AbstractPreferred conformation and types of molecular folding are some of the topics that can be addressed by structure analysis using x‐ray diffraction of single crystals. The conformations of small linear peptide molecules with 2–6 residues are affected by polarity of solvent, presence of water molecules, hydrogen bonding with neighboring molecules, and other packing forces. Larger peptides, both cyclic and linear, have manyintramolecularhydrogen bonds, the effect of which outweighs any intermolecular attractions. Numerous polymorphs of decapeptides grown from a variety of solvents, with different cocrystallized solvents, show a constant conformation for each peptide.Large conformational changes occur, however, upon complexation with metal ions. A new form of free valinomycin grown from DMSO exhibits near three‐fold symmetry with only three intramolecular hydrogen bonds. The peptide is in the form of a shallow bowl with a hydrophobic exterior. Near the bottom of the interior of the bowl are three carbonyl oxygens, spaced and directed so that they are in position to form three ligands to a K+, e.g., complexation can be completed by the three lobes containing the β‐bends closing over and encapsulating the K+ion. In another example, free antamanide and the biologically inactive perhydro analogue, in which four phenyl groups become cyclic hexyl groups, have essentially the same folding of backbone and side chains. The conformation changes drastically upon complexation with Li+or Na+. However, the metal ion complex of natural antamanide has a hydrophobic globlar form whereas the metal ion complex of the inactive perhydro analogue has a polar band around the middle. The structure results indicate that the antamanide molecule is in a complexed form during its biological activity.Single crystal x‐ray diffraction structure analyses have identified the manner in which water molecules are essential to creating minipolar areas on apolar helices. Completely apolar peptides, such as membrane‐active peptides, can acquire amphiphilic character byinsertion of a water moleculeinto the helical backbone of Boc‐Aib‐Ala‐Leu‐Aib‐Ala‐Leu‐Aib‐Ala‐Leu‐Aib‐OMe, for example. The C‐terminal half assumes an α‐helix conformation, whereas the N‐terminal half is distorted by an insertion of a water molecule W(1) between N(Ala5) and O(Ala2), forming and hydrogen bonds N(5)H ⃛ W(1) and W(1) ⃛ O(2). The distortion of the helix exposes CO (Aib1) and CO (Aib4) to the outside environment with the consequence of attracting additional water molecules. The leucyl side chains are on the other side of the molecule. Thus a helix with