Universal quenching of common fluorescent probes by water and alcohols

Although biological imaging is mostly performed in aqueous media, it is hardly ever considered that water acts as a classic fluorescence quencher for organic fluorophores. By investigating the fluorescence properties of 42 common organic fluorophores recommended for biological labelling, we demonstrate that H₂O reduces their fluorescence quantum yield and lifetime by up to threefold and uncover the underlying fluorescence quenching mechanism. We show that the quenching efficiency is significantly larger for red-emitting probes and follows an energy gap law. The fluorescence quenching finds its origin in high-energy vibrations of the solvent (OH groups), as methanol and other linear alcohols are also found to quench the emission, whereas it is restored in deuterated solvents. Our observations are consistent with a mechanism by which the electronic excitation of the fluorophore is resonantly transferred to overtones and combination transitions of high-frequency vibrational stretching modes of the solvent through space and not through hydrogen bonds. Insight into this solvent-assisted quenching mechanism opens the door to the rational design of brighter fluorescent probes by offering a justification for protecting organic fluorophores from the solvent via encapsulation.

Overtones and combinations of O–H vibrations in the solvent efficiently quench red-emitting fluorophores by resonant energy transfer.     

Enantioselective addition of aryllithium reagents to aromatic imines mediated by 1,2-diamine ligands

A variety of optically enriched amines have been obtained by addition of aryllithium reagents to aromatic imines using N,N′-tetramethylcyclohexane-1,2-diamine as chiral ligands. Enantiomeric excesses up to 90% could be obtained.     

Antiedematogenic activity of two thiazolidine derivatives: N-tryptophyl-5-(3,5-di-tert-butyl-4-hydroxybenzylidene) rhodanine (GS26) and N-tryptophyl-5-(3,5-di-tert-butyl-4-hydroxybenzylidene)-2,4-thiazolidinedione (GS28)

The search for new anti-inflammatory drugs has been constant in several research centers. The use of the Bioisostery concept allows the elaboration of new bioactive compounds with different properties through the introduction of substitute groups in one or more positions of a main molecule with known biological activity. Preliminary works accomplished at our laboratory with 2,4-thiazolidinedione isosters demonstrated inhibitory activity on edema formation for N-tryptophyl-5-(3,5-di-tert-butyl-4-hydroxybenzylidene)-2,4-thiazolidinedione (GS28) and N-tryptophyl-5-(3,5-di-tert-butyl-4-hydroxybenzylidene) rhodanine (GS26). We verified the antiedematogenic and ulcerogenic activity of these two compounds in Wistar rats. The carrageenan induced paw edema suffered significant (p<0.05) inhibition (28.36% on average) for GS28 (100 mg/kg; v.o.) during the entire time of the experiment. GS26 (50 and 100 mg/kg; v.o.) significantly inhibited (p<0.05) the paw edema dextran induced (22.1 and 27.8%, for the respective doses) after 180 min. The compounds GS26 and GS28 did not show ulcerogenic activity on gastric mucous. The results suggest antiedematogenic action for both compounds without the appearance of gastric lesions.     

Formation of π-coupled organic wire on the Si(0 0 1)[2 × 1] surface

The stability and electronic properties of highly packed 1-hexyl-naphthalene (HNap) molecular wire on Si(0 0 1) have been studied with first principles DFT method. HNap assembles into a 1D arrangement on the Si(0 0 1)[2 × 1] surface on which molcules adopt a commensurate structure along a dimer row with an intermolecular distance of 3.8 Å. HNap is attached to the surface through the hexyl chain, and stands normal to the surface. This highly packed structure leads to the formation of delocalized π-orbitals over the entire wire but essentially localized on the naphthalene counterpart, and well separated from the Si surface states. Cohesion energy within the wire arises from a significant attraction between hexyl chains, and to a weaker stabilizing π–π interaction between naphthalenes.     

Discrimination between ethanol inhibition models in a continuous alcoholic fermentation process using flocculating yeast

Discrimination between different rival models for describing the inhibitory effect of ethanol both on yeast growth and on fermentation was studied for a continuous process of alcoholic fermentation in a tower reactor with recycling of flocculating cells. Models tested include linear, parabolic, hyperbolic, exponential, and generalized nonlinear power-law types. The best expressions were identified under the criteria that all the kinetic parameters should assume acceptable values in a feasible range and should result in the best fit of the experimental data. The kinetic parameters were estimated from steady-state data of several sugar concentrations in feeding stream (S₀ = 160, 170, 180, 190, 200 g/L), constant dilution rate (D = 0.2 h^-1), recycle ratio (α = 13.6), and temperature (T = 30°C). The best model for the yeast growth was of power-law type, whereas for the product formation the best model was of linear type. These models were able to reproduce the trends of the process variables satisfactorily.     

