Multiple conformational equilibria of cyclic octapeptide, cyclo (L‐Pro‐Sar)4 in solution

Abstract

Cyclo (l‐Pro‐Sar)4 is asynthetic cyclic octapeptide composed of only N ‐substituted amino acids. The conformation of this peptide in different solvents was examined by 1H‐and 13C‐n.m.r. spectroscopy, 1H‐n.m.r. data of this cyclic peptide demonstrated that multiple conformational equilibria take place in solution and they vary with solvent polarity. Three conformers are interconverting with each other in the nonpolar chloroform (CDCl3); one C4‐symmetric conformer (49%) and two C2‐symmetric conformers (37% and 14%). While three C2‐symmetric (59%, 19%, and 18%) and one asymmetric conformer (4%) are detected to coexist in acetonitrile (CD3CN), one largely populated C2‐symmetric one (97%) is favored in the polar dimethyl sulfoxide (Me2SO‐d 6). N.m.r. measurement employing various strategy predicted the occurrence of trans‐cis‐cis‐cis‐trans‐cis‐cis‐cis (two Sar‐Pro bonds: trans) (tccctccc) peptide bond sequence in a predominant C2‐symmetric conformer of Me2SO‐d 6 solution. In the same way, tccctccc and ctttcttt (two Sar‐Pro bonds: cis) peptide unit arrangement was proposed for the first and the secondly populated conformer in CD3CN, respectively. In CDCl3 a conformer having tctctctc (four Sar‐Pro bonds: trans) C4‐symmetric peptide bond sequence was deduced.