Peter Gilch

Fluorescence spectra of trans- and cis-azobenzene – emission from the Franck–Condon state

Abstract The photodynamics of the two isomers of azobenzene in DMSO after nπ * excitation are monitored via transient absorption and steady state emission spectroscopy. The excited state decay of the cis -form is close to single-exponential with a time constant of 0.1 ps. The trans -isomer decays in a biphasic fashion with characteristic times of 0.34 and 3.0 ps. This biphasic behaviour has no impact on its fluorescence intensity. It is concluded that the fluorescence probes the dynamics in the vicinity of the Franck–Condon region.

Thymine Dimerization in DNA Model Systems : Cyclobutane Photolesion Is Predominantly Formed via the Singlet Channel

UV-induced formation of cylcobutane pyrimidine dimers (CPD) in all thymine DNA models have been studied by femtosecond IR spectroscopy. CPDs are shown to form within approximately 1 ps during the decay of the initially excited (1)pi pi * state. The quantum yields phi(D)(ps) determined after the (1)pi pi * decay equal the final yield phi(D)(cw). This gives evidence for a predominance of the singlet channel in CPD formation.

Early Events of DNA Photodamage

Ultraviolet (UV) radiation is a leading external hazard to the integrity of DNA. Exposure to UV radiation triggers a cascade of chemical reactions, and many molecular products (photolesions) have been isolated that are potentially dangerous for the cellular system. The early steps that take place after UV absorption by DNA have been studied by ultrafast spectroscopy. The review focuses on the evolution of excited electronic states, the formation of photolesions, and processes suppressing their formation. Emphasis is placed on lesions involving two thymine bases, such as the cyclobutane pyrimidine dimer, the (6-4) lesion, and its Dewar valence isomer.

Loop formation in unfolded polypeptide chains on the picoseconds to microseconds time scale

Intrachain loop formation allows unfolded polypeptide chains to search for favorable interactions during protein folding. We applied triplet–triplet energy transfer between a xanthone moiety and naphthylalanine to directly measure loop formation in various unfolded polypeptide chains with loop regions consisting of polyserine, poly(glycine–serine) or polyproline. By combination of femtosecond and nanosecond laserflash experiments loop formation could be studied over many orders of magnitude in time from picoseconds to microseconds. The results reveal processes on different time scales indicating motions on different hierarchical levels of the free energy surface. A minor (<15%) very fast reaction with a time constant of ≈3 ps indicates equilibrium conformations with donor and acceptor in contact at the time of the laserflash. Complex kinetics of loop formation were observed on the 50- to 500-ps time scale, which indicate motions within a local well on the energy landscape. Conformations within this well can form loops by undergoing local motions without having to cross major barriers. Exponential kinetics observed on the 10- to 100-ns time scale are caused by diffusional processes involving large-scale motions that allow the polypeptide chain to explore the complete conformational space. These results indicate that the free energy landscape for unfolded polypeptide chains and native proteins have similar properties. The presence of local energy minima reduces the conformational space and accelerates the conformational search for energetically favorable local intrachain contacts.

The ketene intermediate in the photochemistry of ortho-nitrobenzaldehyde.

The first intermediate of the photochemical transformation of ortho-nitrobenzaldehyde to ortho-nitrosobenzoic acid in acetonitrile solvent has been characterized by femtosecond spectroscopy and time-dependent density functional theory (TDDFT) calculations. Femtosecond stimulated Raman spectroscopy (FSRS) indicates that this intermediate adopts a ketene structure. This assignment is supported by the TDDFT results. A kinetic analysis of FSRS and transient absorption data points to two channels for the formation of the ketene. For the predominating first channel the formation takes 0.4 ps. For the second channel it is much slower and takes 220 ps. We assign the first channel to a reaction via an excited singlet state. The second one might involve a triplet state.

A 75 MHz Light Source for Femtosecond Stimulated Raman Microscopy

In femtosecond stimulated Raman microscopy (FSRM) a spectrally broad pulse (Raman probe) and a spectrally narrow pulse (Raman pump) interact in a sample and thereby generate a Raman spectrum of the focal volume. Here a novel light source for FSRM is presented. It consists of an 8-fs laser (repetition rate of 75 MHz) operating as Raman probe. A Yb(3+) based fiber amplifier generates the Raman pump light at 980 nm. The amplifier is seeded by the spectral wing of the 8-fs laser output which ensures synchronisation of pump and probe pulses. Spectral and temporal characteristics of these pulses are reported and simultaneous recording of broadband Raman spectra relying on these pulses is demonstrated.

