Observation of sub-100ps conformational changes in photolyzed carbonmonoxy-myoglobin probed by time-resolved circular dichroism

Time-resolved circular dichroism of excitonic systems: theory and experiment on an exemplary squaraine polymer

Experimental and theoretical foundations for femtosecond time-resolved circular dichroism (TRCD) spectroscopy of excitonic systems are presented. In this method, the system is pumped with linearly polarized light and the signal is defined as the difference between the transient absorption spectrum probed with left and with right circularly polarized light. We present a new experimental setup with a polarization grating as key element to generate circularly polarized pulses. Herein the positive (negative) first order of the diffracted light is left-(right-)circularly polarized and serves as a probe pulse in a TRCD experiment. The grating is capable of transferring ultrashort broadband pulses ranging from 470 nm to 720 nm into two separate beams with opposite ellipticity. By applying a specific chopping scheme we can switch between left and right circular polarizations and detect transient absorption (TA) and TRCD spectra on a shot-to-shot basis simultaneously. We perform experiments on a squaraine polymer, investigating excitonic dynamics, and we develop a general theory for TRCD experiments of excitonically coupled systems that we then apply to describe the experimental data in this particular example. At a magic angle of 54.7° between the pump-pulse polarization and the propagation direction of the probe pulse, the TRCD and TA signals become particularly simple to analyze, since the orientational average over random orientations of complexes factorizes into that of the interaction with the pump and the probe pulse, and the intrinsic electric quadrupole contributions to the TRCD signal average to zero for isotropic samples. Application of exciton theory to linear absorption and to linear circular dichroism spectra of squaraine polymers reveals the presence of two fractions of polymer conformations, a dominant helical conformation with close interpigment distances that are suggested to lead to short-range contributions to site energy shifts and excitonic couplings of the squaraine molecules, and a fraction of unfolded random coils. Theory demonstrates that TRCD spectra of selectively excited helices can resolve state populations that are practically invisible in TA spectroscopy due to the small dipole strength of these states. A qualitative interpretation of TRCD and TA spectra in the spectral window investigated experimentally is offered. The 1 ps time component found in these spectra is related to the slow part of exciton relaxation obtained between states of the helix in the low-energy half of the exciton manifold. The dominant 140 ps time constant reflects the decay of excited states to the electronic ground state.

Molecular Chirality and Its Monitoring by Ultrafast X-ray Pulses

Major advances in X-ray sources including the development of circularly polarized and orbital angular momentum pulses make it possible to probe matter chirality at unprecedented energy regimes and with Ångström and femtosecond spatiotemporal resolutions. We survey the theory of stationary and time-resolved nonlinear chiral measurements that can be carried out in the X-ray regime using tabletop X-ray sources or large scale (XFEL, synchrotron) facilities. A variety of possible signals and their information content are discussed.

Multiscale conformational dynamics probed by time-resolved circular dichroism from seconds to picoseconds

Time-resolved circular dichroism has been developed for a few decades to investigate rapid conformational changes in (bio)molecules. In our group, we have come up with several experimental set-ups allowing us to study pico-nanosecond local phenomena in molecular systems as well as much slower effects occurring in proteins and DNA in the folding processes. After an overview of the worldwide realizations in this domain, we present emblematic experiments that we have carried out, spanning time domain from picoseconds to seconds.

Ultrafast Broadband Transient Absorption and Circular Dichroism Reveal Relaxation of a Chiral Copolymer

We present a study of the photoinduced dynamics of the chiral polyfluorene-phenylene copolymer PFPh in THF and in cholesteric thin films. After photoexcitation at 370 nm in THF, ultraviolet-visible-near-infrared (UV-vis-NIR) transient absorption spectra show fast subpicosecond to picosecond intrachain migration of singlet excitons, solvation dynamics, and an exciton lifetime of 410 ps. The PFPh thin film features also interchain singlet exciton migration and exhibits shorter (2.1 and 240 ps) and longer lifetime components (2800 ps, interchain recombination). Furthermore, a setup for ultrafast UV-vis broadband transient circular dichroism (TrCD) spectroscopy has been developed. Fast supramolecular relaxation processes are observed, which are linked to changes in the anisotropic polarizability and pitch length of the cholesteric film. Such combined ultrafast transient CD and absorption experiments hold promise to reveal not only details of relaxation processes in supramolecular arrangements but also structural rearrangements of chiral molecular systems featuring CD signals in the UV-vis region.

