Contents Other Important Ligands

Just as the auto mechanic sometimes has parts left over, electron-density maps occasionally show clear, empty density after all known contents of the crystal have been located. Apparent density can appear as an artifact of missing Fourier terms, but this density disappears when a more complete set of data is obtained. Among the possible explanations for density that is not artifactual are ions like phosphate and sulfate from the mother liquor reagents like mercaptoethanol, dithiothreitol, or detergents used in purification or crystallization or cofactors, inhibitors, allosteric effectors, or other small molecules that survived the protein purification. Later discovery of previously unknown but important ligands has sometimes resulted in subsequent interpretation of empty density. 

Many crystals grown from aqueous solution are hydrates (see Chapter 7). Dehydration often precedes loss of other ligands or decomposition of other constituent ions. If water loss is the reaction to be studied, the possibility that partial dehydration may have occurred during storage must be investigated. Information on the water content of the reactant and the importance of any intermediate steps in dehydration must be considered. The conditions during dehydration can influence the reactivity of the anhydrous salt, or the lower hydrate, obtained. The product formed during dehydration in vacuum is often amorphous or microcrystalline, while... 

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