SynopsisA newly designed host-guest approach is introduced as a experimental tool to explore the relationship between the sequence of peptides and their secondary structure. From the CD spectra of the host-guest peptides studied, a tentative scale for the a-helix potential in 2,Z.Z-trifluoroethanol of guest amino acids is delineated. The conformational preferences are also examined in &structure supporting media (solid state, CH,Cl,, CH,OH, H20) using ir-absorption and CD techniques. Scales for the &forming tendency of guest amino acid residues in the different media are delineated. It is shown that the preferred conformation of the host-guest peptides is a function of the medium, the chain length, and the protecting groups. Given the fact that conformational effects are important in peptide synthesis, the tentative scales may serve as a guideline to predict secondary structures of side-chain-protected or -deprotected peptides in a given solvent, complementing the well-known empirical conformational prediction parameters.