A Photocontrolled β‐Hairpin Peptide

Abstract

Beta-hairpins constitute the smallest beta-type structures in peptides and proteins. The development of highly stable, yet monomeric beta-hairpins based on the tryptophan zipper motif was therefore a remarkable success [A. G. Cochran, N. J. Skelton, M. A. Starovasnik, Proc. Natl. Acad. Sci USA 2001, 98, 5578-5583]. We have been able to design, synthesize and characterize a hairpin based on this motif which incorporates an azobenzene-based photoswitch, that allows for time-resolved folding studies of beta-structures with unprecedented time resolution. At room temperature the trans-azo isomer exhibits a mostly disordered structure; however, light-induced isomerization to the cis-azo form leads to a predominantly extended and parallel conformation of the two peptide parts, which are linked by the novel photoswitch, [3-(3-aminomethyl)phenylazo]phenylacetic acid (AMPP). While in the original sequence the dipeptide Asn-Gly forms a type I' beta-turn which connects the two strands of the hairpin, this role is adopted by the AMPP chromophore in our photoresponsive beta-hairpin that can apparently act as a beta I'-turn mimetic. The beta-hairpin structure was determined and confirmed by NMR spectroscopy, but the folding process can be monitored by pronounced changes in the CD, IR and fluorescence spectra. Finally, incorporation of the structurally and functionally important beta-hairpin motif into proteins by chemical ligation might allow for the photocontrol of protein structures and/or functions.

An Azobenzene Photoswitch Sheds Light on Turn Nucleation in Amyloid-β Self-Assembly

“…In the context of peptides, cis-AMPP has been shown to be a type I′ β-hairpin backbone peptidomimetic, whereas trans-AMPP lies in a planar conformation ( Figure 1B). 35 We hypothesized that, in the context of the Aβ42 peptide, the trans-azobenzene conformer would reduce selfassembly propensity (rate, thermodynamic stability of fibrils), while the cis-conformer would promote β-turn formation resulting in accelerated fibrillization. The position of the AMPP turn was chosen based on models proposed by Tycko and Smith, which place the turn between a D23/K28 salt bridge.…”

“…16,17,22 NMR analysis of AMPPcontaining peptides by Dong et al has shown that the trans-AMPP moiety in the context of a Trp zipper can form a nonhairpin turn. 35 This implies that β-hairpin formation is not a strict prerequisite for fibril nucleation and that formation of a nonhairpin turn is sufficient to nucleate fibrillogenesis. That trans AMPP facilitates the formation of nonhairpin turns in Aβ is consistent with the spectroscopic data obtained herein, which correlates to solid-state NMR models of Aβ40 proposed by Tycko and co-workers that predict a nonhairpin turn within the putative turn region.…”

“…36−38 AMPP can be selectively photoisomerized into cis or trans conformations; the cis conformer has been shown to nucleate β-hairpin turns in the context of peptides. 35 We hypothesized that nucleating the turn in Aβ42 using the AMPP photoswitch would nucleate fibrillization. We conjectured that, in the trans-conformation, the turn region would lie in an extended conformation that precludes efficient self-assembly.…”

Light-triggered β-hairpin folding and unfolding

“…We and others (8)(9)(10) have recently focused on the design of photocontrolled ␤-hairpin peptides. To enable an ultrafast initiation of ␤-hairpin folding and unfolding the azobenzene derivative 3-(3-aminomethylphenylazo)phenylacetic acid (AMPP) was used as a reversible light switch (8,10), which, in the cis isomeric state, acts as ␤-turn mimetic (see Fig. 1A).…”

Ultrafast Unzipping of a Beta-Hairpin Peptide

“…A general feature of the disastrous conformational changes is a transition from native and "healthy" structures to |3-extended precursors of insoluble fibrils. We have now succeeded in designing, synthesizing and investigating a special Phairpin model [1], where the introduction of a femtosecond light switch in a (3-hairpin peptide allowed the observation of ultrafast structural changes -orders of magnitude faster than in previous experiments on P-structures. We show here that an optical trigger starts a series of picosecond conformational transitions of the pstructure (see Fig.…”