Synthesis of hemithioindigo-based chromopeptides by using the Tmb auxiliary in native chemical ligation studies.

The 4,5,6-trimethoxy-2-mercaptobenzyl auxiliary was used in auxiliary-based native chemical ligation reactions with Boc-protected pHTI and mHTI ω-amino acid thioesters 2a,b for the construction of small hemithioindigo (HTI)-based chromopeptides 6a,b with a class 1 PDZ binding motif. While reversible tris(2-carboxyethyl)phosphine (TCEP)-HTI adduct formation required moderate use of access TCEP, the Na ascorbate concentration was broadly varied for optimization of the reaction conditions. In the studies presented, the mHTI ω-amino acid thioester 2b proved to be slightly less reactive than the pHTI ω-amino acid thioester 2a. Ligated products 5a-d were isolated in 35-81% yield, and also cleavage of the auxiliary proceeded smoothly, furnishing peptides 6a-d in 48-61% yield. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

Ultrafast coherent oscillations reveal a reactive mode in the ring-opening reaction of fulgides.

The ultrafast ring-opening reaction of photochromic fulgides proceeds via conical intersections to the ground state isomers involving activation barriers in the excited state. The coherent oscillations observed in the femtosecond transient absorption signal of a methyl-substituted indolylfulgide were analysed in the framework of vibrational wavepackets to expose a dominant low-frequency mode at ∼80 cm(-1). The quantum chemical calculations in the relaxed excited state geometry of this fulgide revealed that the experimentally observed vibrational normal mode has a dominant contribution to the relevant ring-opening reactive coordinate.

Light-switchable hemithioindigo-hemistilbene-containing peptides: ultrafast spectroscopy of the Z → E isomerization of the chromophore and the structural dynamics of the peptide moiety.

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 ß-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.

Photochromic bis(thiophen-3-yl)maleimides studied with time-resolved spectroscopy.

The dynamics of the ring-closure reaction of three different bis(thiophen-3-yl)maleimides are investigated using ultrafast spectroscopy in the visible range. The structures of the molecules differ with respect to substitution of the thiophene ring and the maleimide. The experiments reveal reaction kinetics which point to the population of an excited electronic state for several nanoseconds. In the case of completely unsubstituted thiophene rings, a long excited-state lifetime (biexponential decay with 3 and 15 ns) can be observed. The remaining ultrafast absorption transients of this molecule are due to relaxational processes on the excited electronic potential energy surface. The ring-closure reaction has a small yield (<1%) and does not show up in the ultrafast absorption experiments. A dimethyl substitution of the thiophene ring results in completely different behavior: after transients related to relaxation in the excited electronic state, one finds pronounced absorption transients with tau = 16 ps which represent the partial decay of the excited electronic state and the formation of the ring-closed isomer. Another fraction of the emitting excited electronic state decays again on the few nanosecond time scale. The experiments suggest that the open isomer of the dimethyl-substituted imides exists in two conformations.

Reducing the molecule-substrate coupling in C60-based nanostructures by molecular interactions.

Codeposition of C60 and the three-dimensional molecular hydrocarbon 1,3,5,7-tetraphenyladamantane (TPA) on Au(111) leads to the spontaneous formation of molecular nanostructures in which each fullerene is locked into a specific orientation by three surrounding TPA. Scanning tunneling spectroscopy shows that the electronic coupling of C60 with the surface is significantly reduced in these nanostructures, enhancing the free-molecule properties. As evidenced by density functional theory simulations, the nanostructures are stabilized by 18 local electrostatic forces between C60 and TPA, resulting in a lifting of the C60 cage from the surface.