Nicolae Viorel Pavel

An extended x‐ray absorption fine structure study of aqueous solutions by employing molecular dynamics simulations

Bromine–oxygen radial distribution functions [g(r)] have been calculated by means of molecular dynamics simulations for aqueous solutions of rubidium bromide, 2‐bromopropane and bromoethane. X‐ray absorption spectra at the bromine K edge have been recorded for these solutions. The water contribution to the extended x‐ray absorption fine structure spectra has been calculated starting from the gBr,O(r) distribution function. Fits of the x‐ray absorption spectra have been performed directly on the raw experimental data, allowing the reliability of the g(r) distribution functions to be verified. The agreement between theoretical and experimental spectra is satisfactory. A procedure to improve model g(r) functions on the basis of the short‐range structural information provided by extended x‐ray absorption fine structure data is proposed.

Structural investigation of copper(II) chloride solutions using x-ray absorption spectroscopy

The local environment of the Cu2+ ion in copper chloride solutions has been investigated by x-ray absorption spectroscopy. Three aqueous solutions of CuCl2 with increasing Cl−/Cu2+ ratio have been examined. An advanced data analysis including multiple-scattering effects produced quantitative information on the chlorocuprate complexes present in solution and provided evidence for an increasing degree of complex formation between Cu2+and Cl− ions with increasing chloride concentration. The presence of Cu–Cl interactions at the axial site has been detected in a 0.1 M CuCl2 solution for the first time. At higher chloride concentrations (Cl−/Cu2+ratios equal 10 and 30) the equatorial positions in the distorted octahedral copper coordination are occupied by 3.2 and 3.0 oxygens and 0.8 and 1.0 chloride ions, respectively, while the axial positions are occupied by 1.2 and 1.0 oxygens and 0.8 and 1.0 chloride ions, respectively. The results are found to be consistent with previous x-ray and neutron diffraction stu...

Catanionic Tubules with Tunable Charge

Thethree-dimensionalstructureswithnanoscopicdimensionsthat are yielded by the self-assembly of lipids and surfactantsare of particular interest for their applications in nano-technology.Intheseapplications,thepossibilityofcontrollingthe charge of the particles allows the regulation of funda-mental aspects, such as the ability of the particles to loadmolecules (drugs, DNA, proteins, etc.), to aggregate, and topenetrate membranes. Within the possible surfactant supra-molecular architectures, tubular structures have recentlydrawn much research interest.

Synthesis and Characterization of a New Gemini Surfactant Derived from 3α,12α-Dihydroxy-5β-cholan-24-amine (Steroid Residue) and Ethylenediamintetraacetic Acid (Spacer)

A new gemini steroid surfactant derived from 3alpha,12alpha-dihydroxy-5beta-cholan-24-amine (steroid residue) and ethylenediamintetraacetic acid (spacer) was synthesized and characterized in aqueous solution by surface tension, fluorescence intensity of pyrene, and light scattering (static and dynamic) measurements. These techniques evidence the existence of a threshold concentration (cac), below which a three layers film is formed at the air-water interface. Above the cac, two types of aggregates--micelles and vesicle-like aggregates--coexist in a metastable state. Filtration of a solution with a starting concentration of 2.6 mM (buffer 150 mM, pH 10) allows isolation of the micelles, which have an average aggregation number of 12, their density being 0.28 g cm(-3). Under conditions where only the vesicle-like aggregates are detected by dynamic light scattering, a value of 5.5 x 10(4) was obtained for their aggregation number at 30 microM, their density being 6.8 x 10(-4) g cm(-3). At high concentrations, the intensity ratio of the vibronic peaks of pyrene, I1/I3, (=0.68) is very close to published values for deoxycholate micelles, indicating that the probe is located in a region with a very low polarity and far from water. A hypothesis to explain the observed aggregation behavior (small aggregates are favored with increasing gemini concentration) is outlined.

Kinetics of formation of supramolecular tubules of a sodium cholate derivative

We report a kinetic study of the supramolecular tubule formation of the bile salt derivative [3β,5β,7α,12α]-3-(4-t-butylbenzoilamine)-7,12-dihydroxycholan-24-oic acid sodium salt (Na-tbutPhC). At high bicarbonate buffer concentration (pH∼10) this salt shows gelator properties. Starting from gels or viscous solutions, the tubule formation is triggered by increasing the temperature beyond the critical value of 34–36 °C. For gels, when the process takes place, the transition to sols occurs. The process is easily triggered and can be followed by several techniques. We used static light scattering (SLS), circular dichroism (CD), small angle X-ray scattering (SAXS) along with transmission electron (TEM) and optical microscopies. The CD results show that fibrils with a clockwise arrangement of the bile salt derivative are present in the samples at room temperature. When the tubule formation starts, evolutions of the CD and SLS profiles are observed indicating that the formation process begins with the aggregation of the fibrils accompanied by a simultaneous peculiar reciprocal reorientation of the surfactant molecules. After that, as pointed out by the long time evolution of the curves, a slow transformation towards the final well defined tubules occurs, involving an adjustment of the molecular packing. In the meanwhile, the slow ordering of the tubule walls in well spaced layers takes place, as inferred by SAXS. The TEM images show that short disordered tubules are formed, because of the aggregation of fibrils, in the beginning. Moreover they highlight a final elongation of the tubules taking place without a further aggregation of fibrils. Optical microscopy frames, collected during the process, point out that the tubules grow singly even at quite a high concentration, thus supporting the data interpretation.

