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The measurement of chirality and its temporal evolution are crucial for the understanding of a large range of biological functions and chemical reactions. Steady-state circular dichroism (CD) is a standard analytical tool for measuring chirality in chemistry and biology. Nevertheless, its push into the ultrafast time domain and in the deep-ultraviolet has remained a challenge, with only some isolated reports of subnanosecond CD. Here, we present a broadband time-resolved CD spectrometer in the deep ultraviolet (UV) spectral range with femtosecond time resolution. The setup employs a photoelastic modulator to achieve shot-to-shot polarization switching of a 20 kHz pulse train of broadband femtosecond deep-UV pulses (250–370 nm). The resulting sequence of alternating left- and right-circularly polarized probe pulses is employed in a pump-probe scheme with shot-to-shot dispersive detection and thus allows for the acquisition of broadband CD spectra of ground- and excited-state species. Through polarization scrambling of the probe pulses prior to detection, artifact-free static and transient CD spectra of enantiopure [Ru(bpy)_3]^2+ are successfully recorded with a sensitivity of <2×10^−5 OD (≈0.7  mdeg). Due to its broadband deep-UV detection with unprecedented sensitivity, the measurement of ultrafast chirality changes in biological systems with amino-acid residues and peptides and of DNA oligomers is now feasible.

We introduce a novel configuration for the broadband measurement of the optical activity of molecules, combining time-domain detection with heterodyne amplification. A birefringent common-path polarization-division interferometer creates two phase-locked replicas of the input light with orthogonal polarization. The more intense replica interacts with the sample, producing a chiral free-induction decay field, which interferes with the other replica, acting as a time-delayed phase-coherent local oscillator. By recording the delay-dependent interferogram, we obtain by a Fourier transform both the circular dichroism and circular birefringence spectra. Our compact, low-cost setup accepts ultrashort light pulses, making it suitable for measurement of transient optical activity.

Several ways to electronically synchronize different types of amplified femtosecond laser systems are presented based on a single freely programmable electronics hardware: arbitrary-detuning asynchronous optical sampling (ADASOPS), as well as actively locking two femtosecond laser oscillators, albeit not necessarily to the same round-trip frequency. They allow us to rapidly probe a very wide range of timescales, from picoseconds to potentially seconds, in a single transient absorption experiment without the need to move any delay stage. Experiments become possible that address a largely unexplored aspect of many photochemical reactions, in particular in the context of photo-catalysis as well as photoactive proteins, where an initial femtosecond trigger very often initiates a long-lasting cascade of follow-up processes. The approach is very versatile and allows us to synchronize very different lasers, such as a Ti:Sa amplifier and a 100 kHz Yb-laser system. The jitter of the synchronization, and therewith the time-resolution in the transient experiment, lies in the range from 1 to 3 ps, depending on the method. For illustration, transient IR measurements of the excited state solvation and decay of a metal carbonyl complex as well as the full reaction cycle of bacteriorhodopsin are shown. The pros and cons of the various methods are discussed, with regard to the scientific question one might want to address, and also with regard to the laser systems that might be already existent in a laser lab.

We describe an optimized setup for two-dimensional (2D) IR spectroscopy, which can be implemented at low additional cost and with standard optics in any laboratory equipped for femtosecond mid-IR spectroscopy. An interferometer produces a pair of intense pump pulses, whose interferogram is simultaneously recorded and directly yields the relative phase needed for the calculation of absorptive 2D spectra. We analyze different sampling methods based on a realistic noise model and introduce fast population time modulation as an alternative to the use of choppers in the suppression of scatter. Signal levels are compared to those of a photon-echo setup.

Linear dichroism and birefringence artifacts are a major source of concern in transient circular dichroism measurements. They mainly arise from interaction of an imperfectly circular polarized probe beam with a nonisotropic sample. We present in this article a procedure to generate mid-IR pulses of highly symmetric left and right handed circular or elliptical polarization states for transient VCD measurements. An infrared femtosecond laser source is synchronized to the natural frequency of a photo elastic modulator. Residual static birefringence of the modulator and the sample cell can be largely compensated by carefully controlling the arrival time of the ultrashort probe pulses at the modulator. Chirality 21:E298–E306, 2009. © 2009 Wiley-Liss, Inc.

