Marco Milanesio

Structural Characterization of Siliceous Spicules from Marine Sponges

Siliceous sponges, one of the few animal groups involved in a biosilicification process, deposit hydrated silica in discrete skeletal elements called spicules. A multidisciplinary analysis of the structural features of the protein axial filaments inside the spicules of a number of marine sponges, belonging to two different classes (Demospongiae and Hexactinellida), is presented, together with a preliminary analysis of the biosilicification process. The study was carried out by a unique combination of techniques: fiber diffraction using synchrotron radiation, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetric (DSC), Fourier transform infrared spectroscopy (FTIR), and molecular modeling. From a phylogenetic point of view, the main result is the structural difference between the dimension and packing of the protein units in the spicule filaments of the Demospongiae and the Hexactinellida species. Models of the protein organization in the spicule axial filaments, consistent with the various experimental evidences, are given. The three different species of demosponges analyzed have similar general structural features, but they differ in the degree of order. The structural information on the spicule axial filaments can help shed some light on the still unknown molecular mechanisms controlling biosilicification.

Kinematic diffraction on a structure with periodically varying scattering function.

A theory is developed to describe the kinematic diffraction response of a crystal when it is subjected to a periodically varying external perturbation. It is shown that if a part of the local electron density varies linearly with an external stimulus, the diffracted signal is not only a function of the stimulation frequency Ω, but also of its double 2Ω. These frequency components can provide, under certain conditions, selective access to partial diffraction contributions that are normally summed up in the interference pattern. A phasing process applied to partial diffraction terms would allow recovery of the substructure actively responding to the stimulus. Two ways of frequency filtering are discussed (demodulation and correlation) with respect to extracting information from such an experiment. Also considered is the effect of the variation of different structural parameters on the diffraction intensity that have to be accounted for while planning modulation-enhanced experiments. Finally, the advantages and limitations of the proposed concept are discussed, together with possible experiments.

Structural characterization and thermal and chemical stability of bioactive molecule–hydrotalcite (LDH) nanocomposites

Layered double hydroxides (LDH) are versatile materials used for intercalating bioactive molecules, both in pharmaceutical and cosmetic fields, with the purpose of protecting them from degradation, enhancing their water solubility to increase bioavailability, and/or obtaining modified release properties. The properties of the intercalation compounds of Mg/Al_LDH and Zn/Al_LDH with different drugs and sunscreens, namely diclofenac, ketoprofen, gliclazide, retinoic acid, furosemide, para-aminobenzoic acid and 2-phenylbenzimidazolsulfonic (Eusolex) acid, have been studied by crystallographic, spectroscopic and thermogravimetric techniques and by solid state NMR, to shed light on their structure, their molecular interactions and their stability from the thermal and chemical viewpoint. The structural features were described with particular attention to the interaction between the organic and inorganic components and to the stability of the intercalation products. For the first time two synchrotron radiation powder diffraction patterns of organic-containing LDH were solved and refined by Rietveld methods to obtain an experimental crystal structure.

Patterson selectivity by modulation-enhanced diffraction

Modulation excitation spectroscopy is a powerful and well established technique for investigating the dynamic behaviour of chemical and physical systems. Recently, an expansion of this technique for diffraction was proposed and the theory deriving the diffraction response of a crystal subjected to a periodically varying external perturbation was developed [Chernyshov, van Beek, Emerich, Milanesio, Urakawa, Viterbo, Palin & Caliandro (2011). Acta Cryst. A67, 327–335]. The result of this is that a substructure composed of atoms actively responding to the stimulus may be separated out by analysing the diffraction signal at a frequency twice that of the stimulus. This technique is called modulation-enhanced diffraction. Here, a version of the theory dealing with the modulation of the site occupancies of a selected subset of atoms is formulated, and this is supported by experiments carried out at the Swiss–Norwegian Beam Lines at the ESRF, involving periodic variation of the xenon content of a polycrystalline zeolite as a function of temperature. The data analysis involves three steps: (i) data selection is carried out to mimic a linear response; (ii) phase-sensitive detection is applied to obtain contributions both from the responding part of the electron density associated with the Xe atoms and from the interference term; (iii) a phasing procedure is applied to both. A Patterson deconvolution technique has been successfully used to phase the demodulated diffraction patterns and obtain the active substructure.

Rationalization of Dye Uptake on Titania Slides for Dye‐Sensitized Solar Cells by a Combined Chemometric and Structural Approach

A model photosensitizer (D5) for application in dye-sensitized solar cells has been studied by a combination of XRD, theoretical calculations, and spectroscopic/chemometric methods. The conformational stability and flexibility of D5 and molecular interactions between adjacent molecules were characterized to obtain the driving forces that govern D5 uptake and grafting and to infer the most likely arrangement of the molecules on the surface of TiO2. A spectroscopic/chemometric approach was then used to yield information about the correlations between three variables that govern the uptake itself: D5 concentration, dispersant (chenodeoxycholic acid; CDCA) concentration, and contact time. The obtained regression model shows that large uptakes can be obtained at high D5 concentrations in the presence of CDCA with a long contact time, or in absence of CDCA if the contact time is short, which suggests how dye uptake and photovoltaic device preparation can be optimized.

