Supramolecular Control of the Temperature Responsiveness of Fluorescent Macrocyclic Molecular Rotamers.

To fully harness the potential of molecular machines, it is crucial to develop methods by which to exert control over their speed of motion through the application of external stimuli. A conformationally strained macrocyclic fluorescent rotamer, CarROT, displays a reproducible and linear fluorescence decrease towards temperature over the physiological temperature range. Through the external addition of anions, cations or through deprotonation, the compound can access four discreet rotational speeds via supramolecular interactions (very slow, slow, fast and very fast) which in turn stop, reduce or enhance the thermoluminescent properties due to increasing or decreasing non-radiative decay processes, thereby providing a means to externally control the temperature sensitivity of the system. Through comparison with analogues with a higher degree of conformational freedom, the high thermosensitivity of CarROT over the physiological temperature range was determined to be due to conformational strain, which causes a high energy barrier to rotation over this range. Analogues with a higher degree of conformational freedom display lower sensitivities towards temperature over the same temperature range. This study provides an example of an information rich small molecule, in which programable rotational speed states can be observed with facile read-out.

A macrocyclic fluorescent probe for the detection of citrate.

We present a macrocyclic fluorescent probe for the detection of citrate. This receptor binds citrate through hydrogen-bonding interactions in aqueous solutions, and exhibits a turn-on in fluorescence in response to binding. The presence of common biologically relevant dicarboxylate species does not significantly impact the fluorescence response. We have demonstrated the utility of this probe with the staining of murine splenocytes, and identified different basal levels of citrate present in immune cell subsets via flow cytometry analysis.

Lipid-Oriented Live-Cell Distinction of B and T Lymphocytes.

The identification of each cell type is essential for understanding multicellular communities. Antibodies set as biomarkers have been the main toolbox for cell-type recognition, and chemical probes are emerging surrogates. Herein we report the first small-molecule probe, CDgB, to discriminate B lymphocytes from T lymphocytes, which was previously impossible without the help of antibodies. Through the study of the origin of cell specificity, we discovered an unexpected novel mechanism of membrane-oriented live-cell distinction. B cells maintain higher flexibility in their cell membrane than T cells and accumulate the lipid-like probe CDgB more preferably. Because B and T cells share common ancestors, we tracked the cell membrane changes of the progenitor cells and disclosed the dynamic reorganization of the membrane properties over the lymphocyte differentiation progress. This study casts an orthogonal strategy for the small-molecule cell identifier and enriches the toolbox for live-cell distinction from complex cell communities.

Diaminomethylenemalononitriles and Diaminomethyleneindanediones as Dual Hydrogen Bond Donors for Anion Recognition.

Diaminomethylenemalononitriles (DMMs) and diaminomethyleneindanediones (DMIs) are dual H-bond donors that have previously been used as organocatalysts, but their anion binding ability has not been investigated. We report the synthesis of both alkyl- and aryl-substituted DMMs and DMIs, together with a comparison of their anion binding ability with that of the analogous thioureas. The DMMs display affinity for monovalent anions, with similar anion binding affinities observed to that of the thioureas in acetonitrile, albeit with differing trends for the N,N'-dialkyl versus N,N'-diaryl compounds. In contrast, the DMIs do not bind to monovalent anions under similar conditions as a result of conformational locking through the formation of intramolecular H-bonds. This can be overcome upon addition of sulfate ions, and binding of sulfate is enhanced in a more competitive solvent (DMSO).

Boronic Acid: A Bio-Inspired Strategy To Increase the Sensitivity and Selectivity of Fluorescent NADH Probe.

Fluorescent probes have emerged as an essential tool in the molecular recognition events in biological systems; however, due to the complex structures of certain biomolecules, it remains a challenge to design small-molecule fluorescent probes with high sensitivity and selectivity. Inspired by the enzyme-catalyzed reaction between biomolecule and probe, we present a novel combination-reaction two-step sensing strategy to improve sensitivity and selectivity. Based on this strategy, we successfully prepared a turn-on fluorescent reduced nicotinamide adenine dinucleotide (NADH) probe, in which boronic acid was introduced to bind with NADH and subsequently accelerate the sensing process. This probe shows remarkably improved sensitivity (detection limit: 0.084 µM) and selectivity to NADH in the absence of any enzymes. In order to improve the practicality, the boronic acid was further modified to change the measurement conditions from alkalescent (pH 9.5) to physiological environment (pH 7.4). Utilizing these probes, we not only accurately quantified the NADH weight in a health care product but also evaluated intracellular NADH levels in live cell imaging. Thus, these bio-inspired fluorescent probes offer excellent tools for elucidating the roles of NADH in biological systems as well as a practical strategy to develop future sensitive and selective probes for complicated biomolecules.

pH-Regulated Nonelectrogenic Anion Transport by Phenylthiosemicarbazones.

