The structure of cellulose microfibrils in wood is not known in detail, despite the abundance of cellulose in woody biomass and its importance for biology, energy, and engineering. The structure of the microfibrils of spruce wood cellulose was investigated using a range of spectroscopic methods coupled to small-angle neutron and wide-angle X-ray scattering. The scattering data were consistent with 24-chain microfibrils and favored a "rectangular" model with both hydrophobic and hydrophilic surfaces exposed. Disorder in chain packing and hydrogen bonding was shown to increase outwards from the microfibril center. The extent of disorder blurred the distinction between the I alpha and I beta allomorphs. Chains at the surface were distinct in conformation, with high levels of conformational disorder at C-6, less intramolecular hydrogen bonding and more outward-directed hydrogen bonding. Axial disorder could be explained in terms of twisting of the microfibrils, with implications for their biosynthesis.crystallinity | infrared | deuterium exchange | nuclear magnetic resonance | spin diffusion

In the primary walls of growing plant cells, the glucose polymer cellulose is assembled into long microfibrils a few nanometers in diameter. The rigidity and orientation of these microfibrils control cell expansion; therefore, cellulose synthesis is a key factor in the growth and morphogenesis of plants. Celery (Apium graveolens) collenchyma is a useful model system for the study of primary wall microfibril structure because its microfibrils are oriented with unusual uniformity, facilitating spectroscopic and diffraction experiments. Using a combination of x-ray and neutron scattering methods with vibrational and nuclear magnetic resonance spectroscopy, we show that celery collenchyma microfibrils were 2.9 to 3.0 nm in mean diameter, with a most probable structure containing 24 chains in cross section, arranged in eight hydrogen-bonded sheets of three chains, with extensive disorder in lateral packing, conformation, and hydrogen bonding. A similar 18-chain structure, and 24-chain structures of different shape, fitted the data less well. Conformational disorder was largely restricted to the surface chains, but disorder in chain packing was not. That is, in position and orientation, the surface chains conformed to the disordered lattice constituting the core of each microfibril. There was evidence that adjacent microfibrils were noncovalently aggregated together over part of their length, suggesting that the need to disrupt these aggregates might be a constraining factor in growth and in the hydrolysis of cellulose for biofuel production.Growth and form in plants are controlled by the precisely oriented expansion of the walls of individual cells. The driving force for cell expansion is osmotic, but the rate and direction of expansion are controlled by the mechanical properties of the cell wall (Szymanski and

Cellulose is the most familiar and most abundant strong biopolymer, but the reasons for its outstanding mechanical performance are not well understood. Each glucose unit in a cellulose chain is joined to the next by a covalent C–O–C linkage flanked by two hydrogen bonds. This geometry suggests some form of cooperativity between covalent and hydrogen bonding. Using infrared spectroscopy and X-ray diffraction, we show that mechanical tension straightens out the zigzag conformation of the cellulose chain, with each glucose unit pivoting around a fulcrum at either end. Straightening the chain leads to a small increase in its length and is resisted by one of the flanking hydrogen bonds. This constitutes a simple form of molecular leverage with the covalent structure providing the fulcrum and gives the hydrogen bond an unexpectedly amplified effect on the tensile stiffness of the chain. The principle of molecular leverage can be directly applied to certain other carbohydrate polymers, including the animal polysaccharide chitin. Related but more complex effects are possible in some proteins and nucleic acids. The stiffening of cellulose by this mechanism is, however, in complete contrast to the way in which hydrogen bonding provides toughness combined with extensibility in protein materials like spider silk.

C o-crystallization is an increasingly promising approach to accessing important solid forms of, for example, pharmaceutical materials. The work reported here represents a breakthrough in using such a technique-the presence of a coforming molecule-to generate an industrially significant solid form of an important pharmaceutical molecule with favorable physical properties for its formulation and delivery ( Figure 1). The selectivity and yield offered by this multicomponent route is remarkable and makes production of this desirable form routinely accessible for the first time. The range of related cocomponents capable of accessing the desired form of paracetamol is unprecedented, allowing the optimization of both comolecule and solvent in a systematic manner using a library of compounds to generate 100% yields. The generalization of this approach offers enormous potential for the facile production of similarly challenging but desirable solid forms.p-Hydroxyacetanilide (paracetamol) is an important bioactive compound and active pharmaceutical ingredient (API), previously studied intensively in the crystalline state. 1À7 The metastable orthorhombic form II polymorph has been shown to be more soluble 1 and more readily compressible into tablets than form I, due to the layered nature of its packing in the solidstate. These physical characteristics make it far more attractive in formulation. 2 However, existing routes to selective crystallization of this phase are complex, and solid form II products from these tend to convert into form I over time; 7 form II is a metastable polymorph. The favorable physical properties of form II make it desirable to find a route to selective crystallization of this polymorph that would lend itself to production on an industrial scale.We present a method for selectively controlling polymorph growth utilizing cocrystallization methodology. By introducing a second component into the crystallization environment in addition to the solvent, it is possible to selectively grow paracetamol form II under ambient conditions and to 100% yields ( Figure 2). Furthermore, the crystals obtained are stable for periods of greater than one year. The critical importance of generating form II at 100% is emphasized by the fact that the presence of small quantities of form I in the final product encourages the conversion of the metastable form II into the more stable form I over a period of weeks to months. The simplicity of the method and the use of ambient conditions make this a highly desirable route to the selective growth of paracetamol form II with greater potential for scale-up to industrial processes than the methods identified previously. The technique also has more general applicability in accessing elusive but valuable solid forms of active molecular ingredients.It is relatively common to use additives to encourage polymorph selection, 4,8 where it is possible to engineer the polymorph obtained by considering the faces of the seeding additives. 9 Co-crystallization, however, where the relative quan...

Selective, robust and cost-effective chemical sensors for detecting small volatile-organic compounds (VOCs) have widespread applications in industry, healthcare and environmental monitoring. Here we design a Pt(II) pincer-type material with selective absorptive and emissive responses to methanol and water. The yellow anhydrous form converts reversibly on a subsecond timescale to a red hydrate in the presence of parts-per-thousand levels of atmospheric water vapour. Exposure to methanol induces a similarly-rapid and reversible colour change to a blue methanol solvate. Stable smart coatings on glass demonstrate robust switching over 104 cycles, and flexible microporous polymer membranes incorporating microcrystals of the complex show identical vapochromic behaviour. The rapid vapochromic response can be rationalised from the crystal structure, and in combination with quantum-chemical modelling, we provide a complete microscopic picture of the switching mechanism. We discuss how this multiscale design approach can be used to obtain new compounds with tailored VOC selectivity and spectral responses.