From molecular level to macroscopic properties: A solid-state NMR biomineralization and biomimetic exploration

Through biomineralization, calcareous composites are produced with exceptional properties, evolution-optimized for specific function. The bioinspired quest to understand how properties are controlled and enhanced is motivated by their fundamental and technological significance. The incorporation of small molecules and/or biopolymers as inter- and intra-crystalline additives in the CaCO₃ matrix, is widely employed by organisms to achieve diverse functions. The interactions between the components during the early events within the precipitation medium, and when entrapped through precipitation-crystallization, are key players of process–property regulation. In addition to identifying the bulk matrices and the incorporated molecules, we show how solid-state NMR methods are tailored to directly report the chemical-structural details of the inorganic interface that surrounds an occlusion. Solid-state NMR is uniquely suited for that and is applicable to stable or spontaneously transforming lattices, crystalline or amorphous. Our findings are grouped to highlight the connection between the molecular level and tunability of macroscopic properties.     

From crystal to crystal: A reconstructive solid-state reaction

The crystal structure of the Lewis acid-base adduct of 2-chloro-2-boraindane and diethylether was determined by X-ray diffraction. After abstraction of the boron-coordinated Et₂O molecule in vacuo, the crystal and molecular structure of the solvent-free boraindane, earlier synthesized by Kaufmann and Schacht [1], was studied on the resulting crystalline material. The different crystal packing in both structures and a diffraction experiment on an oriented single crystal showed that the reaction is reconstructive and that a rearrangement of the molecules parallels the change in molecular geometry.     

A solid-state approach to SCF Xα molecular equations

A recent method to calculate molecular states using self-consistent pseudopotentials and a plane wave basis set is explored further. Results of calculations for equilibrium bond length, stretching force constant and molecular binding energy for Si₂ are compared with experiment.     

From a spin-crossover molecule to the bulk electronic behaviour of a material: the paramount role of hydrogen bonds in molecular electronics

The exact role of the hydrogen bond network in a spin-crossover material is unambiguously shown: changing the −N-H…O(nitro)salicylidene− ‘communication wire’ between adjacent Fe ions in the unidi-mensional chains of a two-step spin-transition material to the poorer −N-C-H…O(nitro)salicylidene− pathway results in a gradual and partial spin conversion of Fe^II in otherwise similar unidimensional chains of complex molecules. Cooperativity in the spin-crossover phenomenon indeed results from the effectiveness of the inter-molecular communication.     

From chemical topology to molecular machines

The chemistry of molecules displaying novel topologies has experienced an explosive development in the course of the last 25 years. The fast growth of this field originates to a large extent from the new templated synthetic methods which allow one to prepare these compounds at a real macroscopic level. Our group, in particular, has proposed a particularly efficient copper(I)-based template synthesis of a large variety of catenanes and rotaxanes at an early stage, participating in the revival of molecular topology. One of the highlights of the field has been the synthesis of the trefoil knot, a particularly challenging target. This object is not only an aesthetically attractive molecule but it also displays interesting properties in relation to coordination chemistry and chirality. A highly promising extension of molecular topology is that of molecular machines. By combining the specific properties of catenanes and rotaxanes, i.e., marked flexibility and propensity to undergo large amplitude motions, and coordination chemistry, it has been possible to elaborate and study a large variety of molecular machines. A recent example is that of an adjustable receptor, based on a [3]rotaxane attached to two mobile porphyrinic plates. This compound and related molecules will lead to “molecular presses” and, eventually, to molecular machines usable in solution to catalyse reactions or change the conformation of given substrates.     

Structural understanding of a molecular material that is accessed only by a solid-state desolvation process: the scope of modern powder X-ray diffraction techniques

Many molecular materials cannot be prepared as a "pure" (nonsolvate) crystalline phase by conventional crystal growth from solution due to the facile formation of solvate structures. In such cases, it may be possible to obtain the pure phase by a solid-state desolvation process, although such processes are generally associated with loss of crystal integrity, yielding a microcrystalline powder of the pure phase. This paper demonstrates the utility of modern powder X-ray diffraction techniques for obtaining structural understanding in such cases, focusing on a particular member of a structural family that is of wider relevance within the context of crystal engineering and design.     