Urea-Induced Denaturation Process on Defatted Human Serum Albumin and in the Presence of Palmitic Acid

We report a study on the unfolding behavior of the most abundant protein contained in plasma, human serum albumin. The unfolding mechanisms in denaturing conditions induced by urea are studied for the defatted form (HSA) and for the palmitic acid:albumin (HSAPalm) complex. We employed the singular value decomposition method to determine the minimum number of structural states present in the unfolding processes. Low-resolution three-dimensional structures are reconstructed from the one-dimensional small-angle X-ray scattering patterns and are correlated with the parameters obtained from static and dynamic light scattering experiments. The unfolding process is pointed out by both ab initio and rigid body fitting methods that highlight a stepwise evolution of the protein structure toward open conformations. The superimpositions of the 3D structures provided independently by the two methods show very good agreements. The hydrodynamic radii estimated for the protein best fitting conformations are in satisfactory agreement with the experimental ones. The results show that the HSA unfolding process is consistent with previous spectroscopic studies that suggest a multistep unfolding pathway. In particular, a scheme in which domains I and II are opened in sequence and the presence of two intermediates are evidenced is presented. The opening sequence is different from that found using guanidine hydrochloride as denaturant agent. The stabilizing role of the fatty acids in the urea denaturation process is evident. The palmitic acid ligand strongly stabilizes the protein, which remains in the native form up to high denaturant concentrations. In this case, the unfolding process is characterized by a single-step mechanism.

Human serum albumin binding ibuprofen: A 3D description of the unfolding pathway in urea

Small angle X-ray scattering (SAXS) technique, supported by light scattering measurements and spectroscopic data (circular dichroism and fluorescence) allowed us to restore the 3D structure at low resolution of defatted human serum albumin (HSA) in interaction with ibuprofen. The data were carried out on a set of HSA solutions with urea concentrations between 0.00 and 9.00M. The Singular Value Decomposition method, applied to the complete SAXS data set allowed us to distinguish three different states in solution. In particular a native conformation N (at 0.00M urea), an intermediate I1 (at 6.05M urea) and an unfolded structure U (at 9.00M urea) were recognized. The low-resolution structures of these states were obtained by exploiting both ab initio and rigid body fitting methods. In particular, for the protein without denaturant, a conformation recently described (Leggio et al., PCCP, 2008, 10, 6741-6750), very similar to the crystallographic heart shape, with only a slight reciprocal movement of the three domains, was confirmed. The I1 structure was instead characterized by only a closed domain (domain III) and finally, the recovered structure of the U state revealed the characteristic feature of a completely open state. A direct comparison with the free HSA pointed out that the presence of the ibuprofen provokes a shift of the equilibrium towards higher urea concentrations without changing the unfolding sequence. The work represents a type of analysis which could be exploited in future investigations on proteins in solution, in the binding of drugs or endogenous compounds and in the pharmacokinetic properties as well as in the study of allosteric effects, cooperation or anticooperation mechanisms.

Triplet correlations in the hydration shell of aquaions

Abstract The presence of angular O—ion—O correlations in the hydration shell of dilute water solutions of Cu 2+ and Br − is investigated by means of an X-ray absorption experiment. An advanced data analysis including multiple-scattering effects poroduced quantitative information on these triplet correlations; the complementarity with X-ray and neutron diffraction is discussed. The experimental spectra indicate that Cu 2+ has a Jahn—Teller distorted octahedral coordination, while the hydration shell of Br − is more disordered and substantially dominated by excluded volume effects.

The molecular and crystal structure of an allylpalladium- (II) triazenido complex: [(1-3-η-C3H5)Pd(II)(p-CH3C6H4NNNC6H4CH3-p)]2

Abstract For the class of compounds mentioned in the title it is difficult to the stucture unambiguously on the molecular and crystal structure of CH 3 - p )] 2 by single crystal X-ray analysis. The crystal data are: M == 743 a.m.u. Space group P 2 1 / c . a = 8.510(2), b = 40.652(9), c = 9.762(2) A, β = 103.61(2)° D c = 1.50 g/cm 3 . R = 0.041, R = 0.060, based on 3978 independent reflections. The two π-allylpalladium residues are bridged azenido groups, gaining an approximate square planar coordination around each heavy atom. The two allyl units are stereochemically equivalent, with the central carbon atoms pointing outwars. The rigid triazenido groups force the two palladium atoms into close contact (2.86 A). The aromatic rings are somewhat rotated with respect to the bonded NNN planes, but some π-conjugation over the whole ligand is still retained.

About the albumin structure in solution: cigar Expanded form versus heart Normal shape.

A structural comparison between the Normal and the Expanded isomers of the human serum albumin has been carried out by using small angle X-ray scattering (SAXS) and light scattering (LS) techniques. Geometrical bodies, recovered structures (GA_STRUCT code) and rigid body modeling (CRYSOL and BUNCH software) were used to obtain low-resolution 3D structures from one-dimensional scattering patterns. These restored shapes were also exploited to perform a correlation between SAXS and LS data. By attempting a detailed description of globular and unfolded protein structures in solution, we tried to propose a suitable approach to follow the path of folding/unfolding processes and to isolate and characterize possible partially folded intermediate states.