Cooperative structural stabilization and the deceleration of chirality interconversion of an octapeptide helix are achieved by a single intramolecular side-chain crosslinking, as revealed by X-ray crystallography, H-1 NMR spectroscopy, and computational analyses. The helical inversion is slow on the NMR time scale even at 80 degrees C. This is the first case of decelerated chirality inversion for helices based on the biological backbone, and the sequence -Api-Alb(2)-Api-bridged with p-phenylenediacetic acid may serve as a key structural unit for the design of optically active peptides composed of only achiral building blocks and/or a variety of peptides with enhanced structural rigidity.