From molecules to crystal engineering: supramolecular isomerism and polymorphism in network solids.

The aim of this critical review is to provide a broad but digestible overview of mechanochemical synthesis, i.e. reactions conducted by grinding solid reactants together with no or minimal solvent. Although mechanochemistry has historically been a sideline approach to synthesis it may soon move into the mainstream because it is increasingly apparent that it can be practical, and even advantageous, and because of the opportunities it provides for developing more sustainable methods. Concentrating on recent advances, this article covers industrial aspects, inorganic materials, organic synthesis, cocrystallisation, pharmaceutical aspects, metal complexes (including metal–organic frameworks), supramolecular aspects and characterization methods. The historical development, mechanistic aspects, limitations and opportunities are also discussed (314 references).

/pdf/mechanochemistry-opportunities-for-new-and-cleaner-synthesis-15fcwynvnd.pdf

Mechanochemistry: opportunities for new and cleaner synthesis

In recent years there has been immense interest in studying gels derived from low molecular mass gelators (supramolecular, or simply molecular gels). The motivation for this is not only to understand the fundamental aggregate structures in the gels at different length scales, but also to explore their potential for futuristic technological applications. Gels have been made sensitive to external stimuli like light and chemical entities by incorporating a spectroscopically active or a receptor unit as part of the gelator molecule. This makes them suitable for applications such as sensing and actuating. The diversity of gel structural architectures has allowed them to be utilized as templates to prepare novel inorganic superstructures for possible applications in catalysis and separation. Gels derived from liquid crystals (anisotropy gels) that can act as dynamically functional materials have been prepared, for example, for (re-writable) information recording. Supramolecular gels can be important in controlled release applications, in oil recovery, for gelling cryogenic fuels etc. They can also serve as media for a range of applications. This tutorial review highlights some of the instructive work done by various groups to develop smart and functional gels, and covers a wide spectrum of scientific interest ranging from medicine to materials science.

/pdf/supramolecular-gels-functions-and-uses-51lemkjn44.pdf

Supramolecular gels: Functions and uses

Ca(2+) regulation of contraction in vertebrate striated muscle is exerted primarily through effects on the thin filament, which regulate strong cross-bridge binding to actin. Structural and biochemical studies suggest that the position of tropomyosin (Tm) and troponin (Tn) on the thin filament determines the interaction of myosin with the binding sites on actin. These binding sites can be characterized as blocked (unable to bind to cross bridges), closed (able to weakly bind cross bridges), or open (able to bind cross bridges so that they subsequently isomerize to become strongly bound and release ATP hydrolysis products). Flexibility of the Tm may allow variability in actin (A) affinity for myosin along the thin filament other than through a single 7 actin:1 tropomyosin:1 troponin (A(7)TmTn) regulatory unit. Tm position on the actin filament is regulated by the occupancy of NH-terminal Ca(2+) binding sites on TnC, conformational changes resulting from Ca(2+) binding, and changes in the interactions among Tn, Tm, and actin and as well as by strong S1 binding to actin. Ca(2+) binding to TnC enhances TnC-TnI interaction, weakens TnI attachment to its binding sites on 1-2 actins of the regulatory unit, increases Tm movement over the actin surface, and exposes myosin-binding sites on actin previously blocked by Tm. Adjacent Tm are coupled in their overlap regions where Tm movement is also controlled by interactions with TnT. TnT also interacts with TnC-TnI in a Ca(2+)-dependent manner. All these interactions may vary with the different protein isoforms. The movement of Tm over the actin surface increases the "open" probability of myosin binding sites on actins so that some are in the open configuration available for myosin binding and cross-bridge isomerization to strong binding, force-producing states. In skeletal muscle, strong binding of cycling cross bridges promotes additional Tm movement. This movement effectively stabilizes Tm in the open position and allows cooperative activation of additional actins in that and possibly neighboring A(7)TmTn regulatory units. The structural and biochemical findings support the physiological observations of steady-state and transient mechanical behavior. Physiological studies suggest the following. 1) Ca(2+) binding to Tn/Tm exposes sites on actin to which myosin can bind. 2) Ca(2+) regulates the strong binding of M.ADP.P(i) to actin, which precedes the production of force (and/or shortening) and release of hydrolysis products. 3) The initial rate of force development depends mostly on the extent of Ca(2+) activation of the thin filament and myosin kinetic properties but depends little on the initial force level. 4) A small number of strongly attached cross bridges within an A(7)TmTn regulatory unit can activate the actins in one unit and perhaps those in neighboring units. This results in additional myosin binding and isomerization to strongly bound states and force production. 5) The rates of the product release steps per se (as indicated by the unloaded shortening velocity) early in shortening are largely independent of the extent of thin filament activation ([Ca(2+)]) beyond a given baseline level. However, with a greater extent of shortening, the rates depend on the activation level. 6) The cooperativity between neighboring regulatory units contributes to the activation by strong cross bridges of steady-state force but does not affect the rate of force development. 7) Strongly attached, cycling cross bridges can delay relaxation in skeletal muscle in a cooperative manner. 8) Strongly attached and cycling cross bridges can enhance Ca(2+) binding to cardiac TnC, but influence skeletal TnC to a lesser extent. 9) Different Tn subunit isoforms can modulate the cross-bridge detachment rate as shown by studies with mutant regulatory proteins in myotubes and in in vitro motility assays. (ABSTRACT TRUNCATED)

