Crystals and in Solution

We will first treat ionic crystals formed by positive ions from the first (alkali) and second (alkaline earth) group in the periodic table, together with negative ions from the sixth (chalcogen) group and seventh (halogen) group in the periodic table. 

To ionic crystals also belong the transition metal compounds, where the negative ions come from group 6 or 7, and the transition metal is from periods 4-6, where the d subshells are being filled. Transition metal ions are often involved in electron transfer reactions. 

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The electronic spectrum of the complex consists of a combination of the spectra of the parent compounds plus one or more higher wavelength transitions, responsible for the colour. Charge transfer is promoted by a low ionization energy of the donor and high electron affinity of the acceptor. A potential barrier to charge transfer of Va = Id — Ea is predicted. The width of the barrier is related to the intermolecular distance. Since the same colour develops in the crystal and in solution a single donor-acceptor pair should be adequate to model the interaction. A simple potential box with the shape... 

Amino acids are represented in two ways either as H2N-CHR-COOH or as the zwitterionic form H3N-CHR-COO . Although the second of these forms is overwhelmingly predominant in the crystal and in solution, it is generally more convenient to name them and their derivatives from the first form. They are normal organic compounds and are treated as such as far as numbering and naming are concerned, although trivial names are retained for all natural amino acids. 

MNDO calculation of AH for 3-amino-1,2,4-dithiazole-5-thione indicated that, of its possible isomeric forms (7), (8), and (9) (R = H), (7) is the most favorable. This structure had been found experimentally in the crystal and in solution but the differences are small and structures (8) and (9) may also take part in reactions. In fact, methylation of (7 R = H) gives (9 R = Me) shown by MNDO calculation to possess the lowest AHf <82JCR(S)65>. The MNDO method was used to calculate the structure and energy of 3-amino-1,2,4-dithiazole-5-one, its molecular cation and also its fragmentation ions <82MI 4i3-oi>. 

The conformation that a furanoid compound will adopt in the crystal is not readily predicted. A furanoid compound can adopt one of four envelope conformations in which C-2 or C-3 is endo or exo, namely, 2E, E2, 3E, and E3 less commonly, it may adopt a twist conformation. The energy difference between the four common envelope conformations may be so slight that other effects predominate in determining the conformation that the compound will adopt, both in the crystal and in solution. Such effects include those caused by hydrogen bonding, solvation, and close contacts with an aglycon. 

Cytochrome c can exist in two forms, oxidized (ferric form) or reduced (ferrous form). After careful refinement (and removal of errors) both X-ray39 and nmr38 data show that in crystals and in solution there is only a very small conformation change on oxidation state change. In fact, the change of the internal conformation of the protein is far too small for this to be used as a through-bond trigger or relay for electron transport. 

The change in conformation seen here (both in the crystal and in solution) may not be due to the zinc so much as to the anion. The X-ray study showed that the change in zinc coordination took place with uptake of an anion in a very hydrophobic pocket. This is consistent with the binding strength order observed SCbT > I" > CD > SO4". In the solution studies two thiocyanate anions are required for the overall change. Apart from the dependence on cation and anion concentration, the equilibrium position between the conformations in solution is temperature and pH dependent. 

The structure of hexahydro-l,2,4,5-tetrazines has been studied both by X-ray crystallography and by NMR and photoelectron spectroscopy. The results obtained by the different methods agree very well. All monocyclic hexahydro-l,2,4,5-tetrazines are chair-shaped in the crystal and in solution. 

When these drastic measures are required to yield good crystals, the crys-tallographer is faced with the question of whether the resulting fragment is worthy of the arduous effort to determine its structure. This question is similar to the basic issue of whether a protein has the same structure in crystal and in solution, and the question must be answered in the same way. Specifically, it may be possible to demonstrate that the fragment maintains at least part of the biological function of the intact molecule, and further, that this function is retained after crystallization. 