Ultrashort laser pulses in biology and medicine

Ultrafast Lasers in Medicine.- Ultrahigh-Resolution Optical Coherence Tomography Using Femtosecond Lasers.- Two-Photon Laser Scanning Microscopy.- Femtosecond Lasers in Ophthalmology: Surgery and Imaging.- Ultrafast Lasers in Biology.- Ultrafast Peptide and Protein Dynamics by Vibrational Spectroscopy.- Photosynthetic Light-Harvesting.- Primary Photosynthetic Energy Conversion in Bacterial Reaction Centers.- Ultrafast Primary Reactions in the Photosystems of Oxygen-Evolving Organisms.- Primary Photochemistry in the Photoactive Yellow Protein: The Prototype Xanthopsin.- Structure Based Kinetics by Time-Resolved X-ray Crystallography.- Primary Reactions in Retinal Proteins.- Ultrashort Laser Pulses in Single Molecule Spectroscopy.

Femtosecond spectroscopy on the photochemistry of ortho-nitrotoluene

The photo tautomerisations of ortho-nitrotoluene (oNT) and its methylated derivative ortho-ethylnitrobenzene (oENB) have been studied by means of femtosecond spectroscopy and (TD)-DFT computations. In UV/Vis transient absorption spectroscopy a band peaking at 400 nm is seen to rise in a bi-modal manner with time constants of 1-10 ps and 1500 ps. Femtosecond stimulated Raman experiments clearly identify aci-nitro forms as the spectroscopic carriers of the 400 nm band. The assignment of the Raman spectra is based on TD-DFT computations. The quantum yields of the aci-nitro forms after 3 ns are 0.08 (oNT) and 0.3 (oENB). The aci-nitro forms are formed via a singlet channel (1-10 ps) and a triplet channel (1500 ps). There are indications that the triplet channel involves a bi-radical intermediate. In between 3 ns and 1 ms the spectrum of the aci-nitro form shifts from 400 to 390 nm. This could indicate a tautomerisation from Z-aci-nitro to an E form.

Electronic States of o-Nitrobenzaldehyde: A Combined Experimental and Theoretical Study

The experimental UV/vis absorption spectrum of ortho-nitrobenzaldehyde (o-NBA) has been assigned by means of MS-CASPT2/CASSCF, TD-DFT, and RI-CC2 theoretical computations. Additional information on the nature of the absorbing bands was obtained by comparing the o-NBA spectrum with that of related compounds, as, e.g., nitrobenzene and benzaldehyde. For wavelengths larger than approximately 280 nm, the absorption spectrum of o-NBA is dominated by a series of weak n pi* absorptions from the NO2 and CHO groups. These weak transitions are followed in energy by a more intense band, peaking at 250 nm and arising from charge transfer pi pi* excitations involving mainly benzene and nitro orbitals. Finally, the most intense band centered at 220 nm has its origin in the overlap of two different absorptions: the first one localized in the NO2 substituent and the second one arising from a charge transfer excitation involving the NO2 and the CHO fragments, respectively.

Mechanism of the Decay of Thymine Triplets in DNA Single Strands.

The decay of triplet states and the formation of cyclobutane pyrimidine dimers (CPDs) after UV excitation of the all-thymine oligomer (dT)18 and the locked dinucleotide TLpTL were studied by nanosecond IR spectroscopy. IR marker bands characteristic for the CPD lesion and the triplet state were observed from ∼1 ns (time resolution of the setup) onward. The amplitudes of the CPD marker bands remain constant throughout the time range covered (up to 10 μs). The triplet decays with a time constant of ∼10 ns presumably via a biradical intermediate (lifetime ∼60 ns). This biradical has often been invoked as an intermediate for CPD formation via the triplet channel. The present results lend strong support to the existence of this intermediate, yet there is no indication that its decay contributes significantly to CPD formation.