Untangling diffraction intensity: modulation enhanced diffraction on ZrO2 powder

This paper describes a new method for extracting the individual contributions to the diffracted intensity of subsets of atoms in the crystal structure. The periodic perturbation of the scattering process, required for untangling the scattered intensity, is provided by altering the resonant contributions. The theory of modulation enhanced diffraction (MED) is briefly recalled in the context of resonant scattering. A periodic variation in the atomic form factor has been achieved by changing the X-ray energy in such a way that the MED theory holds. Simulated results and experimental data are presented, together with necessary corrections. Two data analysis schemes are presented, both illustrating the advantages and drawbacks of the novel modulation technique.

Studying modifications and reactions in materials by simultaneous Raman and X-ray powder diffraction at non-ambient conditions: methods and applications

X-ray powder diffraction played a key role in materials science over the past 25 years. It is now possible to employ powder diffraction at in situ conditions to study solid-state transformations. “Real” materials science samples often show deviations from the “perfectly crystalline” state and hence powder diffraction reaches its limits. Spectroscopic techniques can often provide useful complementary information. With the aim of gaining information from this aspect, we combined Raman spectroscopy with diffraction. A novel experimental setup for simultaneous in situ Raman and XRPD experiments was designed and realized at the SNBL line at ESRF Synchrotron (Grenoble-F). High-resolution XRPD data can be collected with a time resolution in the range of minutes. A permanently installed Raman spectrometer can now be used to measure the same sample at the same spot under the same conditions. A wide variety of non-ambient conditions are available. The setup is described with limitations and potentialities, by describing one example of application. Finally some hints on future improvements are described.

Synthesis, structural and spectroscopic study of the donor–acceptor complexes between fluorene and D2h cyano molecular building blocks

Three solid molecular complexes of fluorene with electron-withdrawing tetracyanoethylene, 1,2,4,5-tetracyanobenzene and 7,7,8,8-tetracyanoquinodimethane were synthesized. Single-crystal X-ray diffraction data elucidated the order–disorder aspects of the crystal structures ascribed to the different molecular symmetries of the employed building blocks. This hypothesis was confirmed by the structural and energetic results of ab initio periodic calculations. Donor–acceptor solid state interactions between molecular counterparts have been highlighted by electron and vibrational IR and Raman solid state spectroscopy, indicating a significant extent of electron density transfer from the fluorene unit towards the cyano-molecules. The experimental evidences of donor–acceptor interactions between molecular counterparts were compared to the lattice energies and solid state band-gaps, obtained by periodic calculations, and to the cluster HOMO–LUMO differences, obtained by isolated cluster calculations. A good agreement between spectral and theoretical data was found.

CONFORMATIONAL AND DYNAMICAL STUDY OF SQUALENE DERIVATIVES. III : AZASQUALENES AND SOLVATED SQUALENE

Abstract A detailed conformational analysis of six squalene derivatives (19-azasqualene 1, 19-azasqualene N-oxide 2, azasqualene alcohol 3, azasqualene alcohol N-oxide 4, squalene ethylendiamine 5 and 22,23-oxidoazasqualene N-oxide 6) is presented. Most of these compounds are potent inhibitors of 2,3-oxidosqualene cyclases, key enzymes in the biosynthesis of sterols, mimicking the different carbonium ion intermediates formed during the cyclization of 2,3-oxidosqualene. They were studied by mono- and bi-dimensional NMR methods (both 1H and 13C NMR) and by molecular mechanics and molecular dynamics techniques, in order to obtain some insight into the conformational and dynamical behavior of these molecules in chloroform solution. NMR data indicate that azasqualene derivatives 1–6 behave similarly to squalene itself and to other previously studied analogues: the mid portion of all these squalene derivatives appears to be comparatively stiff with respect to the rather mobile endings. Molecular dynamic calculations on squalene in chloroform solution at 300K were also performed, using explicit atomic representation of the solvent, which permits: (i) reproducing and explaining the increase of mobility of the individual torsion angles in going from the center to the end of the squalene chain, indicating that the solvent plays a major role in dictating the conformational properties of squalene and its derivatives; (ii) achieving direct evidence of relatively stable, solvent induced folded conformations. Finally, the chemical and biological implications of the results are discussed.

A Combined High-Resolution X-ray Powder Diffraction, Computational, and XPS Study of the Local Structure of Extra-Framework Copper Ions in Over-Exchanged Cu-MCM22 Zeolite

Local structure and site distribution of extra-framework copper ions in over-exchanged Cu-MCM22 zeolite were determined by a combination of high resolution X-ray powder diffraction and computational analysis. X-ray diffraction data suggested the presence of three Cu sites in six-membered rings and one site in a five-membered ring close to the interlamellar region, inside the MCM-22 supercage, whereas no Cu ions were found within the sinusoidal channels. First principle molecular orbital DFT calculations were employed to obtain, for the first time, an accurate structural description of the Cu(I) sites in the supercage, adding a structural and energetic interpretation to previous IR and EPR studies. The combined experimental and computational study suggested that Cu(I) sites facing 6-MRs are particularly stable. In general 5- or 4-fold coordination sites are located in 6-MRs while 2- or 3-fold coordination sites are located in 5-MRs. Three preferentially occupied sites were found in copper-exchanged MCM-22. X-ray photoelectron spectroscopy suggested the formation of dispersed Cu close to the surface of MCM-22 crystallites, easily reduced to Cu(I) under ultrahigh vacuum conditions.