Concentration-Dependent Br···O Halogen Bonding between Carbon Tetrabromide and Oxygen-Containing Organic Solvents

Competition between electrostatic interactions and halogen bonding in the protein-ligand system: structural and thermodynamic studies of 5,6-dibromobenzotriazole-hCK2α complexes

CK2 is a member of the CMGC group of eukaryotic protein kinases and a cancer drug target. It can be efficiently inhibited by halogenated benzotriazoles and benzimidazoles. Depending on the scaffold, substitution pattern, and pH, these compounds are either neutral or anionic. Their binding poses are dictated by a hydrophobic effect (desolvation) and a tug of war between a salt bridge/hydrogen bond (to K68) and halogen bonding (to E114 and V116 backbone oxygens). Here, we test the idea that binding poses might be controllable by pH for ligands with near-neutral pK_a, using the conditionally anionic 5,6-DBBt and constitutively anionic TBBt as our models. We characterize the binding by low-volume Differential Scanning Fluorimetry (nanoDSF), Isothermal Calorimetry (ITC), Hydrogen/Deuterium eXchange (HDX), and X-ray crystallography (MX). The data indicate that the ligand pose away from the hinge dominates for the entire tested pH range (5.5-8.5). The insensitivity of the binding mode to pH is attributed to the perturbation of ligand pK_a upon binding that keeps it anionic in the ligand binding pocket at all tested pH values. However, a minor population of the ligand, detectable only by HDX, shifts towards the hinge in acidic conditions. Our findings demonstrate that electrostatic (ionic) interactions predominate over halogen bonding.

Direct matrix isolation IR spectroscopic evidence of halogen bonding from a comparative study of complexes of CBr4 and CCl4with acetone and formic acid

Binary complexes of acetone and formic acid with tetrahalomethanes CBr₄ and CCl₄ have been isolated in argon matrix. Spectral shifts in the characteristic ν_C=O region of acetone, as well as in the fingerprint regions, are unambiguously assigned to the formation of halogen bond involving one of the halogen atoms on CBr₄/CCl₄ as donor, and the carbonyl oxygen of acetone as acceptor. The higher magnitude of shifts of ν_C=O and the fingerprint vibrations for the CBr₄ complex, as compared to the CCl₄ complex, is consistent with theoretical predictions of higher value of positive electrostatic potential in the "σ-hole" region of the former, and hence its higher susceptibility to halogen bonding. The formation of halogen bonded complexes involving formic acid as acceptor and CBr₄/CCl₄ as donors is also being reported for the first time. In this case too, distinct shifts are obtained in the ν_C=O as well as ν_C-O regions of formic acid, which again are significantly larger in magnitude for the CBr₄ complex, as compared to the CCl₄ complex. Electronic structure calculations have been carried out using different theoretical methods to identify the various possible structural isomers of the halogen bonded complexes, and to obtain relevant information regarding their energies and intermolecular geometrical parameters. In addition, NBO and AIM analysis have been carried out to understand the role of local interactions at the halogen bonded interface. Such predicted data are found to be consistent with experimental observations, and re-assert the stronger nature of CBr₄ as halogen bond donor, as compared to CCl₄.

Ligand-Induced Tunable Dual-Color Emission Based on Lead Halide Perovskites for White Light-Emitting Diodes

Cesium lead halide perovskites (CsPbX₃, X = Cl, Br, and I) have emerged as an important class of color-tunable light-emitting materials in the past 4 years. However, single-CsPbX₃ nanostructures with dual-color emission remain scarce. Here, we demonstrate dual-color emission from lead halide perovskite nanowires induced by the surface ligands, that is, oligomeric methoxypolyethylene glycol (MEOPEG). In addition to the characteristic emission from the host lattice, an unprecedented emission from the expanded band gap caused by MEOPEG is observed. The ratio of the two emission intensities can be easily adjusted by changing the concentration of the surface ligands. Moreover, the band gaps of CsPbX₃-MEOPEG can be further fine-tuned by a simple postsynthetic anion exchange process. As a result, white light-emitting diodes (WLEDs) with high-quality CIE coordinates of (0.33, 0.29) and a high color rendering index value (84) are realized. These CsPbX₃-MEOPEG materials, with tunable dual-color emission, may serve as ideal model systems for WLEDs, which will undoubtedly expand the applications of cesium lead halide perovskites.

Controlled α-mono- and α,α-di-halogenation of alkyl sulfones using reagent–solvent halogen bonding

The direct and selective α-halogenation of alkyl sulfones was achieved via base-mediated electrophilic halogenation, where reagent–solvent halogen bonding was found to control the selectivity through alteration of the effective size of the halogen source.

Thermodynamics parameters for binding of halogenated benzotriazole inhibitors of human protein kinase CK2α

