MONOCLINIC UNCOMPLEXED DOUBLE-STRANDED, ANTIPARALLEL, LEFT-HANDED BETA-5.6-HELIX (UP-OR-DOWN-BETA-5.6) STRUCTURE OF GRAMICIDIN-A - ALTERNATE PATTERNS OF HELICAL ASSOCIATION AND DEFORMATION

A comparison of the monoclinic and orthorhombic crystal structures of the uncomplexed double-stranded, antiparallel, left-handed beta-helix (5.6 amino acid residues per turn) (up-or-down beta-5.6) conformers of gramicidin A reveals marked differences in the tryptophan side-chain orientations and the degree of helical uniformity of the dimer and in the manner in which these helical dimers associate with one another in the crystal. The helix of the orthorhombic dimer exhibits a regular pattern of bulges and constrictions that appears to be induced by crystal packing forces affecting tryptophan side chains that are aligned parallel to the helix axis. The monoclinic dimer is more uniform than the orthorhombic dimer as a consequence of pi-stacking interactions between dimers in which orientation of tryptophan side chains is normal to the helix axis to relieve the lateral crystal packing forces that may locally twist and deform the helix. It may be inferred from these observations that lipid interactions may be expected to destabilize the up-or-down beta-5.6 helix when it is inserted into a membrane bilayer.