Peptide is disclosed which comprises D-enantiomers of amino acids and is capable of interacting with other .beta.-strand structure to form .beta.-sheet, wherein said peptide is selectively N.alpha.-substituted in one edge (first) of the .beta.-strand-forming section of said peptide while the other edge (second) in the opposite orientation to the first edge in view of peptide backbone plane remains N.alpha.-unsubstituted. Such the N.alpha.-substituted peptide is capable of preventing association of said peptide with other .beta.-strand (target) but permits interaction of said peptide with target .beta.-strand in separate peptide-containing molecules through the N.alpha.-unsubstituted edge. The peptide is useful for preventing .beta.-strand association or aggregation.