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With an increasing number of biological macromolecule structures solved at ultra-high resolution and with the advances of supramolecular chemistry, it becomes necessary to extend to large systems experimental charge-density study methods that are usually applied to small molecules. The latest developments in the refinement program MoPro (Molecular Properties), dedicated to the chargedensity refinement at (sub)atomic resolution of structures ranging from small molecules to biological macromolecules, are presented. MoPro uses the Hansen & Coppens [Acta Cryst. (1978), A34, 909-921] multipolar pseudo-atom model for the electron-density refinement. Alternative methods are also proposed, such as modelling bonding and lone-pair electron density by virtual spherical atoms. For proteins at atomic resolution, a charge-density database developed in the laboratory enables the transfer of multipolar parameters. The program allows complex refinement strategies to be written and has numerous restraints, constraints and analysis tools for use in the structure and electron-density analysis. New kappa and multipolar parameter restraints/constraints are also implemented and discussed. Furthermore, constraints on the electron density, such as local symmetry and atom equivalence, are easily defined. Some examples of applications, from small molecules to large unit cells (including the enzyme aldose reductase), are given in order to guide the MoPro user and to show the large field of applicability of this code.

Topological analysis of the experimental electron density &(r) in hydrogen-bonding regions has been carried out for a large number of organic compounds using different multipole models and techniques. Relevant systematic relationships between topological properties at the critical points and the usual geometric parameters are pointed out. Results involving X-ray data only and joint X-ray and neutron data, as well as special hydrogen bonding cases (symmetric, bifurcated, peptide bonds, etc.) are included and analysed in the same framework. A new classi®cation of hydrogen bonds using the positive curvature of the electron density at the critical point ! 3 r CP is proposed.

Slightly attractive: The attractive and anisotropic nature of the ClCl interaction in C(6)Cl(6) is experimentally demonstrated from an expansion of the electron density rho(r) around the chlorine nuclei. The interaction is explained in a model in which there is a bonding attraction involving electron-deficient (see picture, blue) and electron-rich (red) regions of adjacent Cl atoms.

Highly ordered and vertically oriented mesoporous silica films can be generated by electro-assisted self-assembly (EASA). The method involves the electrogeneration of hydroxide ions at an electrode surface immersed in an hydrolyzed sol solution (containing typically tetraethoxysilane, TEOS, and cetyltrimethylammonium bromide, CTAB) in order to catalyze polycondensation of the precursors and self-assembly of hexagonally packed one-dimensional channels that grow perpendicularly to the support. Vertically aligned mesostructures have been demonstrated by TEM imaging and by grazing incidence X-ray diffraction (GIXD), this latter technique enabling characterization of thin films directly on their underlying electrode surface. The influence of the electrosynthesis medium composition (precursor and surfactant concentrations, surfactant chain length) on the mesostructural order and film thickness has been thoroughly examined. It was shown that the highly ordered and oriented mesoporous silica films can be obtained over a wide composition of the starting sol (i.e., 10−200 mM CTAB and 50−350 mM TEOS) and that the lattice parameter can be moderately tuned by changing the chain length of the surfactant template. Thickness of these films can be accurately controlled by applying galvanostatic conditions and by varying the deposition time, which offer the versatility to be applied in the same way to electrodes of different nature without overpotential problems encountered in the potentiostatic mode. Thin mesoporous films are often covered with an additional byproduct made of particulate aggregates arising from bulk gelification at the electrode/solution interface. Getting aggregate-free thin films is possible by working in diluted solutions (i.e., [TEOS] < 125 mM and CTAB/TEOS ratio <0.32) and with a short deposition time (∼10 s). Voltammetric experiments carried out on these films deposited onto planar indium−tin-oxide electrodes, after template extraction, have revealed very sensitive responses to solution-phase redox probes as a result of fast mass transport from the external solution through the film to the electrode surface. Quantitative characterization of these mass transfer processes reveals that apparent diffusion coefficients as high as about 1 × 10−7 cm2 s−1 can be reached but great care should be taken in defining the film synthesis conditions that may lead to some additional limiting effects.

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Crystallography at subatomic resolution permits the observation and measurement of the non-spherical character of the atomic electron density. Charge density studies are being performed on molecules of increasing size. The MOPRO least-squares re®nement software has thus been developed, by extensive modi®cations of the program MOLLY, for protein and supramolecular chemistry applications. The computation times are long because of the large number of re¯ections and the complexity of the multipolar model of the atomic electron density; the structure factor and derivative calculations have thus been parallelized. Stereochemical and dynamical restraints as well as the conjugate gradient algorithm have been implemented. A large number of the normal matrix off-diagonal terms turn out to be very small and the block diagonal approximation is thus particularly ef®cient in the case of large structures at very high resolution.

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