Host packing

Solid state host-guest compounds or clathrates are examples of co-crystals where the host packs in such a way as to leave a gap in the crystal lattice suitable for inclusion of the guest. 

T. Jacobs, G.O. Lloyd, M.W. Bredenkamp, L.J. Barbour, Breaking the trigonal host packing motif of Dianin s compound, CrystEngComm 11 (2009) 1545-1548. 

Clathrates, or inclusion components (host), pack the molecules of second component (guest) into lattice, generating a definite crystalline stmcture. The most important aspect of polymerization in clathrates deals with possible topochemical control. 

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The major stmctural feature of the HAz chain (blue in Figure 5.20) is a hairpin loop of two a helices packed together. The second a helix is 50 amino acids long and reaches back 76 A toward the membrane. At the bottom of the stem there is a i sheet of five antiparallel strands. The central i strand is from HAi, and this is flanked on both sides by hairpin loops from HAz. About 20 residues at the amino terminal end of HAz are associated with the activity by which the vims penetrates the host cell membrane to initiate infection. This region, which is quite hydrophobic, is called the fusion peptide. 

These carbides, also known as interstitial carbides, are crystalline compounds of a host metal and carbon. The host-metal atoms are generally arranged in a close-packed structure and the carbon occupies specific interstitial sites in that structure. Such a structure sets size restrictions on the two elements in order for the carbon atom to fit into the available sites and the population of these sites (if all are occupied) determines the stoichiometry of the carbide. 

This entry to supramolecular chemistry allows one, at the same time, to develop a host-guest chemistry because the space inside the cucurbituril barrel is sufficient to confine small "guesf molecules. This is illustrated with the crystallization of the supramolecular adduct [W3S4(H20)8Cl](pyH ccuc) Cl4a 5.5 H2O with a pyridinium cation inside the cucurbituril cavity [48]. The introduction of guest molecules may vary the net charge on the assembly and consequently the whole packing in the solid state. 

In selected cases, the effect of solvation on the crystalline structure formed is, however, considerably more pronounced. For example, the observed packing in the crystal of 2,4,6-tris( 1,3-propylenediamine-N,N -)cyclotriphosphazene (4) dihydrate (Fig. 6) is due to strong intermolecular hydrogen bonds between molecules of water and suitable couples of N-H groups on the host moiety M). The HzO species form also continuous H-bonded layers of solvation around the cyclophosphazene derivatives, thus stabilizing the crystal lattice. 

Crystallization of 5 in the open air from an initially aprotic solvent (N,N-dimethyl-acetamide) led to a non-layered structure which is characterized by a three-dimensional lattice of loosely-packed host species interspaced by channel-type zones accommodating the solvent guest components (Fig. 9). 

Fig. 32a and b. Schematic illustration of intermolecular arrangements in the crystalline complexes of host 25 (taken from Ref.25>) a the two-dimensional hydrogen bonding pattern parallel to the ab plane (the shaded area represents the 1,1-diphenylcyclohexane framework) b the van der Waals type packing of the hydrogen bonded layers along the c axis (R represents the cyclohexyl ends of the host species)... 

A substitution of the methyl group in the para-position (Fig. 36) causes a larger expansion of the unit cell along c. The crystal packing of the resulting structure is still an efficient one, however the p-substituted methyl introduces already some steric hindrance into the structure the shortest distance of the methyl from its surroundings (adjacent phenyl walls ) is 3.56 A as compared to the normal van der Waals methyl — — phenyl distance of about 3.7 A49>. This steric misfit could be responsible for the preferential complexation of host 25 with m-cresol rather than with p-cresol. 

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