Small molecules have long been used for the selective recognition of a wide range of analytes. The ability of these chemical receptors to recognise and bind to specific targets mimics certain biological processes (such as proteinsubstrate interactions) and has therefore attracted recent interest. Due to the abundance of biological molecules possessing polyhydroxy motifs, boronic acids-which form fivemembered boronate esters with diols-have become increasingly popular in the synthesis of small chemical receptors. Their targets include biological materials and natural products including phosphatidylinositol bisphosphate, saccharides and polysaccharides, nucleic acids, metal ions and the neurotransmitter dopamine. This review will focus on the many ways in which small chemical receptors based on boronic acids have been used as biochemical tools for various purposes, including sensing and detection of analytes, interference in signalling pathways, enzyme inhibition and cell delivery systems. The most recent developments in each area will be highlighted.

Inositol phospholipids have emerged as important key players in a wide variety of cellular functions. Among the seven existing inositol phospholipids, phosphatidylinositol (4,5)-bisphosphate (PI(4,5)P 2 ) has attracted much attention in recent years due to its important role in numerous cellular signaling events and regulations, which in turn impact several human diseases. This particular lipid is recognized in the cell by specific lipid binding domains, such as the Pleckstrin-homology (PH) domain, which is also employed as a tool to monitor this important lipid. Here, we describe the synthesis and biological characterization of a small molecule that mimics the PH domain as judged by its ability to bind specifically to only PI(4,5)P 2 and effectively compete with the PH domain in vitro and in a cellular environment.The binding constant of this small molecule PH domain mimetic (PHDM) was determined to be 17.6 ± 10.1 µM, similar in potency to the PH domain. Using NIH 3T3 mouse fibroblast cells we demonstrated that this compound is cell permeable and able to modulate PI(4,5)P 2 -dependent effects in a cellular environment such as the endocytosis of the transferrin receptor, loss of mitochondria as well as stress fiber formation. This highly PI(4,5)P 2 -specific chemical mimetic of a PH domain, is not only a powerful research tool, but might also be a lead compound in future drug developments targeting PI(4,5)P 2 -dependent diseases such as Lowe syndrome.

Arylstibonates structurally resemble phosphotyrosine side chains in proteins and here we addressed the ability of such compounds to act as inhibitors of a panel of mammalian tyrosine and dual-specificity phosphatases. Two arylstibonates both possessing a carboxylate side chain were identified as potent inhibitors of the protein tyrosine phosphatase PTP-β. In addition, they inhibited the dual-specificity, cell cycle regulatory phosphatases Cdc25a and Cdc25b with sub-micromolar potency. However, the Cdc25c phosphatase was not affected demonstrating that arylstibonates may be viable leads from which to develop isoform specific Cdc25 inhibitors.

Vanadium complexes have been previously utilised as potent inhibitors of cysteine based phosphatases (CBPs). Herein, we present the synthesis and characterisation of two new fluorescently labelled vanadyl complexes (14 and 15) with bridged di-picolinic acid ligand.These compounds differ significantly from previous vanadyl complexes with phosphatase inhibition properties in that the metal-chelating part is a single tetradentate unit, which should afford greater stability and scope for synthetic elaboration then the earlier complexes. These new complexes inhibit a selection of cysteine based phosphatases (CBPs) in the nM range with some selectivity. Fluorescence spectroscopic studies (including fluorescence anisotropy)were carried out to demonstrate that the complexes are not simply acting as vanadyl delivery vehicles but they interact with the proteins. Finally, we present preliminary fluorescence microscopy studies to demonstrate that the complexes are cell permeable and localise throughout the cytoplasm of NIH3T3 cells.

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