Solid-state NMR, crystallographic, and computational investigation of bisphosphonates and farnesyl diphosphate synthase-bisphosphonate complexes

Bisphosphonates are a class of molecules in widespread use in treating bone resorption diseases and are also of interest as immunomodulators and anti-infectives. They function by inhibiting the enzyme farnesyl diphosphate synthase (FPPS), but the details of how these molecules bind are not fully understood. Here, we report the results of a solid-state (13)C, (15)N, and (31)P magic-angle sample spinning (MAS) NMR and quantum chemical investigation of several bisphosphonates, both as pure compounds and when bound to FPPS, to provide information about side-chain and phosphonate backbone protonation states when bound to the enzyme. We then used computational docking methods (with the charges assigned by NMR) to predict how several bisphosphonates bind to FPPS. Finally, we used X-ray crystallography to determine the structures of two potent bisphosphonate inhibitors, finding good agreement with the computational results, opening up the possibility of using the combination of NMR, quantum chemistry and molecular docking to facilitate the design of other, novel prenytransferase inhibitors.     

Modulation of the photochromic property in an organoboron-based diarylethene by a fluoride ion

Organoboron-based diarylethene was successfully synthesized and exhibited photochromic property. Furthermore, its photochromic property can be modulated by a fluoride ion. The absorption maximum of the photostationary state shifted from 655 to 490 nm upon addition of a fluoride ion. The modulation mechanism is attributed to the special Lewis acid-base interaction between a trivalent boron atom and a fluoride ion.     

An ab initio simulation of the UV/Visible spectra of substituted chalcones

The electronic absorption spectra of 29 phenyl-ring substituted chalcones have been investigated with the time-dependent density functional theory (TD-DFT) and polarizable continuum TD-DFT (PCM-TD-DFT). It turns out that the hybrid PBE1PBE functional with the 6-31G basis set provide reliable λ_max when the solvent effects are included in the model. Comparisons with experimental values lead to a mean absolute error of 12 nm (0.136 eV). Moreover, the observed substituent effects are reproduced by calculation qualitatively. The λ_max of substituted chalcone in phenyl ring A is less sensitive to substitution than that in ring B. The linear correlation of Hammett’s substituent constants (σ_P) with LUMO energies is better with respect to HOMO energies. The calculation reveals that the maximum absorption band mainly results from the π→π* transition from HOMO to LUMO. The analysis of the electron density plots of frontier molecular orbitals show that most transitions should be of valence excitation nature.      

Nonconjugated dendritic iridium(III) complexes with tunable pyridine-based ligands: synthesis, photophysical, electrochemical, and electroluminescent properties

A simple synthetic route was developed for nonconjugated dendritic iridium(III) complex based on tunable pyridine-based ligands. From an intermediate 2-bromopyridyl-4-methanol, three series of polybenzyloxy dendritic pyridine-based ligands with 2-phenyl, 2-benzothienyl, and 2,4-difluorophenyl subsitituents were easily synthesized via two-step reactions (Suzuki reaction and etherifying reaction). Using these pyridine derivatives as the CwedgeN ligands, these dendritic iridium(III) complexes exhibiting tunable photoluminescence from blue to red were obtained. The photoluminescence quantum yields of these dendritic complexes in neat films increased with the increasing generation number of dendritic CwedgeN ligands. Importantly, these iridium complexes were used as dopants for successfully fabricating polymer-based electrophosphorescent light-emitting diodes (PLEDs) with the highest external quantum efficiency of 12.8%.     

Determination of free and protein-bound glutathione in HepG2 cells using capillary electrophoresis with laser-induced fluorescence detection

A rapid method using capillary electrophoresis with laser-induced fluorescence detection (CE-LIF) was developed to determine free and protein-bound glutathione (GSH) in human HepG2 hepatocarcinoma cells. The samples were derivatized with 5-iodoacetamidofluorescein (5-IAF), and analyzed at 22 kV using sodium phosphate buffer (10mM, pH 11.4) and an uncoated 58 cm x 75 microm I.D. fused silica capillary. The analysis time was less than 10 min and N-acetylcysteine was used as internal standard. The derivatization conditions, such as reaction time, 5-IAF concentration, running buffer and cartridge temperature were optimized. Argon gas was used in the study to prevent the oxidization of GSH during sample preparation. The optimized method required only 30-40 nl sample per analysis and was fast and sensitive. The method was applied to the analyses of HepG2 cells treated with the small metal chelating agent, pyrrolidine dithiocarbamate (PDTC). The results demonstrate that the amount of protein-bound GSH, which reflects the amount of protein S-glutathionylation, increased in a time-dependent manner upon cell treatment with PDTC, reaching a maximum of over 50% increase 2h post-PDTC.     