From molecular dynamics to hydrodynamics: a novel Galilean invariant thermostat

This paper proposes a novel thermostat applicable to any particle-based dynamic simulation. Each pair of particles is thermostated either (with probability P) with a pairwise Lowe-Andersen thermostat [C. P. Lowe, Europhys. Lett. 47, 145 (1999)] or (with probability 1-P) with a thermostat that is introduced here, which is based on a pairwise interaction similar to the Nosé-Hoover thermostat. When the pairwise Nosé-Hoover thermostat dominates (low P), the liquid has a high diffusion coefficient and low viscosity, but when the Lowe-Andersen thermostat dominates, the diffusion coefficient is low and viscosity is high. This novel Nosé-Hoover-Lowe-Andersen thermostat is Galilean invariant and preserves both total linear and angular momentum of the system, due to the fact that the thermostatic forces between each pair of the particles are pairwise additive and central. We show by simulation that this thermostat also preserves hydrodynamics. For the (noninteracting) ideal gas at P = 0, the diffusion coefficient diverges and viscosity is zero, while for P > 0 it has a finite value. By adjusting probability P, the Schmidt number can be varied by orders of magnitude. The temperature deviation from the required value is at least an order of magnitude smaller than in dissipative particle dynamics (DPD), while the equilibrium properties of the system are very well reproduced. The thermostat is easy to implement and offers a computational efficiency better than (DPD), with better temperature control and greater flexibility in terms of adjusting the diffusion coefficient and viscosity of the simulated system. Applications of this thermostat include all standard molecular dynamic simulations of dense liquids and solids with any type of force field, as well as hydrodynamic simulation of multiphase systems with largely different bulk viscosities, including surface viscosity, and of dilute gases and plasmas.     

Synthesis and solid-state dynamics of molecular dirotors

We report the design, synthesis, thermal properties, and dynamic NMR characterization of four new molecular dirotors, each containing two rotary 1,4-diethynyl phenylene units, a bridging 1,3-bis(diphenylmethyl) benzene stator, and two capping triphenylmethyl groups. These compounds are variations of the well-known wheel and axle structures. Ambient temperature quadrupolar echo ²H NMR spectra of phenylene-deuterated samples revealed line shapes consistent with dynamic processes having exchange rates >10⁸ s⁻¹, and more than the two minima typically observed for most phenylene rotators. Reasonable line-shape simulations were obtained with a model that involves four minima related by angular displacements of 0°, 30°, 180°, and 230°. Cooling the samples to 260 K revealed spectra consistent with samples that contain two or more components. These results indicate that novel wheel and axle structures do create a certain amount of free volume that allows for the desired mobility of the rotating units.     

Split-pool method for synthesis of solid-state material combinatorial libraries

The synthesis and analysis of inorganic material combinatorial libraries by the split-pool bead method were demonstrated at the proof-of-concept level. Millimeter-size spherical beads of porous gamma-alumina, a commonly used support material for heterogeneous catalysts, were modified with Al(13)O(4)(OH)(24)(H(2)O)(12)(7+) cations in order to promote irreversible adsorption of the anionic fluorescent dyes Cascade Blue, Lucifer Yellow, and Sulforhodamine 101. The compositions of individual beads were easily determined through three split-pool cycles using a conventional fluorescence plate reader. Small split-pool material libraries were made by adsorbing noble metal salts (H(2)PtCl(6), H(2)IrCl(6), and RhCl(3)) into the beads. Analysis of these beads by micro-X-ray fluorescence showed that quantitative adsorption of metal salts without cross-contamination of beads could be achieved at levels (0.3 wt % metal loading) relevant to heterogeneous catalysis. The method offers the potential for synthesis of rather large libraries of inorganic materials through relatively simple benchtop split-pool chemistry.     

