Helices and Sheets in vacuo.

The structures and properties of unsolvated peptides large enough to possess secondary structure have been examined by experiments and simulations. Some of the factors that stabilize unsolvated helices and sheets have been identified. The charge, in particular, plays a critical role in stabilizing alpha-helices and destabilizing beta-sheets. Some helices are much more stable in vacuum than in aqueous solution. Factors like helix propensity, context, and the incorporation of specific stabilizing interactions have been examined. The helix propensities in vacuum differ from those found in solution. Studies of the hydration of unsolvated peptides can be performed one water molecule at a time. The first few water molecules only bind weakly to unsolvated peptides, and they bind much more strongly to some conformations than to others. The most favorable binding locations are not the protonation sites, but clefts or pockets where a water molecule can establish a network of hydrogen bonds. Non-covalent interactions between secondary structure elements leads to the formation of tertiary structure. Helical peptides assemble into complexes with a variety of intriguing structures. The intramolecular coupling of helices to make antiparallel coiled-coil geometries has also been investigated with model peptides.