Linear oligopeptides. Part 406.1 Helical screw sense of peptide molecules: the pentapeptide system (Aib)4/L-Val[L-(αMe)Val] in solution

Helical screw sense of peptide molecules: The pentapeptide system (Aib)4/L-Val[L-(αMe)Val] in the crystal state

“…14 -16 The results obtained from our analysis in solution 8 (by CD) strongly supported the view that the position where the single chiral residue is inserted in the main chain is critical in directing the handedness of a fully developed 3 10 -helical structure. More specifically, if the chiral, helical residue is incorporated at the Cterminal position the relationship between ␣-carbon chirality and the screw sense of the preceding OAib 4 O helix tends to be inverse with respect to that found in proteins (i.e., an L-amino acid gives a lefthanded helix).…”

“…Clearly, this result implies that in the right-handed helix the helical LO(␣Me)Val residue is forced to fall in the unfavorable ϩ,ϩ quadrant of the , map. 14 -16 Taking all these results into account, we conclude that the issue of the helical screw sense of peptide molecules, addressed in the previous paper 8 and in this work by studying the conformational preferences of the O(Aib 4 )O/LOVal [L-(␣Me)Val] pentapeptide system, is more complex than originally expected. At this point it is reasonable to assume that all four possible conformations [based on the right-or lefthanded O(Aib) 4 O sequence, and on the positive or negative torsion angle for the C-terminal chiral residue] would populate the equilibrium mixtures in solution, although to a different extent, and that the net results of this phenomenon would be unraveled by the CD spectra.…”

“…8 Not unexpectedly, this phenomenon is slightly more marked in the peptide based on L-Val, where the difference in the molecular mass between the two substituents (R vs H compared to R vs CH 3 ) on the chiral ␣-carbon is more pronounced. We interpreted this finding in terms of the absence of an unfavourable O .…”

“…Further, in a recent survey of crystal structures of all known N ␣ -blocked, 3 10 -helical peptide esters 7 it was confirmed that in 77% of the molecules the sign of the parameter ( torsion angle) responsible for the induction of this C-capping motif is indeed opposite to that of the preceding residue. 8 Interestingly, neither the steric bulkiness of the ester moiety nor the potential involvement of the ester carbonyl in the intermolecular H-bonding scheme seem to play any major role in determining the preferred conformation of the C-terminal residue. On these grounds, we began the first systematic study of the preferred screw sense of the 3 10 -helical peptides by designing a pentapeptide system based on four helicogenic, achiral Aib residues, 4,9 -13 the prototypical C ␣ -tetrasubstituted ␣-amino acid, and a sin-…”

Induction of Unexpected Left‐Handed Helicity by an N‐Terminal L‐Amino Acid in an Otherwise Achiral Peptide Chain

“…For comparison with reported spectra, [9,22] they were also converted into their N-pbromobenzamide derivatives 6. CD spectra of 5 were acquired at 20 8C in methanol ( Figure 1) and in 2,2,2-trifluoroethanol.…”