Concerted molecular displacements in a thermally-induced solid-state transformation in crystals of DL-norleucine

Martensitic transformations are of considerable technological importance, a particularly promising application being the possibility of using martensitic materials, possibly proteins, as tiny machines. For organic crystals, however, a molecular level understanding of such transformations is lacking. We have studied a martensitic-type transformation in crystals of the amino acid DL-norleucine using molecular dynamics simulation. The crystal structures of DL-norleucine comprise stacks of bilayers (formed as a result of strong hydrogen bonding) that translate relative to each other on transformation. The simulations reveal that the transformation occurs by concerted molecular displacements involving entire bilayers rather than on a molecule-by-molecule basis. These observations can be rationalized on the basis that at sufficiently high excess temperatures, the free energy barriers to concerted molecular displacements can be overcome by the available thermal energy. Furthermore, in displacive transformations, the molecular displacements can occur by the propagation of a displacement wave (akin to a kink in a carpet), which requires the molecules to overcome only a local barrier. Concerted molecular displacements are therefore considered to be a significant feature of all displacive transformations. This finding is expected to be of value toward developing strategies for controlling or modulating martensitic-type transformations.     

Direct structure determination of a multicomponent molecular crystal prepared by a solid-state grinding procedure

A three-component molecular cocrystal material has been prepared by a solvent-free route involving mechanical grinding of the pure phases of the individual components. This material is not accessible from conventional solution-state crystallization procedures. Due to the fact that the grinding procedure intrinsically leads to a microcrystalline powder sample, the use of powder X-ray diffraction data is essential for structure determination. This work emphasizes the scope and utility of ab initio structure solution directly from powder X-ray diffraction data for carrying out structural characterization of new materials prepared via the solid-state grinding route, leading to the opportunity to establish structure-property relationships for such materials.     

Copper-related diseases: From chemistry to molecular pathology

The aim of this review is to give a general view on the current status of the scientific basis for the role of copper in human health and disease, outlining the roles of copper in human metabolism and bioenergetics, its coordination chemistry as well as its biological ligands involved in the multiple steps of metal assimilation and distribution. Copper bioavailability depends on four main factors: (1) the absorption of copper from the gastrointestinal tract; (2) copper transport in the portal blood; (3) the extraction of copper by hepatocytes from the portal blood supply; (4) copper uptake by peripheral tissues and by the central nervous system. The most important copper pumps, providing a permeation pathway for copper ions in man, such as hCTR1, hCTR2, DMT1, ATP7A, ATP7B, and MURR1, are extensively discussed. Different theories on the putative role of ceruloplasmin in copper transport in blood are presented. Data on interactions among other trace elements and copper, from the level of enterocyte to other systems in the body, are also shown. Finally, the function of different drugs, chelators and non-chelating agents, utilized in clinical practice in the therapy of Wilson's disease is treated, in particular the “reductive chelation” of penicillamine is discussed.     

From small building blocks to complex molecular architecture

[reaction: see text] We describe a synthesis of a dendrimer-like amphiphile containing a flat rigid core and 12 hydrophobic and hydrophilic arms. We employ a modular approach based on stepwise protection chemistry starting from simple building blocks. The key feature of this approach is the absence of a polymerization step, which makes it applicable for linear monofunctionalized precursors of any kind. This strategy also allows for precise control of the number of arms and ensures their alternating arrangement.     

Paramagnetic pyrrole-based semiconductor molecular material

The electrochemical oxidation of hexa-N-pyrrolylbenzene in organic media leads, via intramolecular coupling of the pyrrole residues, to the deposition of a molecular semiconductor film on an electrode surface. In situ electron spin resonance–electrochemical experiments reveal that the semiconductor is endowed with both properties of conducting polymers (i.e., reversible oxidation) and polyaromatic molecular materials (i.e., highly paramagnetic). The material, which is easy to process as soft homogeneous thin film, shows a tunable 0 to 1 spin concentration per molecule at room temperature by controlling the electrochemical potential. Contribution to the Fall Meeting of the European Materials Research Society, Symposium D: 9th International Symposium on the Electrochemical–Chemical Reactivity of Metastable Materials, Warsaw, 17th–21st September, 2007. An erratum to this article can be found at