| Abstract
| - To clarify which helical screw sense is preferred when one l-residue is introduced into thesecond position from N-terminal of an achiral helical segment, we prepared Boc-Aib-l-Leu-(Aib-ΔZPhe)2-Aib-OMe (1: Boc = t-butoxycarbonyl; Aib = α-aminoisobutyric acid; ΔZPhe = Z-dehydrophenylalanine;OMe = methoxy). Here the segment -(Aib-ΔZPhe)2-Aib-OMe was used as an achiral backbone composedof two “enantiomeric” (left-/right-handed) helices. Peptide 1 was found to form a 310-type helicalconformation in chloroform, from FT IR (the position of amide I band) and 1H NMR (difference nuclearOverhauser effect and solvent accessibility of NH resonances) measurements. Interestingly, CD patternsof peptide 1 changed with types of solvents: i.e., exciton couplets around 280 nm with a negative peakat longer wavelengths in chloroform and with a positive peak at longer wavelengths in methanol or intetrahydrofuran. Consequently, the helical segment prefers a right-handed screw sense in chloroformand a left-handed one in methanol or in tetrahydrofuran. Also, the preference of a screw sense changedreversibly, depending on chloroform−methanol mixtures of varying concentrations. Conformational energycalculation was carried out on acetyl-Aib-l-Leu-(Aib-ΔZPhe)2-Aib-OMe, which was predicted to take bothleft- and right-handed helical conformations. The above experimental and theoretical results werecompared with those of Boc-l-Leu-(Aib-ΔZPhe)2-Aib-OMe, in which the N-terminal l-Leu residue inducesa left-handed helical screw sense preferentially.
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