A STRUCTURAL 2-RING VERSION OF A TUBULAR STACK OF BETA-RINGS IN CRYSTALS OF A CYCLIC D,L-HEXAPEPTIDE

An X-ray analysis of single crystals (from MeOH) of cyclo(-D-Leu-L-MeL eu-D-Leu-L-MeLeu-D-Leu-L-MeLeu-) has been carried out. The analysis re veals that the molecules of the cyclopeptide occur in the crystals wit h two slightly different, almost hexagonal backbone conformations of t he beta-type, and that pairs of molecules with the same conformation i nteract through their nonmethylated face, forming dimeric units (units A and B) with six interannular H-bonds. This kind of pairing reproduc es well that expected for a two-ring element in a stack of antiparalle l beta-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 l ocated 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 id ea that stacks of beta-rings can serve as molecular channels.