FOLDING AND AGGREGATION OF HETERODIMERS OF GRAMICIDIN. W. L. Duax, B. Burkhart, D. Langs and W. Pangborn, Hauptman-Woodward Medical Research Inst., 73 High St., Buffalo, NY 14203-1196 USA

Full-matrix refinement of the three-dimensional structures of two crystal forms of wild type gramicidin, a D,L-pentadecapeptide, reveal the presence of heterodimers. Partially occupied tyrosine residues are found at position eleven on only one strand of the antiparallel double helix. The approximate ratio of 11-tyrosine to 11-tryptophan in the heterodimer agrees with typical estimates for the ratio of gramicidin C to gramicidin A in wild type gramicidin. The environments of the 11-substituent in the two crystal forms are distinctly different and include specific interactions with solvent. In the orthorhombic form, which crystallized from ethanol, a network of hydrogen bonds link the tyrosine in one double helix with the backbone of an adjacent helix through an ethanol molecule and a water molecule. In the monoclinic form there is no comparable system linking helices.

The presence of a heterodimer in crystal forms having significantly different crystal packing suggests that heterodimer formation is a property of the gramicidin and not induced by crystal formation. In our hands, efforts to crystallize pure gramicidin A have invariably failed to produce sizable crystals and crystals prepared from wild type gramicidin do not readily redissolve upon addition of more solvent. The heterodimer appears to be the most stable form of gramicidin and is critical to crystal nucleation. Dimers of gramicidin observed in the solid state are composed of two antiparallel [[beta]]-strands wrapped into a cylindrical tube. Although most of the amino acids in gramicidin A show a statistical preference for [[beta]] conformation, that preference in tyrosine is highest and significantly greater than that of tryptophan. Apparently as a result of the tight coiling, the [[psi]],[[phi]] values of the L residues in the structures are in a sparsely populated region of the Ramachandran plot, while the D-residues are in the most densely populated region corresponding to [[beta]]-sheet geometry. The properties of the gramicidin heterodimer may relate to analogous properties of prion. The enigmatic behavior of prions, the protein responsible for diseases such as scrapie in animals and kuru and Creutzfeldt-Jakob disease in humans, has been attributed to the presence of trace amounts of mutants that induce heterodimer formation or another type of aggregation.

Supported by NIH grant GM32812