Two hemithioindigo-hemistilbene (HTI) derivatives, designed to operate as structural switches in peptides, as well as two HTI peptides are characterized by ultrafast spectroscopy in the visible and the infrared. The two HTI switches follow the reaction scheme published for other HTI compounds with a picosecond excited state reaction (τ(1) ≈ 6 ps) and isomerization from Z to E with τ(2) = 13 and 51 ps. As compared to the isolated chromophores, the isomerization reaction is slowed down in the chromopeptides to τ(2) = 24 and 69 ps. For the smaller peptide containing 6 amino acids, the structural changes of the peptide moiety observed via the IR spectrum in the amide I band follow the isomerization of the molecular switch closely. In the larger cyclic chromopeptide, containing 20 amino acids and mimicking a β-hairpin structure in the Z-form of the chromophore, the peptide moiety also changes its structure during isomerization of the chromophore. However, the IR spectrum at the end of the observation period of 3 ns deviates significantly from the stationary difference spectrum. These signatures indicate that strong additional structural changes, e.g., breaking of interchain hydrogen bonds, also occur on longer time scales.

The application of chemistry to hydrophobic peptides and membrane-spanning helices is hampered by the fact that they are only poorly soluble in aqueous buffers and that they have a tendency for aggregation. These properties lead to difficulties when purifying them after chemical synthesis and particularly interfere with native chemical ligation. Here, we describe native chemical ligation of model peptides in the organic solvent dimethylformamide (DMF) under anhydrous conditions. Best results concerning yields and complete solubility are obtained if thiophenole is used in the presence of LiCl. These conditions might be applicable also for the ligation of transmembrane helices.

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