FRET-based sensor for imaging chromium(III) in living cells

On the basis of fluorescent resonance energy transfer from 1,8-naphthalimide to rhodamine, a fluorophore dyad (FD8) containing rhodamine and a naphthalimide moiety was synthesized as a Cr3+-selective fluorescent probe for monitoring Cr3+ in living cells with ratiometric fluorescent methods.     

Development of a capillary electrophoresis-based assay of sirtuin enzymes

Sirtuins are a family of nicotinamide adenine dinucleotide (NAD(+))-dependent enzymes catalyzing the deacetylation of acetyl-lysine residues of histones and other proteins. Three 9-fluorenylmethoxycarbonyl (Fmoc)-labeled peptide substrates derived from the amino acid sequence of p53, i.e. Fmoc-KK(Ac)-NH(2), Fmoc-KK(Ac)L-NH(2) and Fmoc-RHKK(Ac)-NH(2), were synthesized and evaluated as substrates of the human isoenzyme SIRT1. The acetylated and respective deacetylated peptides as well as nicotinamide as the reaction product of nicotinamide adenine dinucleotide were separated by capillary electrophoresis in a fused-silica capillary using 200 mM phosphate-Tris buffer, pH 2.7. Sodium hydroxide-mediated sample stacking was performed in order to overcome peak asymmetry due to the high salt and acid content of the sample as well as to enhance UV detection sensitivity. The assay was subsequently validated. Upon incubation of the acetylated peptides for 60 min in the presence of 2.5 U of SIRT1 at least 87% of the peptides was deacetylated, indicating that the new derivatives are efficient substrates of the enzyme.     

Effects of conjugation length, electron donor and acceptor strengths on two-photon absorption cross sections of asymmetric zinc-porphyrin derivatives

Exceptionally large two-photon absorption cross sections at the infrared region have been revealed by time-dependent density functional theory calculations for asymmetric charge-transfer conjugated zinc-porphyrin derivatives. The largest two-photon cross section is found to be more than one order of magnitude larger than for the conventional two-photon active organic molecules. The calculations show that the formation of strong charge-transfer states depends on the length of the conjugation bridge between the zinc-porphyrin core and the electron donor/acceptor. The two-photon absorption cross section can be greatly enhanced by increasing the strengths of the electron donor/acceptor.     

Highly selective phosphorescent chemosensor for fluoride based on an iridium(III) complex containing arylborane units

A new phosphorescent iridium(III) complex [Ir(Bpq)2(bpy)](+)PF6(-) based on cyclometalated ligands (Bpq) containing a dimesitylboryl group was synthesized and characterized by photophysical and electrochemical studies. The excited-state properties of Bpq and [Ir(Bpq)2(bpy)](+)PF6(-) were investigated using molecular orbital calculations. Importantly, both Bpq and [Ir(Bpq)2(bpy)](+)PF6(-) could be used as highly selective chemosensors for a fluoride anion (F(-)) detected by the naked eye, owing to the interaction of the dimesitylboryl group (BMes2) with F(-). For the Bpq ligand, a red shift of the emission spectrum was observed upon the addition of F(-), which could be attributed to an excited-state switch from a pi-pi* transition to a charge-transfer transition upon complexation with F(-). The addition of F(-) to a solution of [Ir(Bpq)2(bpy)](+)PF6(-) induced a change in the solution color from yellow to orange-red and phosphorescent quenching, indicating that [Ir(Bpq)2(bpy)](+)PF6(-) could act as an excellent ON-OFF-type phosphorescent chemosensor for F(-).