Solvent Dependence on the Preference of Helical Screw Sense: Effect of l-Leu Residue Second from N-Terminal on Screw Sense in Achiral Peptide

Abstract

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-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 composed of two “enantiomeric” (left-/right-handed) helices. Peptide 1 was found to form a 310-type helical conformation in chloroform, from FT IR (the position of amide I band) and 1H 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 i...