Abstract
An X-ray analysis of single crystals (from MeOH) of cyclo(-D-Leu-L-MeLeu-D-Leu-L-MeLeu-D-Leu-L-MeLeu-) has been carried out. The analysis reveals that the molecules of the cyclopeptide occur in the crystals with two slightly different, almost hexagonal backbone conformations of the β-type, and that pairs of molecules with the same conformation interact through their nonmethylated face, forming dimeric units (units A and B) with six interannular H-bonds. This kind of pairing reproduces well that expected for a two-ring element in a stack of antiparalleβ-rings. The X-ray analysis has also revealed the presence in the A units of two water molecules, each at one of two equivalent sites located on the 3-fold axis of the units and equidistant from the center of gravity, and the presence in the B units of one water molecule at the center of the units. This provides experimental support for the idea that stacks ofβ-rings can serve as molecular channels.
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Saviano, M., Zaccaro, L., Lombardi, A. et al. A structural two-ring version of a tubular stack ofβ-rings in crystals of a cyclic D,L-hexapeptide. J Incl Phenom Macrocycl Chem 18, 27–36 (1994). https://doi.org/10.1007/BF00706936
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DOI: https://doi.org/10.1007/BF00706936