A new perspective on beta-sheet structures using vibrational Raman optical activity: from poly(L-lysine) to the prion protein

The vibrational Raman optical activity (ROA) spectrum of a polypeptide in a model beta-sheet conformation, that of poly(l-lysine), was measured for the first time, and the alpha-helix --> beta-sheet transition monitored as a function of temperature in H(2)O and D(2)O. Although no significant population of a disordered backbone state was detected at intermediate temperatures, some side chain bands not present in either the alpha-helix or beta-sheet state were observed. The observation of ROA bands in the extended amide III region assigned to beta-turns suggests that, under our experimental conditions, beta-sheet poly(L-lysine) contains up-and-down antiparallel beta-sheets based on the hairpin motif. The ROA spectrum of beta-sheet poly(L-lysine) was compared with ROA data on a number of native proteins containing different types of beta-sheet. Amide I and amide II ROA band patterns observed in beta-sheet poly(L-lysine) are different from those observed in typical beta-sheet proteins and may be characteristic of an extended flat multistranded beta-sheet, which is unlike the more irregular and twisted beta-sheet found in most proteins. However, a reduced isoform of the truncated ovine prion protein PrP(94-233) that is rich in beta-sheet shows amide I and amide II ROA bands similar to those of beta-sheet poly(L-lysine), which suggests that the C-terminal domain of the prion protein is able to support unusually flat beta-sheets. A principal component analysis (PCA) that identifies protein structural types from ROA band patterns provides a useful representation of the structural relationships among the polypeptide and protein states considered in the study.     

The Pesticide 3-(3,4-Dichlorophenyl)-1,1-dimethylurea (Diuron) Immobilized on Silica Gel Surface

A route for 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron) immobilization on silica gel was established after reacting at the first stage the precursor silylant agent 3-trimethoxysilylpropylamine to the support. The pesticide was covalently bonded to available amine groups of the precursor, giving 1.03 mmol of amine per gram of silica. Infrared, (13)C, and (29)Si NMR spectra are in agreement with the proposed reaction between nitrogen of the amine group of the previously anchored silica to carbon on the para-position of the aromatic ring of the pesticide. The immobilization is clearly affected by the presence or absence of disprotonating agent, to give 12.50 and 68.40% reaction yield, respectively; these results were confirmed through elemental analysis. Copyright 2001 Academic Press.     

Molecular dynamics in the smectic a and C phases in a long-chain ferroelectric liquid crystal: 2H NMR, dielectric properties, and a theoretical treatment

In this work, the rotational-diffusion coefficients D(parallel) and D(perpendicular) for the ferroelectric smectogen (+)-(S)-4-[4'-(1-methylheptyloxy)] biphenyl 4-(10-undecenyloxy)benzoate have been studied by means of 2H NMR spectroscopy in the smectic C phase, using a new theoretical approach (Domenici,V.; Geppi, M.; Veracini, C. A. Chem. Phys. Lett. 2003, 382, 518). The analysis of spin-lattice relaxation times has been performed in terms of the diffusional constant and the activation energy of the internal and overall molecular-reorientational motions, and the results are compared to the smectic A (SmA) phase. Moreover, from the 2H NMR data in the SmA phase, the dielectric permittivity and the dielectric relaxation time functions are investigated using a theoretical approach. The longitudinal and transverse components of the real Rchigammaomega and imaginary chigammaomega (gamma = parallel, perpendicular) parts of the complex susceptibility tensor and the nematic-like rotational-viscosity coefficients, lambda2 and lambda5, are calculated.     

The role of rotation in the vibrational relaxation of water by hydrogen molecules

Vibrational relaxation cross sections of the H(2)O(upsilon(2) = 1) bending mode by H(2) molecules are calculated on a recent high-accuracy ab initio potential-energy surface using quasiclassical trajectory calculations. The role of molecular rotation is investigated at a collisional energy of 3500 cm(-1) and it is shown that initial rotational excitation significantly enhances the total (rotationally summed) vibrational relaxation cross sections. A strong and complex dependence on the orientation of the water angular momentum is also observed, suggesting the key role played by the asymmetry of water. Despite the intrinsic limitations of classical mechanics, these exploratory results suggest that quantum approximations based on a complete decoupling of rotation and vibration, such as the widely used vibrational close-coupling (rotational) infinite-order-sudden method, would significantly underestimate rovibrationally inelastic cross sections. We also present some rationale for the absence of dynamical chaos in the scattering process.