Boosting the Host-Guest Binding by Programming the Curvature in Geodesic Nanoribbons

The curvature of an aromatic system is an essential parameter that can be used to program the self-assembly and host-guest complementarity in geodesic polyarenes. However, the challenging synthesis of curved aromatics impedes exploration of the related effects on the binding properties. The design and synthesis of a polyarene with programmed curvature fitting to C60 by a stepwise introduction of five-membered rings are presented to solve this challenge. Among several methods explored, the route utilizing cyclodehydrofluorination proved to be the most successful, in terms of the highest product yield. The binding studies suggest that fine-tuning the curvature in acyclic systems leads to a dramatic increase in affinity, embedding specific binding modes and selectivity, as revealed from the comparative studies with C60 and C70. Experimental and theoretical investigations with curved polyarenes of different sizes show that the buried surface area upon binding has a linear correlation with the binding energies. The curvature complementarity appeared to play a decisive role in achieving selective recognition of C70 via the formation of a 2:1 complex along the major axis with an overall constant of 108 M-2 and positive cooperativity. The developed nanoribbons bearing the curvature of C60 is the first all-carbon host showing binding affinities for fullerenes that are comparable with macrocyclic [10]CPP. The obtained data pave the way for understanding the properties of geodesic polyarenes and the design of new self-assembled materials based on fullerenes, nanotubes, and other curved structures.

The effect of hydrogen bonding on the π depletion and the π-π stacking interaction

Non-covalent interactions such as hydrogen bonding and π-π stacking are essential types of interactions governing molecular self-assembly. The π-π stacking ability of aromatic rings depends on the electron density of the π orbitals, which is affected by the electron-withdrawing or electron-donating properties of the substituents. We have here studied the effect of hydrogen bonding on the strength of the π-π stacking interactions by calculating the binding energies at the explicitly correlated Møller-Plesset (MP2-F12) perturbation theory level using polarized triple-ζ quality basis sets. The stacking interactions in the presence of hydrogen bonding are found to be stronger than in the absence of the hydrogen bonding suggesting that hydrogen bonds lead to π depletion, which affects the aromatic character of the aromatic rings and increases the strength of the π-π stacking interaction. We have also studied how hydrogen bonding affects the stacking interaction by calculating local orbital locator integrated pi over plane (LOLIPOP) indices. Comparing LOLIPOP indices with the stacking-interaction energies calculated at the MP2-F12 level shows that there is no clear correlation between the stacking-interaction energies and LOLIPOP indices.

Imidazole-Based Monomer as Functional Unit for the Specific Detection of Paraxanthine in Aqueous Environments

In the context of personalized medicine, the paraxanthine-to-caffeine ratio is an accepted standard for the optimization of the dose-response effect of many pharmaceuticals in individual patients. There is a strong drive towards the development of cheaper and portable devices for the detection of biomarkers, including paraxanthine and caffeine, which requires materials with high binding efficiency and specificity. We designed a recognition unit specific for paraxanthine which can discriminate molecules with small structural differences and can be used to increase the sensitivity of sensors. A number of functional units were screened by nuclear magnetic resonance for their ability to form specific binding interactions with paraxanthine in water and negligible interactions with its structural analogue caffeine. Imidazole was identified as the unit showing the most promising results and its two polymerizable derivatives were evaluated by isothermal titration calorimetry to identify the best monomer. The data suggested that 4-vinylimidazole was the most promising unit forming specific and strong binding interaction with paraxanthine. The calorimetry experiments allowed also the determination of the thermodynamic parameters of all interactions and the association constant values. Optimization of polymerization protocols in water, achieving high monomer conversions and chemical yields, demonstrate the suitability of the selected functional monomer for polymer preparations, targeting the detection of paraxanthine in aqueous environments.

How do London Dispersion Interactions Impact the Photochemical Processes of Molecular Switches?

In the last two decades, linear-response time-dependent density functional theory (LR-TDDFT) has become one of the most widely used approaches for the computation of the excited-state properties of atoms and molecules. Despite its success in describing the photochemistry and the photophysics of a vast majority of molecular systems, its domain of applicability has been limited by several substantial drawbacks. Commonly identified problems of LR-TDDFT include the correct description of Rydberg states, charge-transfer excited states, doubly excited states, and nearly degenerate states. In addition to these widely recognized shortcomings, the approximate functionals used in LR-TDDFT are unable to fully describe London dispersion interactions. In this work, we aim at understanding the impact of van der Waals interactions on the properties of chemical systems beyond their electronic ground state. For this purpose, we compare the results of excited-state energy profiles and dynamic trajectories for the prototypical cis-stilbene molecule with its 3-3',5-5'-tetra-tert-butyl derivative. While the explicit treatment of London dispersion interactions results in negligible changes for the cis-stilbene, we show that these attractive forces have a substantial influence on the energetics and structural evolution of the substituted derivative. In the latter case, intramolecular dispersion interactions impact the outcome of the simulation qualitatively, leading to an increased preference for the photocyclization pathway. The methodological consequences of this work are not uniquely applicable to the illustrative stilbene case. In fact, this molecule is representative of a whole class of chemical situations, where dispersion forces dominate the interactions between the unexcited substituents of a photoexcited chromophore. This is, for instance, a common situation in organic photovoltaics where donor molecules are usually functionalized with long alkyl side chains to improve solubility and assembly.

