ZIF-8 is a zeolitic imidazole-based metal-organic framework with large cavities interconnected by narrow windows. Because the small size of the windows, it allows in principle for molecular sieving of gases such as H(2) and CH(4). However, the unexpected adsorption of large molecules on ZIF-8 suggests the existence of structural flexibility. ZIF-8 flexibility is explored in this work combining different experimental techniques with molecular simulation. We show that the ZIF-8 structure is modified by gas adsorption uptake in the same way as it is at a very high pressure (i.e., 14,700 bar) due to a swing effect in the imidazolate linkers, giving access to the porosity. Tuning the flexibility, and so the opening of the small windows, has a further impact on the design of advanced molecular sieving membrane materials for gas separation, adjusting the access of fluids to the porous network.

The discovery in the late 1990s of ordered, high surface area silicas with pore sizes of 5 nm and above opened the way to the study of well-defined biomolecule-mesoporous silica hybrids. In particular, it has been possible to immobilize a range of small to medium size enzymes, such as proteases, lipases and peroxidases, via physisorption, encapsulation and tethering on the internal surfaces of the solids. Use has also been made of silicas functionalized for this purpose. In many cases the immobilized enzymes are both active and re-usable. Here we review the studies on enzymes immobilized on ordered mesoporous solids and assess the need for careful studies in real applications. Furthermore, we note the emerging applications of related biomolecule-mesoporous solid hybrids in other applications, such as intracellular drug delivery and transfection technology.

A series of univalent cation forms of zeolite Rho (M(9.8)Al(9.8)Si(38.2)O(96), M = H, Li, Na, K, NH(4), Cs) and ultrastabilized zeolite Rho (US-Rho) have been prepared. Their CO(2) adsorption behavior has been measured at 298 K and up to 1 bar and related to the structures of the dehydrated forms determined by Rietveld refinement and, for H-Rho and US-Rho, by solid state NMR. Additionally, CO(2) adsorption properties of the H-form of the silicoalumino-phosphate with the RHO topology and univalent cation forms of the zeolite ZK-5 were measured for comparison. The highest uptakes at 0.1 bar, 298 K for both Rho and ZK-5 were obtained on the Li-forms (Li-Rho, 3.4 mmol g(-1); Li-ZK-5, 4.7 mmol g(-1)). H- and US-Rho had relatively low uptakes under these conditions: extra-framework Al species do not interact strongly with CO(2). Forms of zeolite Rho in which cations occupy window sites between α-cages show hysteresis in their CO(2) isotherms, the magnitude of which (Na(+),NH(4)(+) < K(+) < Cs(+)) correlates with the tendency for cations to occupy double eight-membered ring sites rather than single eight-membered ring sites. Hysteresis is not observed for zeolites where cations do not occupy the intercage windows. In situ synchrotron X-ray diffraction of the CO(2) adsorption on Na-Rho at 298 K identifies the adsorption sites. The framework structure of Na-Rho "breathes" as CO(2) is adsorbed and desorbed and its desorption kinetics from Na-Rho at 308 K have been quantified by the Zero Length Column chromatographic technique. Na-Rho shows much higher CO(2)/C(2)H(6) selectivity than Na-ZK-5, as determined by single component adsorption, indicating that whereas CO(2) can diffuse readily through windows containing Na(+) cations, ethane cannot.

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The prediction and synthesis of new crystal structures enable the targeted preparation of materials with desired properties. Among porous solids, this has been achieved for metal-organic frameworks, but not for the more widely applicable zeolites, where new materials are usually discovered using exploratory synthesis. Although millions of hypothetical zeolite structures have been proposed, not enough is known about their synthesis mechanism to allow any given structure to be prepared. Here we present an approach that combines structure solution with structure prediction, and inspires the targeted synthesis of new super-complex zeolites. We used electron diffraction to identify a family of related structures and to discover the structural 'coding' within them. This allowed us to determine the complex, and previously unknown, structure of zeolite ZSM-25 (ref. 8), which has the largest unit-cell volume of all known zeolites (91,554 cubic ångströms) and demonstrates selective CO2 adsorption. By extending our method, we were able to predict other members of a family of increasingly complex, but structurally related, zeolites and to synthesize two more-complex zeolites in the family, PST-20 and PST-25, with much larger cell volumes (166,988 and 275,178 cubic ångströms, respectively) and similar selective adsorption properties. Members of this family have the same symmetry, but an expanding unit cell, and are related by hitherto unrecognized structural principles; we call these family members embedded isoreticular zeolite structures.

