifications using synthetic molecules is an active area of research. A common post-translational modification is the phosphorylation of serine, threonine or tyrosine residues. The phosphorylation of proximal tyrosine residues occurs in over 1000 proteins in the human proteome, including in disease-related proteins, so the recognition of this motif is of particular interest. We have developed a luminescent europium(III) complex, [Eu.1]+ , capable of the discrimination of proximally phosphorylated tyrosine residues, from analogous mono- and non-phosphorylated tyrosine residues, more distantly-related phosphotyrosine residues and over proximally phosphorylated serine and threonine residues. [Eu.1]+ was used to continuously monitor the phosphatase catalysed dephosphorylation of a peptide containing proximally phosphorylated tyrosine residues.

Spectroscopic and Theoretical Studies of Fluorescence Effects in 2-Methylamino-5-(2,4-dihydroxyphenyl)-1,3,4-thiadiazole Induced by Molecular Aggregation

The article presents the results of fluorescence analyses of 2-methylamino-5-(2,4-dihydroxyphenyl)-1,3,4-thiadiazole (MDFT) in an aqueous environment. MDFT dissolved in aqueous solutions with a pH value in the range from 1 to 4.5 yielded an interesting effect of two clearly separated fluorescence emissions. In turn, a single fluorescence was observed in MDFT dissolved in water solutions with a pH value from 4.5 to 12. As it was suggested in the previous investigations of other 1,3,4-thiadiazole compounds, these effects may be associated with conformational changes in the structure of the analysed molecule accompanied by aggregation effects. Crystallographic data showed that the effect of the two separated fluorescence emissions occurred in a conformation with the –OH group in the resorcyl ring bound on the side of the sulphur atom from the 1,3,4-thiadiazole ring. The hypothesis of aggregation as the mechanism involved in the change in the spectral properties at low pH is supported by the results of (Time-Dependent) Density Functional Theory calculations. The possibility of rapid analysis of conformational changes with the fluorescence spectroscopy technique may be rather important outcome obtained from the spectroscopic studies presented in this article. Additionally, the presented results seem to be highly important as they can be easily observed in solutions and biologically important samples.

ProxyPhos sensors for the detection of negatively charged membranes

ProxyPhos sensors selectively detect negatively charged phospholipid membranes.