Slower processes of the ultrafast photo-isomerization of an azobenzene observed by IR spectroscopy

The photo-induced trans–cis isomerization of the azobenzene derivative 4-nitro-4'-(dimethylamino)azobenzene in polar solution was studied by femtosecond UV/Vis and IR spectroscopy. The UV/Vis experiment reveals two excited state processes; the slower one (1 ps) is the internal conversion to the ground state. The ensuing spectral changes point to vibrational cooling of the nascent cis product and the recovered trans isomer. Judging from the UV/Vis experiment this ~5 ps process seems to terminate the isomerization. The internal conversion and the cooling also find their manifestation in the IR experiment. In addition slower spectral changes lasting until ~50 ps are detected. These slow IR responses are compared with the behavior of a non-isomerizing analogue, para-N,N-dimethyl-nitroaniline. An origin for this discrepancy is suggested and potential molecular processes causing the slow IR response are discussed

Nitro-Phenylalanine: A Novel Sensor for Heat Transfer in Peptides

Femtosecond IR-pump-IR-probe experiments with independently tunable pulses are used to monitor the ultrafast response of selected IR absorption bands to vibrational excitation of other modes of Fmoc-nitrophenylalanine. The absorptions of both NO(2)-bands change rapidly within <2 ps upon excitation of other vibrational modes. The results point to considerable coupling between the monitored NO(2) modes and the initially excited modes or low-frequency modes. The latter are populated by a rapid energy redistribution process. The strong IR absorption of the NO(2) stretching bands and the intense coupling to other modes makes the nitro group of nitrophenylalanine a sensitive monitor for vibrational energy arriving at this amino acid

Folding and Unfolding of Light Triggered beta-Hairpin Model Peptides

Ultrafast spectroscopy in the visible and mid-infrared is used to study the reaction dynamics of two light-triggered model peptides containing an azobenzene derivative as a switching element. One model peptide, the AzoTrpZip2, forms a β-hairpin structure in the cis form of the chromophore. This peptide is compared to the core structure consisting of the chromophore and the two flanking amino acid residues, used as a minimal model. This combination of experiments performed in different spectral ranges on peptides of different sizes allows for improved insight into light triggered reaction dynamics. The kinetics observed for the core structure are directly connected to the switching process of the chromophore and are finished on the 10 ps time scale. The trans-to-cis reaction of AzoTrpZip2, leading to the formation of the β-hairpin structure involves ultrafast processes on the 100 ps time scale, which are directly related to the relaxation of the strain between the isomerized molecular switch and the two peptide strands. IR-signatures characteristic for changes in interstrand interactions are absent on the <1 ns time scale. Thus folding into the β-hairpin structure occurs on a much longer time scale. In the cis-to-trans unfolding reaction, all IR signatures related to changes in interstrand interactions occur within 1 ns, in a time range where visible spectroscopy reveals the final decay of the intramolecular strain. Apparently unfolding of AzoTrpZip2 is to a large extent a fast, driven process

Relaxation time prediction for a light switchable peptide by molecular dynamics

We study a monocyclic peptide called cAPB, whose conformations are light switchable due to the covalent integration of an azobenzene dye. Molecular dynamics (MD) simulations using the CHARMM22 force field and its CMAP extension serve us to sample the two distinct conformational ensembles of cAPB, which belong to the cis and trans isomers of the dye, at room temperature. For gaining sufficient statistics we apply a novel replica exchange technique. We find that the well-known NMR distance restraints are much better described by CMAP than by CHARMM22. In cAPB, the ultrafast cis/trans photoisomerization of the dye elicits a relaxation dynamics of the peptide backbone. Experimentally, we probe this relaxation at picosecond time resolution by IR spectroscopy in the amide I range up to 3 ns after the UV/vis pump flash. We interpret the spectroscopically identified decay kinetics using ensembles of non-equilibrium MD simulations, which provide kinetic data on conformational transitions well matching the observed kinetics. Whereas spectroscopy solely indicates that the relaxation toward the equilibrium trans ensemble is by no means complete after 3 ns, the 20 ns MD simulations of the process predict, independently of the applied force field, that the final relaxation into the trans-ensemble proceeds on a time scale of 23 ns. Overall our explicit solvent simulations cover more than 6 micros

Vibrational Spectra of the Ground and the Singlet Excited pi pi* State of 6,7-Dimethyl-8-ribityllamazine

6,7-Dimethyl-8-ribityllumazine serves as fluorophore in lumazine proteins (LumP) of luminescent bacteria. The molecule exhibits several characteristic vibrational absorption bands between 1300 and 1750 cm(-1) in its electronic ground state. The IR-absorption pattern of the singlet excited ππ* state was monitored via ultrafast infrared spectroscopy after photoexcitation at 404 nm. The comparison of experimentally observed band shifts for a number of isotopologues allows for a clear assignment of several absorption bands--most importantly the two carbonyl bands. This assignment is confirmed by normal-mode calculations by means of either density functional theory (DFT) calculations for the ground state or the configuration interaction singles (CIS) method for the excited singlet state. A good agreement between experiment and calculation is obtained for models including explicitly a first solvation shell. The results provide a basis for further investigations of lumazine protein and demonstrate the necessity of proper accounting for explicit hydrogen bonding in case of strongly polar molecular systems

Ultrafast Unzipping of a Beta-Hairpin Peptide

Light induced switching of a beta-hairpin structure is investigated by femtosecond IR spectroscopy. While the unzipping process comprises ultrafast kinetics and is finished within 1 ns, the folding into the hairpin structure is a much slower process.

Ultrafast Unzipping of a Beta-Hairpin Peptide

Light induced switching of a beta-hairpin structure is investigated by femtosecond IR spectroscopy. While the unzipping process comprises ultrafast kinetics and is finished within 1 ns, the folding into the hairpin structure is a much slower process