1 99311 nterestingy, the effect of betaine may have been just the opposte, that is, to enhance the difference between the forces requ~red to Induce the trans~t~on in AT-arid CG-r~ch regions 24 The f~tted Kuhn segment length of 15 A rnpes a persstence length of 7.5 A for ssDNA Ths vdue s about half that estrnated by E K Achter end G. Fesenfed iB~ooolyt-ies 10. 1625 (1 971)] for apurineted ssDNA by us~ng g h t scattering ancl sedilnentat~on I: was assumed n that study that 1 M NaCl s a theta solvent for ssDNA. In the present study, the contrectie force on e ssDNA molecule n 1 M NaCl. extrepoated to zero extenson, was about 5 pN (see Fig. 6, nset blue). Ths force offset probably Indicates secondat? st1 ucture forlnation or condensailon with~n the --~olecule If such structure forlned n the sednentation studes, then an erroneously large value for the r~gidty of ssDNA end for RNA could hzve been obtzned 25 In the presence of adenosne trphosphate (ATP) or ATPiylS. RecA ~lndergoes an aloster~c change n t o a high-affnty form that bnds dsDNA cooperatvey n a stocholnetrc rato of 1 RecN3 bp of dsDNA to form a r~ght-handed h e c a falnent [S C \!Vest, An!iii Rev B I O C~J~~ 61. 603 (1 992)] There are six RecA molec~~les and 18 6 bp;turn of the DNA molecule that s overstretched by a factor of 1 5 t~mes its B-form contour length 26. A Klug and F H C Cr~ck [h'atatlii -e 255. 530 (1975)l have suqgtisted that formaton of a few highly bent regons or "knks" n DNA mght be energetcaly fawarable relative to srnooth bendng o. er a longer DNA length. The argument requres that after the ensung of a o c a z e d knk n the DNA molec~lle, the energy re3,u red to bend ;he DNA further by an angle 8 at that locat~on, be smaller than the energy needed to bend the DNA by the same angle before the ensuing of the knk Ths s ndeed obsetved n macroscopic eastc medawhen the deformaton goes beyond the eastc nto the "plastc" regme (a plastc straw s a good example) 27 Carboxyate-polystyrene beads (3 56 ILm n dameter. CV = 2.7'0, Spherotech) were covelently coated vlith streptav~d~n uslng l-ethyl-3-(3-d1methylam1nopropy) carbodmde (EDAC) Each molecule was puled both r~ght and left from the p~pette to determ n e the pont of attachment of the molecule on the ppette bead. Because the o p t c a y trapped bead can rotate freely, but the ppette-trapped bead cannot. Ex can be deterlnned n abso ute un~ts jmlcrometers), In each F-Ex cuwe, data representng the folowng four processes s ~~~p e r i m p o s e d : extendng the molecule to right of the ppette, then relaxing t froln the right, extendng t leftward, then relaxng t from the left. Each data point was taken after a -0.5 ILm change n extension and a 2-s waitn g perod. The force sgna was then averaged for an add~t~ona 2 s and recorded A complete r~ghr-left stretch cycle took about 10 m n . 28 A video showing actual bead-DNA-bead assembly IFlg. 1 B) can be viewed on the Word W!de Web at 29. D W! . Ussey. R \VJ Hoepfner, R R Sinden Met!i-0dS E~z J /~o / 21 2. 262 (1 992) 30.The nat...

ZIF-8, a prototypical zeolitic porous coordination polymer, prepared via the self-assembly of tetrahedral atoms (e.g. Zn and Co) and organic imidazolate linkers, presents large cavities which are interconnected by narrow windows that allow, in principle, molecular sieving. However, ZIF-8 shows flexibility due to the swing of the imidazolate linkers, which results in the adsorption of molecules which are too large to fit through the narrow window. In this work, we assess the impact of this flexibility, previously only observed for nitrogen, and the level of agreement between the experimental and simulated isotherms of different energy-related gases on ZIF-8 (CO(2), CH(4) and alkanes). We combine experimental gas adsorption with GCMC simulations, using generic and adjusted force fields and DFT calculations with the Grimme dispersion correction. By solely adapting the UFF force field to reduce the Lennard-Jones parameter ε, we achieve excellent agreement between the simulated and experimental results not only for ZIF-8 but also for ZIF-20, where the transferability of the adapted force field is successfully tested. Regarding ZIF-8, we show that two different structural configurations are needed to properly describe the adsorption performance of this material, demonstrating that ZIF-8 is undergoing a structural change during gas adsorption. DFT calculations with the Grimme dispersion correction are consistent with the GCMC and experimental observations, illustrating the thermodynamics of the CH(4) adsorption sites and confirming the existence of a new adsorption site with a high binding energy within the 4-ring window of ZIF-8.

Imogolite-like nanotubes have been synthesised in which SiCH(3) groups have been introduced in place of the SiOH groups that naturally occur at the inner surface of imogolite, an alumino-silicate with formula (OH)(3)Al(2)O(3)SiOH, forming nanotubes with inner and outer diameter of 1.0 and 2.0 nm, respectively. The new nanotubular material, composition (OH)(3)Al(2)O(3)SiCH(3), has both larger pores and higher specific surface area than unmodified imogolite: it forms as hollow cylinders 3.0 nm wide and several microns long, with a specific surface area of ca. 800 m(2) g(-1) and intriguing surface properties, due to hydrophobic groups inside the nanotubes and hydrophilic Al(OH)Al groups at their outer surface. Adsorption of methane at 30 °C has been studied in the pressure range between 5 and 35 bar on both the new material and unmodified imogolite: it resulted that the new material adsorption capacity is about 2.5 times larger than that of imogolite, in agreement with both its larger pore volume and the presence of a methylated surface. On account of these properties and of its novelty, the studied material has several potential technical applications, e.g. in the fields of gas chromatography and gas separation.