Array-based sensing of purine derivatives with fluorescent dyes

Natural and synthetic purine derivatives such as caffeine, theophylline, 6-mercaptopurine and 8-chlorotheophylline are important drugs. Due to the structural similarity of these compounds, it is intrinsically difficult to prepare chemosensors for their selective optical detection. Here, we describe a sensor array which can be used to differentiate pharmacologically important purine derivatives with good accuracy. The array is composed of four polysufonated fluorescent dyes, all of which can bind purines viaπ-stacking interactions. The complexation of the analytes results in partial quenching of the fluorescence. The fluorescence response of the four dyes provides a characteristic signal pattern, enabling the identification of thirteen purine derivatives at low millimolar concentration. Furthermore, it is possible to use the array for obtaining information about the quantity and purity of purine samples.

Validation of caffeine dehydrogenase from Pseudomonas sp. strain CBB1 as a suitable enzyme for a rapid caffeine detection and potential diagnostic test

Excess consumption of caffeine (>400 mg/day/adult) can lead to adverse health effects. Recent introduction of caffeinated products (gums, jelly beans, energy drinks) might lead to excessive consumption, especially among children and nursing mothers, hence attracting the Food and Drug Administration's attention and product withdrawals. An "in-home" test will aid vigilant consumers in detecting caffeine in beverages and milk easily and quickly, thereby restricting its consumption. Known diagnostic methods lack speed and sensitivity. We report a caffeine dehydrogenase (Cdh)-based test which is highly sensitive (1-5 ppm) and detects caffeine in beverages and mother's milk in 1 min. Other components in these complex test samples do not interfere with the detection. Caffeine-dependent reduction of the dye iodonitrotetrazolium chloride results in shades of pink proportional to the levels in test samples. This test also estimates caffeine levels in pharmaceuticals, comparable to high-performance liquid chromatography. The Cdh-based test is the first with the desired attributes of a rapid and robust caffeine diagnostic kit.

Fluorescence polarization immunoassays for the quantification of caffeine in beverages

Homogeneous fluorescence polarization immunoassays (FPIAs) were developed and compared for the determination of caffeine in beverages and cosmetics. FPIAs were performed in cuvettes in a spectrometer for kinetic FP measurements as well as in microtiter plates (MTPs) on a multimode reader. Both FPIAs showed measurement ranges in the μg/L range and were performed within 2 and 20 min, respectively. For the application on real samples, high coefficients of variations (CVs) were observed for the performance in MTPs; the CVs for FPIAs in cuvettes were below 4%. The correlations between this method and reference methods were satisfying. The sensitivity was sufficient for all tested samples including decaffeinated coffee without preconcentration steps. The FPIA in cuvettes allows a fast, precise, and automated quantitative analysis of caffeine in consumer products, whereas FPIAs in MTPs are suitable for semiquantitative high-throughput screenings. Moreover, specific quality criteria for heterogeneous assays were applied to homogeneous immunoassays.

Make caffeine visible: a fluorescent caffeine "traffic light" detector

Caffeine has attracted abundant attention due to its extensive existence in beverages and medicines. However, to detect it sensitively and conveniently remains a challenge, especially in resource-limited regions. Here we report a novel aqueous phase fluorescent caffeine sensor named Caffeine Orange which exhibits 250-fold fluorescence enhancement upon caffeine activation and high selectivity. Nuclear magnetic resonance spectroscopy and Fourier transform infrared spectroscopy indicate that π-stacking and hydrogen-bonding contribute to their interactions while dynamic light scattering and transmission electron microscopy experiments demonstrate the change of Caffeine Orange ambient environment induces its fluorescence emission. To utilize this probe in real life, we developed a non-toxic caffeine detection kit and tested it for caffeine quantification in various beverages. Naked-eye sensing of various caffeine concentrations was possible based on color changes upon irradiation with a laser pointer. Lastly, we performed the whole system on a microfluidic device to make caffeine detection quick, sensitive and automated.

Monitoring penetratin interactions with lipid membranes and cell internalization using a new hydration-sensitive fluorescent probe

A new hydration-sensitive fluorescent label attached to the N-terminus of a cell-penetrating peptide allows visualization of the nanoscopic environment of its internalization pathway.

A Caveat on SCC-DFTB and Noncovalent Interactions Involving Sulfur Atoms

Accurate modeling of noncovalent interactions involving sulfur today is ubiquitous, particularly with regard to the role played by sulfur-containing heterocycles in the field of organic electronics. The density functional tight binding (DFTB) method offers a good compromise between computational efficiency and accuracy, enabling the treatment of thousands of atoms at a fraction of the cost of density functional theory (DFT) evaluations. DFTB is an approximate quantum chemical approach that is based on the DFT total energy expression. Here, we address a critical issue inherent to the DFTB parametrization, which prevents the use of the DFTB framework for simulating noncovalent interactions involving sulfur atoms and precludes its combination with a dispersion correction. (1-5) Dramatic examples of structural patterns relevant to the field of organic electronics illustrate that DFTB delivers erroneous (i.e., qualitatively wrong) results involving spurious binding.