Gaining an insight into the photoreactivity of a drug in a protein environment: a case study on nalidixic acid and serum albumin

The binding of nalidixic acid (NA) with human and bovine serum albumin (HSA and BSA) in buffer solution at pH 7.4 was investigated using circular dichroism (CD), UV absorption and fluorescence spectroscopy. Global analysis of multiwavelength spectroscopic data afforded the equilibrium constants of the most stable noncovalent drug/protein adducts of 1:1 and 2:1 stoichiometry and their individual CD, UV absorption, and fluorescence spectra. The primary binding site of the drug was located in subdomain IIIA (Sudlow Site II), whereas the secondary one was assigned to subdomain IIA. Conformational and CD calculations afforded the binding geometries. In the complexes, the fluorescence of the protein was strongly quenched by energy transfer and that of the drug was suppressed by electron transfer. Laser flash photolysis at 355 nm evidenced the formation of a radical pair consisting of a tyroxyl radical (lambdamax = 410 nm) and a reduced nalidixate anion radical NA(2-)* (lambdamax = 640 nm) with quantum yield of 0.4-0.5. Strong evidence was obtained that the process that involves Tyr411 in HSA (Tyr409 in BSA). A further transient with lambdamax approximately 780 nm observed in HSA was attributed to oxidation of the -(S200-S246)- bridge upon electron transfer to NA(-)*. Decay of the confined radical pairs occurred with rates approximately 10(7) s(-1). Formation of covalent drug-protein adducts in mixtures irradiated at lambdairr> 324 nm was proved using HPLC with fluorescence detection.     

An Alternate Solution of Fluorescence Recovery Kinetics after Spot-Bleaching for Measuring Diffusion Coefficients. 2. Diffusion of Fluorescein in Aqueous Sucrose Solutions

The traditional analysis of the fluorescence recovery kinetics after spot bleaching yields expressions for the diffusion coefficient of the probe that are not suitable for linear fittings. In a previous work we developed an improved recovery function that is a better alternative for data analysis. To illustrate its application to real cases and compare it with the previous data treatment, we measured the time response of fluorescein in aqueous sucrose solutions, covering the unsaturated and the supercooled region, where decoupling between diffusion and viscosity is observed. The results are compared with the mobility of different types of solutes in aqueous sucrose solutions and are discussed in terms of the classical hydrodynamic model.     

Stereochemical consequences of threefold symmetry in asymmetric catalysis: distorting C3 Chiral 1,1,1-tris(oxazolinyl)ethanes ("trisox") in CuII Lewis acid catalysts

The underlying conceptual differences in exploiting two- and threefold rotational symmetry in the design of chiral ligands for asymmetric catalysis have been addressed in a comparative study of the catalytic performance of bisoxazoline (BOX) and tris(oxazolinyl)ethanes (trisox) containing copper(II) Lewis acid catalysts. The differences become apparent in constructing new catalysts by systematically "deforming" the stereodirecting ligand by inverting chiral centres or replacing chiral by achiral oxazolines. The catalytic alpha-amination of ethyl 2-methylacetoacetate with dibenzyl azodicaboxylate, which occurs with high enantioselectivity for both Ph(2)-BOX and Ph(3)-trisox copper catalysts, has been employed as the test reaction. In the trisox-copper complex [Cu(II)(iPr(3)-trisox)(kappa(2)-O,O'-MeCOCHCOOEt)](+)[BF(4)](-) (1), which was characterised by X-ray diffraction, two of the oxazoline groups are coordinated to the central copper atom, whilst the third oxazoline unit is dangling with the N-donor pointing away from the metal centre. A similar arrangement is found for the stereochemically "mixed" C(1)-trisox complex [Cu(II){(Ph(3)-trisox(R,S,S)}(kappa(2)-O,O'-MeCOCHCOOEt)(H(2)O)](+)[ClO(4)](-) (2), in which the phenyl substituents adopt a first coordination sphere meso arrangement. The almost identical selectivity of the Ph(3)-trisox(R,R,R)- and Ph(2)-BOX(R,R)-derived catalysts is as expected from the proposed model of the active catalyst based on a partially decoordinated podand. The behaviour of the "desymmetrised" trisox-Cu catalysts may be rationalised in terms of a general steady-state kinetic model for the three possible active bisoxazoline-copper species, which are expected to be in rapid exchange with each other in solution. This applies to both the trisox derivatives with stereochemically inverted and achiral oxazoline rings. The study underscores previous observations of superior performance of the catalysts bearing C(3)-chiral stereodirecting ligands as compared to systems of lower symmetry.     

Quantum mechanical calculations on vibronic activity in the MCD spectra of carbonyl compounds

The magnetooptical properties (B terms) vibronically induced have been calculated for a series of carbonyl compounds in the region of the first absorption band. The rules deduced experimentally for the signs and sizes of the B terms induced by vibrations of different symmetry are generally confirmed by these calculations.     

