Markus Löweneck

Light-triggered β-hairpin folding and unfolding

A light-switchable peptide is transformed with ultrashort pulses from a β-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 β-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 μs. The folded state has a defined structure, and its formation requires an extended search for the correct hydrogen-bond pattern of the β-strand.

Redox Potential of Azobenzene as an Amino Acid Residue in Peptides

Photochromic compounds that undergo significant structural changes in a reversible manner when exposed to light of an appropriate wavelength are particularly attractive as molecular switches because they allow the storage of information at the molecular level. Of the many chromophores that fulfil this criterion, azobenzene derivatives have become a popular choice in the various fields of polymer science, material science, chemistry and even in the life sciences. [1] Indeed, by strategically incorporating this light switch into (bio)polymers, the photomodulation of one or another physical, chemical or biochemical property is readily achieved because of the pronounced change in geometry and dipole moment that occurs upon the isomerization of the central diazene double bond. [2] Moreover, the minimal photobleaching of this chromophore, its high quantum yields, the large population differences between Z and E isomers that are achievable and its ultrafast isomerization (within picoseconds) are properties that favour the use of azobenzene for inducing responsiveness to light. [2, 3] We and

Following the energy transfer in and out of a polyproline–peptide†

The intramolecular and intermolecular vibrational energy flow in a polyproline peptide with a total number of nine amino acids in the solvent dimethyl sulfoxide is investigated using time-resolved infrared (IR) spectroscopy. Azobenzene covalently bound to a proline sequence containing nitrophenylalanine as a local sensor for vibrational excess energy serves as a heat source. Information on through-space distances in the polyproline peptides is obtained by independent Förster resonance energy transfer measurements. Photoexcitation of the azobenzene and subsequent internal conversion yield strong vibrational excitation of the molecule acting as a local heat source. The relaxation of excess heat, its transfer along the peptide and to the solvent is monitored by the response of the nitro-group in nitrophenylalanine acting as internal thermometer. After optical excitation, vibrational excess energy is observed via changes in the IR absorption spectra of the peptide. The nitrophenylalanine bands reveal that the vibrational excess energy flows in the peptide over distances of more than 20 Å and arrives delayed by up to 7 ps at the outer positions of the peptide. The vibrational excess energy is transferred to the surrounding solvent on a time scale of 10-20 ps. The experimental observations are analyzed by different heat conduction models. Isotropic heat conduction in three dimensions away from the azobenzene heat source is not able to describe the observations. One-dimensional heat dissipation along the polyproline peptide combined with a slower transversal heat transfer to the solvent surrounding well reproduces the observations.

Energy transfer along a poly(Pro) - peptide

Using a novel molecular thermometer, p-nitro-phenylalanine, we investigate the transport of vibrational excess energy along a poly(Pro) sequence. Time resolved IRspectroscopy reveals that heat transfer proceeds at a speed of several A per picosecond.