The fact that only naked molecules are refined is based on the problem that for crystal lattices at least 27 unit cells would have to be included (with at least one unit per cell, including counter ions and solvents of crystallization), and in solution at least 200 molecules of water must be refined in the solvent sheath interacting with the compound to be modeled. Since CPU time f(m2), where m is the number of nuclei, the time required for a single optimization cycle increases dramatically under these conditions. Even more importantly, the initial configuration of the molecule and its environment is not easy to predict since the intermolecular contacts (crystal lattice, ion-pairing and solvation) of a compound to be modeled are not known beforehand. Thus, inclusion of environmental effects in modeling studies has necessitated the use of some severe approximations176-781. 

In 5-aminoalkyltetrazoles, an intramolecular proton transfer is possible from the NH-tetrazole fragment to the amino group. For instance, 5-(2,2-dimethylaminoethyl)tetrazole 148 is known to exist both in crystals and in solution predominantly as a zwitterion 149 (Equation 9) <1998RJO870>. 

The present chapter reviews the structural chemistry (Section 2) and properties and applications (Section 3) of polyoxometalates that incorporate one or more rare-earth elements. In most cases these are discrete anionic entities within the crystal and in solution, but there are also extended lattices in which POM groups are linked by rare-earth cations. Solids which can best be described as mixed oxides, or which appear to be salts of common polyoxometalate architectures such as the... 

Bentley GA, Delepierre M, Dobson CM, Mason SA, Poulsen FM, Wedin RE (1983) Exchange of individual hydrogens of a protein in a crystal and in solution. J Mol Biol 170 243 -247... 

The manganese(II) cations are derived from manganese(II) oxide. They form colourless salts, though if the compound contains water of crystallization, and in solutions, they are slightly pink this is due to the presence of the hexaquo-manganate(II) ion, [Mn(H20)6]2+. 

In contrast to the flexible C-terminal region, the highly conserved fingerprint residues are located at the N-terminus of helix E and include a disulfide bond that locks the conformation of AB loop and helix E. Residues on this end of the helix bundle are more flexible than the central core but adopt well-defined structures in the crystal and in solution. Thus, structural differences on this end of the bundle are thought to influence the different receptor binding and biological properties of IFN-a and 1EN-/3. 

Cu2+, and a large number of salts of various anions, many of which are water soluble, exist in addition to a wealth of complexes. The aqua ion [Cu(H20)6]2+ is tetragonally distorted both in crystals and in solution. 

The anaesthetic steroid 3a-hydroxy-5a-pregnane-ll,20-dione has normal conformational features, both in the crystal and in solution. X-Ray data show that deoxycholic acid can form an inclusion complex in which alternate molecules of dimethyl sulphoxide and water are held in canals formed by helically arranged host molecules.Six different crystalline forms of 17a-ethynyloestradiol have been recognized. X-Ray structural data are reported for 3-methoxy-2-aza-oestra-l,3,5(10)-trien-17/3-yl acetate, 3/3-hydroxypregn-5-en-20-one (pregnenolone), 5a-cholest-2-ene, 3/3-bromo- and 3/3-chloro-cholest-5-enes, and cholesteryl acetate (at 123 K), benzoate, chloroformate, ° laurate, methyl carbonate, and 24-norcholesteryl acetate. ... 

The proposed schemes for the synthesis of macrobicyclic tris-dioximates have most readily been realized in high yields for alicyclic dioximes, having a cis-conformation both in crystals and in solutions. The change of the acyclic dioxime conformation from trans to cis during complexation decreases the stability of the compounds formed. 

Since sarcophaginates and sepulchrates are relatively easy to crystallize, a great number of these compounds are studied by X-ray crystallography, which together with molecular geometry calculations makes it possible to establish their three-dimensional structures both in crystal and in solution. The optical activity of such clathrochelates enables one routinely to utilize circular dichroism measurements to investigate their structure. The spatial and electronic structures of sarcophaginates and sepulchrates are much more seldomly determined by alternative spectral techniques compared with clathrochelates of other types. 

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