To clarify which helical screw sense is preferred when one L-residue is introduced into the second position from N-terminal of an achiral helical segment, we prepared Boc-Aib-L-Leu-(Aib-Delta(Z)Phe)(2)-Aib-OMe (1: Boc = t-butoxycarbonyl; Aib = alpha-aminoisobutyric acid; Delta(Z)Phe = Z-dehydrophenylalanine; OMe = methoxy). Here the segment -(Aib-Delta(Z)Phe)(2)-Aib-OMe was used as an achiral backbone composed of two "enantiomeric" (left-/right-handed) helices. Peptide 1 was found to form a 3(10)-type helical conformation in chloroform, from FT IR (the position of amide I band) and H-1 NMR (difference nuclear Overhauser effect and solvent accessibility of NH resonances) measurements. Interestingly, CD patterns of peptide 1 changed with types of solvents: i.e., exciton couplets around 280 nm with a negative peak at longer wavelengths in chloroform and with a positive peak at longer wavelengths in methanol or in tetrahydrofuran. Consequently, the helical segment prefers a right-handed screw sense in chloroform and a left-handed one in methanol or in tetrahydrofuran. Also, the preference of a screw sense changed reversibly, depending on chloroform-methanol mixtures of varying concentrations. Conformational energy calculation was carried out on acetyl-Aib-L-Leu-(Aib-Delta(Z)Phe)(2)-Aib-OMe, which was predicted to take both left- and right-handed helical conformations. The above experimental and theoretical results were compared with those of Boc-L-Leu-(Aib-Delta(Z)Phe)2-Aib-OMe, in which the N-terminal L-Leu residue induces a left-handed helical screw sense preferentially.