Synthesis and optical properties of isomeric branched pi-conjugated systems

[structure: see text] Branched conjugated systems with a terminal alkyne function have been prepared starting from 4-(triisopropylsilylethynyl) phenylacetylene by applying the following iterative reaction sequence: (i) metal-catalyzed cross-coupling reaction of the terminal alkyne with 3,4-dibromobenzaldehyde or 2,5- dibromobenzaldehyde; (ii) Corey-Fuchs dibromoolefination and treatment with an excess of LDA. The building blocks thus prepared have been subjected to a Pd-catalyzed cross-coupling reaction with 1,4-diiodobenzene to yield isomeric branched pi-conjugated systems containing 7 (first generation) or 15 (second generation) phenyl units connected by ethynyl spacers. The different pi-conjugation patterns in those isomeric derivatives have a dramatic effect on their electronic properties, as attested by the differences observed in their absorption and emission spectra. Finally, theoretical calculations have been performed to rationalize the optical properties of these compounds.     

Free and bound phenolic compounds in barley (Hordeum vulgare L.) flours. evaluation of the extraction capability of different solvent mixtures and pressurized liquid methods by micellar electrokinetic chromatography and spectrophotometry

The solution speciation of metals is a critical parameter controlling the bioavailability, solution-solid phase distribution and transport of metals in soils. The natural metal-complexing ligands that exist in soil solution include inorganic anions, inorganic colloids, organic humic substances, amino acids (notably phytosiderophores and bacterial siderophores) and low-molecular mass organic acids. The latter two groups are of particular significance in the soil surrounding plant roots (the rhizosphere). A number of analytical methodologies, encompassing computational, spectroscopic, physico-chemical and separation techniques, have been applied to the measurement of the solution speciation of metals in the environment. However, perhaps with the exception of the determination of the free metal cation, the majority of these techniques rarely provide species specific information. High-performance liquid chromatography (HPLC) coupled to a sensitive detection system, such as inductively coupled plasma mass spectrometry (ICP-MS) or electrospray ionisation mass spectrometry (ESI-MS), offers the possibility of separating and detecting metal-organic acid complexes at the very low concentrations normally found in the soil environment. This review, therefore, critically examines the literature reporting the HPLC separation of metal-organic acid complexes with reference to thermodynamic equilibrium and kinetic considerations. The limitations of HPLC techniques (and the use of thermodynamic equilibrium calculations to validate analytical results) are discussed and the metal complex characteristics necessary for chromatographic separation are described.     

Structural Analysis of Host–Guest Systems. Methyl-substituted Phenols in beta;-Cyclodextrin

Complexation trajectories and the variation ofinduced circular dichroism are calculated for thedocking of phenol and 2,4,6-trimethyl-phenol with-cyclodextrin. The results are compared toexperimental chirality data to elucidate themechanism of nonspecific molecular recognitionprocesses in aqueous solution. Large geometricalchanges along nearly isoenergetic Dynamic Monte Carlotrajectories show the conformational flexibility ofsuch host–guest systems. This proves diffuseintermolecular interactions, van der Waals orelectrostatic in nature, as the main contributions to thebinding energy. The number and position of the methylsubstituents of the guest reduces the complexityof the conformational space as the guests positionbecomes fixed by steric constraints.The solvation free energy is calculated from thesolvent accessible surface area weighted byrespective atomic solvation parameters. Consideringthe solvation term in the dynamic simulationsrestricts the conformational flexibility of themacromolecular system. The relative importance ofvarious contributions to the solvation energy isdiscussed and it is shown that those terms arisingfrom the interaction of hydrophobic groups with theaqueous environment are essential for thedetermination of the complex structure. Consideringthese terms in the dynamic simulation model, the signand strength of the calculated rotatory strength isin perfect agreement with induced circular dichroismobtained from experimentally determined averagedspectra. The results demonstrate the accuracy of thegeometrical properties of host–guest systems obtainedfrom these simulations.     

Anreicherung von Spuren Seltener Erden aus Mineralwässern durch Ionenaustausch

In the described analysis of mineral waters, after the throughflow of the sample through a column with Dowex 50 WX 12 in H⁺ form, the main cations are first eluted by 1.6 N HCl and then the rare earths by 6 N HCl. The latter are separated by ion-exchange chromatography on Dowex 50WX8 in NH₄ ⁺ form, the elution being performed with ammonium citrate pH 4.19. The presence of rare earths in the eluate was ascertained spectrophotometrically by means of their reaction with xylenol orange in the presence of cetylpyridinium bromide. In mineral waters from the West-Bohemian spa region it was possible to find traces of trivalent La, Ce, Pr, Nd, Sm + Gd, Y, Dy, Ho, Er, Tm, Yb and Lu.