Light-triggered β-hairpin folding and unfolding

Abstract

A light-switchable peptide is transformed with ultrashort pulses from a ␤-hairpin to an unfolded hydrophobic cluster and vice versa. The structural changes are monitored by mid-IR probing. Instantaneous normal mode analysis with a Hamiltonian combining density functional theory with molecular mechanics is used to interpret the absorption transients. Illumination of the ␤-hairpin state triggers an unfolding reaction that visits several intermediates and reaches the unfolded state within a few nanoseconds. In this unfolding reaction to the equilibrium hydrophobic cluster conformation, the system does not meet significant barriers on the free-energy surface. The reverse folding process takes much longer because it occurs on the time scale of 30 s. The folded state has a defined structure, and its formation requires an extended search for the correct hydrogen-bond pattern of the ␤-strand.density functional theory calculation ͉ peptide folding ͉ TrpZip2 ͉ ultrafast infrared spectroscopy F olding is a key process during the formation of a functional protein after the synthesis of its amino acid chain (1). During folding, the amino acid chains are rearranged in highly complex processes, for which a detailed understanding is still missing. Straightforward solutions of the folding problem are prevented by the high dimensionality of the protein conformational spaces and by the wide range of relevant time scales. Thus, for complexity reduction, one has to focus on typical protein substructures, which are small enough to be analyzed by present-day technology. With corresponding peptide model systems there is a chance to monitor the formation of secondary structures, to identify characteristic intermediate states of these folding dynamics, and to carry out realistic simulations on a molecular level. An interesting class of model systems are light-triggered peptides, within which the incorporation of a photoresponsive element can initiate structural changes.Among corresponding photoresponsive chromophores, azobenzene derivatives have become a popular choice when it comes to selecting a fast conformational trigger for peptide refolding, because their structure significantly changes on time scales of a few hundred femtoseconds after photoexcitation. Indeed, an azobenzene chromophore, used as a backbone element within cyclic peptides, was shown to induce large-scale conformational changes, whose dynamics could be monitored by visible (2) and IR (3) spectroscopy. Moreover, the experimental results clearly showed that the initial, strongly driven conformational changes of the peptide proceeded within a few picoseconds after the trigger event. Externally linked to an ␣-helix, azobenzene also was used for unfolding (4) and refolding (5) of an ␣-helical model peptide in the nanosecond to microsecond time range.A prominent secondary structure motif is the ␤-sheet, for which a ␤-hairpin represents a minimal model (6, 7). We and others (8-10) have recently focused on the design of photocontrolled ␤-hairpin peptides. To enable an ultrafa...

Area Increase and Budding in Giant Vesicles Triggered by Light: Behind the Scene

“…In the past few years, the use of azobenzene derivatives for photocontrol of peptides,15 ion channels,16 nucleic acids, and oligonucleotides17 has been reported. Azobenzene‐modified lipids have been employed to study the membrane properties in model systems such as small unilamellar vesicles (SUVs) and large unilamellar vesicles (LUVs).…”

Light‐Directed Protein Binding of a Biologically Relevant β‐Sheet

“…Photoswitchable w-amino acids based on either azobenzene [7] or hemithioindigo [8] are good candidates for the modulation of peptide conformations [9] because they undergo ultrafast photoisomerization [10] thus allowing monitoring of conformational transitions in the picoto femtosecond timescale. [11] For biological applications, [12] azobenzene has been demonstrated to be very effective in that it shows isomerization around the central N = N bond (transQcis) with high isomerization yields and remarkable changes in geometry. In contrast to the previously reported light-switchable b-hairpins, [9d,e] we embedded the azobenzene-w-amino acid 3-((4'-aminomethyl)phenylazo) benzoic acid (A) as photoswitch because of the high thermal stability of the photostationary state (pss) of the cis form (Scheme 1 b).…”

Conformations of Betanova in aqueous trifluoroethanol

“…3 The time scale for folding-unfolding transitions in b-hairpin and b-sheet structures is microseconds. [15][16][17] As a result, MD simulations sufficient to explore fully the conformational space of b-structures can be challenging. 18 Nonetheless, there have been a number of MD studies of Betanova [18][19][20] or closely related structures.…”