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Certain flexible molecules with important functions in biology are hard to crystallize. Two such molecules are ATP, the carrier of energy to those cellular processes that require the input of energy, and NAD+, the coenzyme in certain reactions e.g. those of dehydrogenases. These two molecules have flexible pyrophosphate linkages. Such structures, not determined to very high resolution, are hard to study because the crystal quality is generally poor and the data may present difficulties in structure solution. However the general conformation of the molecule and its packing with metal ions are found. Since the molecule is flexible it is presumed that one of several conformers with similar energies has been studied in a particular crystal structure determination. ATP was studied as a sodium salt and NAD+ as a lithium salt. The resolution of each structure determination was about 1 Å. The coenzyme NAD+ was found in an `extended' form as seen in enzyme complexes, rather than in the `folded' form.
1. ATP. Kennard, O., Isaacs, N. W., Coppola, J. C., Kirby, A. J., Warren, S., Motherwell, W. D. S., Watson, D. G., Wampler, D. L., Chenery, D. H., Larson, A. C., Kerr, K. A. and Riva di Sanseverino, L., Nature 225 (1977) 333.
2. NAD+. Saenger, W., Reddy, B. S., Muhlegger, K. and Weimann, G., Nature 267 (1977) 225.
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