https://www.physiology.org/doi/pdf/10.1152/physrev.2000.80.2.853

Regulation of Contraction in Striated Muscle

The nanoporous metal-organic framework Fe2(azpy)4(NCS)4.(guest) (azpy is trans-4,4'-azopyridine) displays reversible uptake and release of guest molecules and contains electronic switching centers that are sensitive to the nature of the sorbed guests. The switching of this material arises from the presence of iron(II) spin crossover centers within the framework lattice, the sorbed phases undergoing "half-spin" crossovers, and the desorbed phase showing no switching property. The interpenetrated framework structure displays a considerable flexibility with guest uptake and release, causing substantial changes in the local geometry of the iron(II) centers. The generation of a host lattice that interacts with exchangeable guest species in a switchable fashion has implications for the generation of previously undeveloped advanced materials with applications in areas such as molecular sensing.

https://science.sciencemag.org/content/sci/298/5599/1762.full.pdf

Guest-Dependent Spin Crossover in a Nanoporous Molecular Framework Material

Modern crystal engineering has emerged as a rich discipline whose success requires an iterative process of synthesis, crystallography, crystal structure analysis, and computational methods. By focusing on the molecular recognition events during nucleation and growth, chemists have uncovered new ways of controlling the internal structure and symmetry of crystals and of producing materials with useful chemical and physical properties.

https://science.sciencemag.org/content/sci/295/5564/2410.full.pdf

Crystal Engineering: from Structure to Function

FOR over 50 years urea inclusion compounds (UICs)-in which molecules are entrapped within the cavities of a hydrogen-bonded urea network—have intrigued chemists, crystallographers and spectroscopists1. Most previous work on these and other inclusion compounds has focused on intrachannel interactions and dynamics. We have now found that UICs can serve as useful models to probe long-range interactions and cooperative phenomena in crystals. Here we report the structure of 2,10-undecanedione/urea (1:9) (1/ urea), in which an extended hydrogen-bonded array connecting hosts and guests serves to distort the urea channel away from the hexagonal symmetry normally observed for UIC crystals. The distortion is large enough, and exhibits enough cooperativity, to generate macroscopic domains, but is small enough to allow facile domain reorientation under small compressive stresses. By incorporating a specific impurity (2-undecanone) into 1/urea, we can modify the domain size and optical properties, and make the domain reorientation spontaneously reversible when the uniaxial stress is released. All of these phenomena can be understood in terms of cooperative interactions between different channels of the hydrogen-bonded network formed by urea and its guest.

Stress-induced domain reorientation in urea inclusion compounds

Organic single crystals provide an ideal model for studying the factors that influence chemical processes in structured media. Reaction trajectories are well defined and reveal the influence of spontaneous mechanical stresses equivalent to tens of thousands of atmospheres. Analysis of molecular and crystal structures helps to explain both local mechanical properties, which influence reactions, and bulk properties such as melting point, compressibility, and surface energy.

https://science.sciencemag.org/content/sci/234/4778/830.full.pdf

Mechanical stress and reactivity in organic solids.

X-ray oscillation photographs reveal both diffuse and discrete scattering for alkanone guest molecules within inclusion compounds of 5-undecanonehrea grown from methanol. Guest molecules that give rise to diffuse scattering are ordered only along the channel axis, whereas those guest substructures that give rise to discrete scattering are ordered in three dimensions. Precession photographs and diffractometry show that within domains containing three dimensionally ordered guests, the offset (denoted δg) between the heights of guest molecules in adjacent channels is exactly zero. These guests therefore share overall hexagonal symmetry with the host, as do a large number of n-alkanones of varying chain length and symmetry properties. Although side by side ordering optimizes dipole-dipole interactions between alkanones in adjacent channels, the large inter channel distance (8.22A) and the lack of symmetry in 5-undecanone suggest that other factors could influence guest molecule ordering. A careful di...

An X-Ray Diffraction Study

A template-directed mechanism of crystal growth is demonstrated for urea inclusion compounds (UICs). For UICs containing n-alkanone or α+1,ω−1-alkanedione guests, x-ray diffraction revealed superstructure relations between host and guest repeats along the channel axis for guests containing 8 to 14 carbons. For a favorable structural match between host and guest, UICs typically grow as flat hexagonal plates, and atomic force microscopy of {001} surfaces revealed molecularly smooth terraces differing in height by multiples of the guest repeat. If the match is poor, protrusion of guests from the {001} surfaces nucleates growth along the channel axis to form hexagonal needles.

https://science.sciencemag.org/content/sci/273/5280/1355.full.pdf

Mark D. Hollingsworth

Papers

Crystal Engineering: from Structure to Function

Stress-induced domain reorientation in urea inclusion compounds

Mechanical stress and reactivity in organic solids.

An X-Ray Diffraction Study

Superstructure control in the crystal growth and ordering of urea inclusion compounds