Conformational studies of Aib‐rich peptides containing lactam‐bridged side chains: Evidence of 3₁₀‐helix formation

Abstract

Aib-rich side-chain lactam-bridged oligomers Ac-(Glu-Aib-Aib-Lys)n-Ala-OH with n = 1,2,3 were designed and synthesized as putative models of the 3(10)-helix. The lactam bridge between the side chains of L-Glu and L-Lys in (i)--(i + 3) positions was introduced in order to enhance the structural preference toward the right-handed 3(10)-helix. The conformational properties of the three peptides were studied in trifluoroethanol (TFE) solution by CD, NMR, and computer simulations. The structural information was derived mainly from the analysis of nuclear Overhauser effect spectroscopy spectra. The presence of alpha H(i)-HN(i + 2) and of alpha H(i)-HN(i + 3) connectivities and the absence of alpha H(i)-HN(i + 4) connectivities indicate that these peptides fold into a 3(10)-helix rather than into an alpha-helix. Based on these conformational features, stereospecific assignment of the Aib methyl groups was possible. The results of such experiments and of the subsequent distance geometry and restrained molecular dynamics simulations reveal a marked preference of these peptides for 3(10)-helix. The CD spectra of these peptides indicate that the helix content increases upon chain elongation. The CD spectrum of the trimer is characterized by a negative band at 200 nm and by a weak positive band around 220 nm. The CD spectrum in TFE is different from that observed in aqueous solution in the presence of SDS micelles, reported in our previous work, and from those reported by a different research group for 3(10)-helical peptides. A possible reason for these differences could rest in the presence of different equilibria of the conformer populations of the various peptides in different solvent systems.

Development of small molecules designed to modulate protein–protein interactions

“…Alternatively, the helical structure could be stabilized through the incorporation of covalent or noncovalent linkages between the side chains of two residues separated in sequence, but spatially close in a helix, such as residues i and i+4 of an a-helix or residues i and i+3 of a 3 10 -helix, respectively. Examples of chemical linkages that enhance helical propensity are salt bridges [59] hydrophobic interactions [60,61] aromatic-charge [62] or aromatic-sulfur [63] interactions, disulfide bonds [64,65], lactam bridges [66][67][68][69][70][71][72], hydrocarbon staplings [73][74][75], diaminoalkanes [76] acetylenes [77], and metal ligation between natural [78][79][80][81][82] and unnatural amino acids [83,84]. These cross-linked peptides have been demonstrated to yield a marked enhancement of peptide helicity, stability, and in vitro and in vivo biological activity.…”

Stapling of a 310-Helix with Click Chemistry

“…Examples of i → i +3 and i → i +4 side chain-to-side chain cross-linking in 3 10 -helical peptides by Glu-Lys lactam formation, ferrocenedicarboxylic acid Lys diamides, photoinduced 1,3-dipolar cycloaddition, metathesis derived hydrocarbon bridges, ,, and a p -phenylenediacetic acid bridge between two α,α-disubstituted 4-aminopiperidine-4-carboxylic acid (Api) residues have been reported. However, only two studies , have provided atomic resolution detail of the effect of cyclization on helix regularity, i.e., on backbone dihedral angles and H-bond lengths.…”

“…The p -phenylenediacetic acid bridge on the other hand appears to afford a highly regular Api/Aib based 3 10 -helix . However, α,α-disubstituted amino acids such as Aib and N -acylated Api are generally hydrophobic and tend to distort the dihedral angles of neighboring monosubstituted, proteinogenic residues away from ideality. ,, In the context of a helical peptide primarily consisting of proteinogenic amino acids, mono substituted residues are expected to be better tolerated. Hence, new methodology for cross-linking of monosubstituted residues, which does not significantly distort the regularity of the 3 10 -helix, is highly desirable.…”

The activity of prolactin releasing peptide correlates with its helicity

“…The presence of αN(i,i + 2) NOEs is often associated with i(i + 3) hydrogen bonding characteristic of 3 10 ‐helices. Further, the ratio of αβ(i,i + 3) to αN(i,i + 3) NOEs, as well as the lack of αN(i,i + 4) NOEs, support the idea that the peptide exists in an equilibrium of α‐ and 3 10 ‐helix in SDS micelles17–19 (see D'Ursi, et al for original figures12). An ensemble of twenty low‐energy models of the PrRP20 residues 8–20 consistent with the NMR data obtained for the full‐length peptide was generated and deposited in the Protein Model Database20 (Figure 1; Supporting Information; PM ID: 0078404).…”