The interaction of human CK2α (hCK2α) with nine halogenated benzotriazoles, TBBt and its analogues representing all possible patterns of halogenation on the benzene ring of benzotriazole, was studied by biophysical methods. Thermal stability of protein-ligand complexes, monitored by calorimetric (DSC) and optical (DSF) methods, showed that the increase in the mid-point temperature for unfolding of protein-ligand complexes (i.e. potency of ligand binding to hCK2α) follow the inhibitory activities determined by biochemical assays. The dissociation constant for the ATP-hCK2α complex was estimated with the aid of microscale thermophoresis (MST) as 4.3±1.8 μM, and MST-derived dissociation constants determined for halogenated benzotriazoles, when converted according to known ATP concentrations, perfectly reconstruct IC50 values determined by the biochemical assays. Ligand-dependent quenching of tyrosine fluorescence, together with molecular modeling and DSC-derived heats of unfolding, support the hypothesis that halogenated benzotriazoles bind in at least two alternative orientations, and those that are efficient hCK2α inhibitors bind in the orientation which TBBt adopts in its complex with maize CK2α. DSC-derived apparent heat for ligand binding (ΔΔHbind) is driven by intermolecular electrostatic interactions between Lys68 and the triazole ring of the ligand, as indicated by a good correlation between ΔΔHbind and ligand pKa. Overall results, additionally supported by molecular modeling, confirm that a balance of hydrophobic and electrostatic interactions contribute predominantly (~40 kJ/mol), relative to possible intermolecular halogen/hydrogen bonding (less than 10 kJ/mol), in binding of halogenated benzotriazoles to the ATP-binding site of hCK2α. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases.

Mechanism and application of halogen bond induced fluorescence enhancement and iodine molecule cleavage in solution

Halogen bonding between iodine and ciprofloxacin (I⋯N XB) induces I–I cleavage with fluorescence enhancement.

Halogen bonding at the ATP binding site of protein kinases: preferred geometry and topology of ligand binding

Halogenated ligands have been widely developed as potent, and frequently selective, inhibitors of protein kinases (PK). Herein, all structures of protein kinases complexed with a halogenated ligand, identified in the PDB, were analyzed in the context of eventual contribution of halogen bonding to protein-ligand interactions. Global inspection shows that two carbonyl groups of residues located in the hinge region are the most abundant halogen bond acceptors. In contrast to solution data, well-defined water molecules, located at sites conserved across most PK structures, are also involved in halogen bonding. Analysis of cumulative distributions of halogen-acceptor distances shows that structures displaying short contacts involving a halogen atom are overpopulated, contributing together to clearly defined maxima of 2.82, 2.91 and 2.94Å for chlorine, bromine and iodine, respectively. The angular preference of a halogen bond favors ideal topology (180°, 120°) for iodine. For bromine the distribution is much more dispersed, and no such preference was found for chlorine. This article is part of a Special Issue entitled: Inhibitors of Protein Kinases (2012).

Competition and interplay between σ-hole and π-hole interactions: a computational study of 1:1 and 1:2 complexes of nitryl halides (O2NX) with ammonia

Quantum calculations at the MP2/cc-pVTZ, MP2/aug-cc-pVTZ, and CCSD(T)/cc-pVTZ levels have been used to examine 1:1 and 1:2 complexes between O(2)NX (X = Cl, Br, and I) with NH(3). The interaction of the lone pair of the ammonia with the σ-hole and π-hole of O(2)NX molecules have been considered. The 1:1 complexes can easily be differentiated using the stretching frequency of the N-X bond. Thus, those complexes with σ-hole interaction show a blue shift of the N-X bond stretching whereas a red shift is observed in the complexes along the π-hole. The SAPT-DFT methodology has been used to gain insight on the source of the interaction energy. In the 1:2 complexes, the cooperative and diminutive energetic effects have been analyzed using the many-body interaction energies. The nature of the interactions has been characterized with the atoms in molecules (AIM) and natural bond orbital (NBO) methodologies. Stabilization energies of 1:1 and 1:2 complexes including the variation of the zero point vibrational energy (ΔZPVE) are in the ranges 7-26 and 14-46 kJ mol(-1), respectively.

Halogen bonding in iodo-perfluoroalkane/pyridine mixtures

Mole fraction and temperature studies of halogen bonding between 1-iodo-perfluorobutane, 1-iodo-perfluorohexane, or 2-iodo-perfluoropropane and pyridine were performed using noisy light-based coherent anti-Stokes Raman scattering (I((2)) CARS) spectroscopy. The ring breathing mode of pyridine both is highly sensitive to halogen bonding and provides a strong I((2)) CARS signal. As the lone pair electrons from the pyridinyl nitrogen interact with the sigma-hole on the iodine from the iodo-perfluoroalkane, the ring breathing mode of pyridine blue-shifts proportionately with the strength of the interaction. The measured blue shift for halogen bonding of pyridine and all three iodo-perfluoroalkanes is comparable to that for hydrogen bonding between pyridine and water. 2-Iodo-perfluoropropane displays thermodynamic behavior that is different from that of the 1-iodo-perfluoroalkanes, which suggests a fundamental difference at the molecular level. A potential explanation of this difference is offered and discussed.

Synthesis, structural analysis, and chiral investigations of some atropisomers with EE-tetrahalogeno-1,3-butadiene core

The atropenantiomers of stable 1,2,3,4-tetrahalo-1,3-butadiene derivatives (where halogeno stands for bromine or iodine) were separated with use of chiral HPLC. The barriers for the enantiomerization process were determined on-line by dynamic HPLC (DHPLC) or off-line by classical kinetic measurements. In the case of the tetrachloro compound, the barrier was too low for DHPLC and its value was obtained by dynamic NMR experiments. The obtained barriers for chloro, bromo, and iodo derivatives correlate with the van der Waals radii of the halogens. The absolute configuration of the isolated enantiomers of the tetraiodo and tetrabromo compounds was assigned by comparison of the experimental and conformations averaged calculated VCD spectra. The identification of a signature band of the absolute configuration of the butadiene core, the sign and location of which are independent from the different conformations and substituents, allowing the safe assignment of the absolute configuration of the enantiomers of chiral 1,3-butadienes, is also reported.