The conformations of a protected tetrathiopeptide have been analysed by isotope labelling and two‐dimensional infrared spectroscopy (2D‐IR). It has been found that Boc‐Ala‐Gly(S)‐Ala‐Aib‐OMe in acetonitrile, as well as its oxopeptide analogue, can adopt a hydrogen‐bonded loop conformation in coexistence with less restricted structures. The two types of conformations interconvert too quickly to be resolved on the nuclear magnetic resonance (NMR) timescale, but give rise to different cross peaks in two‐dimensional infrared spectra. The hydrogen bond between the Boc terminal group and the amide proton of Aib can be broken by photoisomerisation of the thioamide bond.

The dynamics of the ferric CN complexes of the heme proteins Myoglobin and Hemoglobin I from the clam Lucina pectinata upon Soret band excitation is monitored using infrared and broad band visible pump-probe spectroscopy. The transient response in the UV-vis spectral region does not depend on the heme pocket environment and is very similar to that known for ferrous proteins. The main feature is an instantaneous, broad, short-lived absorption signal that develops into a narrower red-shifted Soret band. Significant transient absorption is also observed in the 360–390nm range. At all probe wavelengths the signal decays to zero with a longest time constant of 3.6ps. The infrared data on MbCN reveal a bleaching of the C≡N stretch vibration of the heme-bound ligand, and the formation of a five-times weaker transient absorption band, 28cm−1 lower in energy, within the time resolution of the experiment. The MbC≡N stretch vibration provides a direct measure for the return of population to the ligated electronic (and vibrational) ground state with a 3–4ps time constant. In addition, the CN-stretch frequency is sensitive to the excitation of low frequency heme modes, and yields independent information about vibrational cooling, which occurs on the same timescale.

We discuss the cage induced stabilisation of fragments in excited electronic states following the UV-dissociation of ICN in cryogenic matrices. Emission spectra recorded upon &z.Atilde;-band excitation of ICN in solid neon, argon and krypton exhibit a long progression of broad bands due to a weakly bound electronically excited state, presumably one of the low-lying triplet states ³Π₁ or ³Π₂ of ICN. A lifetime analysis favours the ³Π₂ state. Molecular dynamics with quantum transitions (MDQT) simulations were conducted on six coupled electronic potential energy surfaces in a matrix of 498 argon atoms. Although a complete potential energy surface for the ³Π₂ state is not available, it is known to be very similar to the ³Π₁ one. Therefore only the 6 available [³Π₁ (A', A"), ³Π_0⁺, ¹Π₁ (A', A"), X¹Σ⁺] ab initio electronic potential energy surfaces were considered. The results predict a 2% probability of stabilisation in the shallow minimum of the triplet excited state. The molecule adopts a linear ICN configuration with a mean value of the I–CN distance far away from the absorption Franck–Condon region. The simulations also deliver insight into the mechanism of cage-induced population trapping in excited state surfaces, which is not accessible in the gas phase.

We studied the stability and folding and unfolding kinetics of the tryptophan zipper, containing different double thioamide subsitutions. Conformation change was triggered by photoisomerization of an integrated AMPP photoswitch in the turn region of the hairpin, and transient spectra were recorded in the deep UV and the mid-IR, covering the time window of the (un)folding transition from picoseconds to tens of microseconds. Thio-substitution of inward-pointing backbone carbonyls was found to strongly destabilize the β-hairpin structures, whereas molecules with two outward pointing thio-carbonyls showed similar or enhanced stability with respect to the unsubstituted sequence, which we attribute to stronger interstrand hydrogen bonding. Thiolation of the two Trp residues closest to the turn can even prevent the opening of the hairpin after cis–trans isomerization of the switch. The circular dichroism due to the two thioamide ππ* transitions is spectrally well-separated from the aromatic tryptophan signal. It changes upon photoswitching, reflecting a local change in coupling and geometry.

We present an experimental setup to detect transient vibrational circular dichroism signals. A femtosecond laser system is synchronized to a photoelastic modulator to produce alternating left- and right-handed circularly polarized mid-IR pulses at 1 kHz repetition rate. Transient changes in the circular dichroism of the CH-stretch vibrations of a cobalt-sparteine complex were probed in a proof-of-principle experiment and are clearly distinct from conventional transient absorption changes.