To understand the terminal effect of chiral residue for determining a helical screw sense, we adopted five kinds of peptides I-V containing N-and/or C-terminal chiral Leu residue(s): Boc-D-Leu-(Aib-⌬Phe) 2 -L-Leu-OMe (IV), and Boc-D-Leu-(Aib-⌬Phe) 2 -Aib-OMe (V). The segment -(Aib-⌬Phe) 2 -was used for a backbone composed of two "enantiomeric" (left-/right-handed) helices. Actually, this could be confirmed by 1
H-nmr [nuclear Overhauser effect (NOE) and solvent accessibility of NH resonances] and CD spectroscopy on
Boc-(Aib-⌬Phe) 2 -Aib-OMe, which took a left-/right-handed 3 10 -helix. Peptides I-V were also found to take 3 10 -type helical conformations in CDCl 3 , from difference NOE measurement and solvent accessibility of NH resonances. Chloroform, acetonitrile, methanol, and tetrahydrofuran were used for CD measurement. The CD spectra of peptides I-III in all solvents showed marked exciton couplets with a positive peak at longer wavelengths, indicating that their main chains prefer a left-handed screw sense over a right-handed one. Peptide V in all solvents showed exciton couplets with a negative peak at longer wavelengths, indicating it prefers a right-handed screw sense. Peptide IV in chloroform showed a nonsplit type CD pattern having only a small negative signal around 280 nm, meaning that left-and right-handed helices should exist with almost the same content. In the other solvents, peptide IV showed exciton couplets with a negative peak at longer wavelengths, corresponding to a right-handed screw sense. From conformational energy calculation and the above 1 H-nmr studies, an N-or C-terminal L-Leu residue in the lowest energy left-handed 3 10 -helical conformation was found to take an irregular conformation that deviates from a left-handed helix. The positional effect of the L-residue on helical screw sense was discussed based on CD data of peptides I-V and of Boc-(L-Leu-⌬Phe) n -L-Leu-OMe (n ϭ 2 and 3).