An Unnatural Amino Acid that Induces β-Sheet Folding and Interaction in Peptides

Abstract

This paper introduces a unique amino acid that can readily be incorporated into peptides to make them fold into beta-sheetlike structures that dimerize through beta-sheet interactions. This new amino acid, Orn(i-PrCO-Hao), consists of an ornithine residue with the beta-strand-mimicking amino acid Hao [J. Am. Chem. Soc. 2000, 122, 7654-7661] attached to its side chain. When Orn(i-PrCO-Hao) is incorporated into a peptide, or appended to its N-terminus, the Hao group hydrogen bonds to the three subsequent residues to form a beta-sheetlike structure. The amino acid Orn(i-PrCO-Hao) is readily used in peptide synthesis as its Fmoc derivative, Fmoc-Orn(i-PrCO-Hao)-OH (3). Fmoc-Orn(i-PrCO-Hao)-OH behaves like a regular amino acid in peptide synthesis and was uneventfully incorporated into the peptide o-anisoyl-Val-Orn(i-PrCO-Hao)-Phe-Ile-Leu-NHMe (4) through standard automated Fmoc solid-phase peptide synthesis, with DIC and HOAt as the coupling agent for Fmoc-Orn(i-PrCO-Hao)-OH and o-anisic acid and HATU as the coupling agent for all other couplings. A second synthetic strategy was developed to facilitate the preparation of peptides with N-terminal Orn(i-PrCO-Hao) residues, which avoids the need for the preparation of Fmoc-Orn(i-PrCO-Hao)-OH. In this strategy, Boc-Orn(Fmoc)-OH is used as the penultimate amino acid in the peptide synthesis, and i-PrCO-Hao-OH (2) is used as the final amino acid. N-Terminal Orn(i-PrCO-Hao) peptide H-Orn(i-PrCO-Hao)-Phe-Ile-Leu-NHMe.TFA (5) was prepared in a fashion similar to that for 4, using DIC and HOAt as the coupling agent for i-PrCO-Hao-OH and HATU as the coupling agent for all other couplings. 1H NMR transverse-ROESY, coupling constant, and chemical shift studies establish that peptide 4 forms a dimeric beta-sheetlike structure in CDCl3 solution. The 1H NMR studies also suggest that the ornithine unit adopts a well-defined turn conformation. Analogous 1H NMR studies of peptide 5 indicate that this TFA salt folds but does not dimerize in CD3OD solution. Collectively, these synthetic and spectroscopic studies establish that the amino acid Orn(i-PrCO-Hao) induces beta-sheet structure and interactions in peptides in suitable organic solvents. Unlike the Hao amino acid, which acts as a prosthetic to replace three residues of the peptide strand, the Orn(i-PrCO-Hao) amino acid acts as a splint that helps enforce a beta-sheetlike structure without replacing the residues and their side chains. This feature of Orn(i-PrCO-Hao) is important, because it allows the creation of beta-sheet structure with minimal perturbation of the peptide sequence.

Sequence‐Selective Molecular Recognition between β Sheets

“…Strong NOEs between the Thr and Val a protons in the Tr-ROESY spectra of unmixed 1 b and 1 c, similar to those we reported previously for peptide 1 a, [4] help establish the formation of homodimers. A strong NOE between the two different Val a protons of the new species formed upon mixing 1 b and 1 c confirms the formation of the heterodimer.…”

“…[3] We had previously reported that peptides such as 1 a, which contain the amino acid Orn(iPrCO-Hao), fold into bsheetlike structures that dimerize through b-sheet interactions in organic solvents (Scheme 1). [4] In the present study, we chose to replace the valine and leucine residues of 1 a with threonine and valine in all possible orders: Thr-Val (1 b), ValThr (1 c), Thr-Thr (1 d), and Val-Val (1 e). We selected these residues with the expectation that Val would pair preferentially with Val, and that Thr would pair preferentially with Thr, through self-complementary noncovalent interactions and with the knowledge that these pairings frequently occur in the non-hydrogen-bonded rings of antiparallel b sheets.…”

Inhibition of β-amyloid1-40 Peptide Aggregation and Neurotoxicity by Citrate

“…Hence, human Aβ1-40 is amphiphilic, meaning that possess both hydrophobic and hydrophilic groups, like surfactants (Soreghan et al, 1994;Ji et al, 1995;Bokvist et al, 2004). Many reagents have been developed to modulate the aggregation of Aβ peptide (Pallitto et al, 1999;Lowe et al, 2001;Nowick et al, 2002;Ban et al, 2004), and, surfactants belong to one important category of the reagents.…”

Molecular mimicry of conformationally diverse β-sheets by using α/β-peptide foldamers

“…Structural elements such as the fibrillous -peptidic nanostructures 47, 79-80 64 , short turns [81][82][83][84] , and a three stranded sheet in organic solvent 85 are described among -peptide foldamers. Sheet-structures that were stabilized by macrocyclization and a non-peptidic template showed bioactivity [6][7][8][9][10][11][12] but only a few other bioactive strand or -sheet like structures are known.…”

“…[3][4][5] The mimicry of -sheets therefore requires further investigations, only a few sheet-like structures are known with biological activity. [6][7][8][9][10][11][12] Our goal was to establish design strategies for conformationally diverse -sheet folding systems by using -peptide foldamers based on a selected peptide participating in proteinprotein and membrane interactions. The antiangiogenic and antimicrobial peptide anginex was chosen as a model system, [13][14][15] which exhibits diverse structural features.…”