Gated ion transport across biological membranes is an intrinsic process regulated by protein channels. Synthetic anion carriers (anionophores) have potential applications in biological research; however, previously reported examples are mostly nonspecific, capable of mediating both electrogenic and electroneutral (nonelectrogenic) transport processes. Here we show the transmembrane Cl(-) transport studies of synthetic phenylthiosemicarbazones mimicking the function of acid-sensing (proton-gated) ion channels. These anionophores have remarkable pH-switchable transport properties with up to 640-fold increase in transport efficacy on going from pH 7.2 to 4.0. This "gated" process is triggered by protonation of the imino nitrogen and concomitant conformational change of the anion-binding thiourea moiety from anti to syn. By using a combination of two cationophore-coupled transport assays, with either monensin or valinomycin, we have elucidated the fundamental transport mechanism of phenylthiosemicarbazones which is shown to be nonelectrogenic, inseparable H(+)/Cl(-) cotransport. This study demonstrates the first examples of pH-switchable nonelectrogenic anion transporters.

Aromatic isophthalamides aggregate in lipid bilayers: evidence for a cooperative transport mechanism.

The synthesis and anion transport properties of a series of transmembrane anion transporters based on an isophthalamide scaffold with phenyl, naphthyl or anthracenyl central rings are reported. Anion transport studies using POPC vesicles, showed that the compounds have Hill coefficients >1. This is indicative of higher order complex formation, evidence that leads us to suggest that the compounds are not functioning solely as mobile carriers but rather that a cooperative transport mechanism is being observed. Fluorescence spectroscopy was used to show that the compounds aggregate in the phospholipid bilayer, which provides evidence that these compounds function as a self-assembled anion-conducting aggregate.

Conformationally adaptable macrocyclic receptors for ditopic anions: analysis of chelate cooperativity in aqueous containing media.

The effect of chelate cooperativity on the binding of several ditopic anions to two tetrathiourea macrocycles has been analysed in competitive solvent mixtures (H2O : DMSO 1 : 9 v/v). The semi-flexible receptors bind dicarboxylates with high affinity dependent on the length and flexibility of the guest. Chemical double mutant cycle (DMC) analysis allowed the chelate cooperativity effects to be measured in detail and revealed both positive and negative cooperativity effects which were dependent on guest size, flexibility and spacer interactions between guest and macrocycle. 1H NMR and crystallographic studies confirmed the macrocycle hosts are adaptable, changing conformation to match their pore size to a selected guest.

Receptors for sulfate that function across a wide pH range in mixed aqueous-DMSO media.

Water soluble squaramide macrocycles (MSQs) display high sulfate binding affinities in aqueous DMSO mixtures. The introduction of pyridine spacers into the macrocycles resulted in increased sulfate binding affinity in comparison to compounds with benzene spacers. [3]MSQ 6 was found to be a selective ligand for SO42- in highly competitive conditions and over a wide pH range (3.2-14).

Fluorescent squaramides as anion receptors and transmembrane anion transporters.

A series of squaramide-based anion transporters functionalised with the 1,8-naphthalimide fluorophore has been developed for improved ionophoric activity and fluorescent imaging in cells.

A new approach for turn-on fluorescence sensing of l-DOPA.

A novel design strategy for the fluorescence sensing of l-DOPA is reported. Resa-Sulf displays a significant turn-on fluorescence response to l-DOPA due to its reduction properties; this sensing mechanism was fully confirmed by mechanistic studies. Furthermore, Resa-Sulf was successfully utilized to quantitatively detect l-DOPA concentrations from a commercially available source.

Tris-ureas as transmembrane anion transporters.

Nine tris-urea receptors (L(1)-L(9)) have been synthesised and shown to coordinate to a range of anionic guests both by (1)H NMR titration techniques and single crystal X-ray structural analysis. The compounds have been shown to be capable of mediating the exchange of chloride and nitrate and also chloride and bicarbonate across POPC or POPC : cholesterol 7 : 3 vesicle bilayer membranes at low transporter loadings. An interesting dependency of anion transport on the nature of the cation is evidence to suggest that a M(+)/Cl(-) cotransport process may also contribute to the release of chloride from the vesicles.

Fluorescent transmembrane anion transporters: shedding light on anionophoric activity in cells.

A series of fluorescent anion transporters consisting of a urea or thiourea group linked to a naphthalimide fluorophore have been synthesised and their anion transport properties studied. The compounds possess similar anion transport properties to (thio)urea-based anionophores that have previously been reported. Fluorescence studies in cells show all anionophores cross the plasma membrane and localise within the interior of cells. The most lipophilic, aromatic substituted transporters localise homogeneously throughout the cell and are toxic towards cancer cells with the highly fluorinated compound 6 being the most effective. The least lipophilic, alkyl substituted transporters localise in specific vesicles and are non-toxic to cells. This work provides new insight to the actions of anionophores in cells and may be useful in the design of novel antineoplastic agents.

Small neutral molecular carriers for selective carboxylate transport.

A series of neutral thiourea receptors were found to mediate the antiport of chloride with a range of biologically relevant carboxylate anions across phospholipid bilayers. Simple structural modification of the carriers resulted in a change in the lactate/pyruvate transport selectivity.