Peptide hairpins with strand segments containing α‐ and β‐amino acid residues: Cross‐strand aromatic interactions of facing Phe residues

Abstract

The incporation of beta-amino acid residues into the strand segments of designed beta-hairpin leads to the formation of polar sheets, since in the case of beta-peptide strands, all adjacent carbonyl groups point in one direction and the amide groups orient in the opposite direction. The conformational analysis of two designed peptide hairpins composed of alpha/beta-hybrid segments are described: Boc-Leu-betaPhe-Val-(D)-Pro-Gly-Leu-betaPhe-Val-OMe (1) and Boc-betaLeu-Phe-betaVal-D-Pro-Gly-betaLeu-Phe-betaVal-OMe (2). A 500-MHz 1H-NMR (nuclear magnetic resonance) analysis in methanol supports a significant population of hairpin conformations in both peptides. Diagnostic nuclear Overhauser effects (NOEs) are observed in both cases. X-ray diffraction studies on single crystals of peptide 1 reveal a beta-hairpin conformation in both the molecules, which constitute the crystallographic asymmetric unit. Three cross-strand hydrogen bonds and a nucleating type II' beta-turn at the D-Pro-Gly segment are observed in the two independent molecules. In peptide 1, the betaPhe residues at positions 2 and 7 occur at the nonhydrogen-bonding position, with the benzyl side chains pointing on opposite faces of the beta-sheet. The observed aromatic centroid-to-centroid distances are 8.92 A (molecule A) and 8.94 A (molecule B). In peptide 2, the aromatic rings must occupy facing positions in antiparallel strands, in the NMR-derived structure. Peptide 1 yields a normal "hairpin-like" CD spectrum in methanol with a minimum at 224 nm. The CD spectrum of peptide 2 reveals a negative band at 234 nm and a positive band at 221 nm, suggestive of an exciton split doublet. Modeling of the facing Phe side chains at the hydrogen-bonding position of a canonical beta-hairpin suggests that interring separation is approximately 4.78 A for the gauche+ gauche- (g+ g-) rotamer. A previously reported peptide beta-hairpin composed of only alpha-amino acids, Boc-Leu-Phe-Val-D-Pro-Gly-Leu-Phe-Val-OMe also exhibited an anomalous far-UV (ultraviolet) CD (circular dichroism) spectrum, which was interpreted in terms of interactions between facing aromatic chromophores, Phe 2 and Phe 7 (C. Zhao, P. L. Polavarapu, C. Das, and P. Balaram, Journal of the American Chemical Society, 2000, Vol 122, pp. 8228-8231).

Family of pH (low) insertion peptides for tumor targeting

“…No shift in the position of the peptide fluorescence maximum was seen for either of the K-pHLIPs in the presence of lipid as a result of pH decrease, and the CD spectra do not change and do not exhibit the minima at 208 and 225 nm that are characteristic signals reporting α-helical conformations. The CD spectra reveal a negative band at 234 nm and a positive band at 221 nm, suggestive of an exciton split doublet (2,14) possibly arising from peptide aggregation.…”

pHLIP Peptide Interaction with a Membrane Monitored by SAXS

“…The exciton (sharing of electronic density) is observed, when rings of several aromatic residues form stacks. 38 The exciton formation was monitored previously for 50 µM of the peptide. 37 At the same time, we do not observe the appearance of any elements of the secondary structure for the WT peptide at 150 µM at pH 8.0.…”

Tuning the β‐turn segment in designed peptide β‐hairpins: Construction of a stable type I′ β‐turn nucleus and hairpin–helix transition promoting segments

“…Peptide hairpins can be readily constructed by positioning a type II′ or I′ β‐turn forming segment at the centre of model sequences 3–12, 15–22. While cross strand interactions can lend additional stability to a hairpin structure,35–38 these conformations are reasonably tolerant to amino acid substitutions.…”

“…The use of D Pro at the i + 1 position facilitates formation of either type II′ or I′ turns, as a consequence of the restriction of ϕ ( D Pro) to values of +60° ± 20°. The D Pro‐Gly segment has found extensive application in the design of isolated hairpins3–12, 15–22 and multistranded β‐sheets 23–28. Figure 1 illustrates two examples of peptide β‐hairpins characterized in crystals, which provide examples of fold stabilization by centrally positioned type I′ or II′ β‐turn structures.…”