Time resolved transient circular dichroism spectroscopy using synchrotron natural polarization

Ultraviolet (UV) synchrotron radiation circular dichroism (SRCD) spectroscopy has made an important contribution to the determination and understanding of the structure of bio-molecules. In this paper, we report an innovative approach that we term time-resolved SRCD (tr-SRCD), which overcomes the limitations of current broadband UV SRCD setups. This technique allows accessing ultrafast time scales (down to nanoseconds), previously measurable only by other methods, such as infrared (IR), nuclear magnetic resonance (NMR), fluorescence and absorbance spectroscopies, and small angle X-ray scattering (SAXS). The tr-SRCD setup takes advantage of the natural polarization of the synchrotron radiation emitted by a bending magnet to record broadband UV CD faster than any current SRCD setup, improving the acquisition speed from 10 mHz to 130 Hz and the accessible temporal resolution by several orders of magnitude. We illustrate the new approach by following the isomer concentration changes of an azopeptide after a photoisomerization. This breakthrough in SRCD spectroscopy opens up a wide range of potential applications to the detailed characterization of biological processes, such as protein folding and protein-ligand binding.

Ultrafast photoinduced energy and charge transfer

After presenting the basic theoretical models of excitation energy transfer and charge transfer, I describe some of the novel experimental methods used to probe them. Finally, I discuss recent results concerning ultrafast energy and charge transfer in biological systems, in chemical systems and in photovoltaics based on sensitized transition metal oxides.

Broad-Band Ultraviolet CD Spectroscopy of Ultrafast Peptide Backbone Conformational Dynamics

The far-UV spectral window widely used for the conformational analysis of biomolecules is not easily covered with broad-band lasers. This has made it difficult to use circular dichroism (CD) spectroscopy to directly follow fast structure changes. By combining transient CD spectroscopy in the deep-UV with thioamide substitution, we demonstrate a method to overcome this difficulty. We investigated a dipeptide whose two carbonyl oxygen atoms were replaced by sulfur, red-shifting the strong lowest-lying ππ* transitions into the more accessible 250-370 nm spectral window. Coupling of the two thioamide units cannot be resolved by achiral 2D-UV spectroscopy, but it gives rise to a pronounced bisignate CD spectrum. The transient CD spectra reveal weakening of this coupling in the electronically excited state, where conformational constraints are released. Our results show that direct local probing of fast backbone conformational change via CD spectroscopy is possible in combination with site-selective thio substitution in peptides and proteins.

Ultrafast myoglobin structural dynamics observed with an X-ray free-electron laser

Light absorption can trigger biologically relevant protein conformational changes. The light-induced structural rearrangement at the level of a photoexcited chromophore is known to occur in the femtosecond timescale and is expected to propagate through the protein as a quake-like intramolecular motion. Here we report direct experimental evidence of such 'proteinquake' observed in myoglobin through femtosecond X-ray solution scattering measurements performed at the Linac Coherent Light Source X-ray free-electron laser. An ultrafast increase of myoglobin radius of gyration occurs within 1 picosecond and is followed by a delayed protein expansion. As the system approaches equilibrium it undergoes damped oscillations with a ~3.6-picosecond time period. Our results unambiguously show how initially localized chemical changes can propagate at the level of the global protein conformation in the picosecond timescale.

Optical discrimination of racemic from achiral solutions

We demonstrate purely optical discrimination between achiral and racemic solutions by selectively triggering an asymmetric photoreaction with femtosecond laser pulses.

Sub-100-ps structural dynamics of horse heart myoglobin probed by time-resolved X-ray solution scattering

Here we report sub-100-ps structural dynamics of horse heart myoglobin revealed by time-resolved X-ray solution scattering. By applying the time-slicing scheme to the measurement and subsequent deconvolution, we investigate the protein structural dynamics that occur faster than the X-ray temporal pulse width of synchrotrons (~100 ps). The singular value decomposition analysis of the experimental data suggests that two structurally distinguishable intermediates are formed within 100 ps. In particular, the global structural change occurring on the time scale of 70 ps is identified.

Broadband near UV to visible optical activity measurement using self-heterodyned method

We demonstrate that broadband electronic optical activity can be measured with supercontinuum light pulse generated by a femtosecond pump (800 nm). It is the self-heterodyned detection technique that enables us to selectively measure the real (optical rotatory dispersion, ORD) or imaginary (circular dichroism, CD) part of the chiroptical susceptibility by controlling the incident polarization state. The single-shot-based measurement that is capable of correcting power fluctuations of the continuum light is realized by using a fast CCD detector and a polarizing beam splitter. Particularly, non-differential scheme used does not rely on any polarization-switching components. We anticipate that this broadband CD/ORD spectrometry with intrinsically ultrafast time-resolution will be applied to a variety of ultrafast chiroptical dynamics studies.

Picosecond primary structural transition of the heme is retarded after nitric oxide binding to heme proteins

We investigated the ultrafast structural transitions of the heme induced by nitric oxide (NO) binding for several heme proteins by subpicosecond time-resolved resonance Raman and femtosecond transient absorption spectroscopy. We probed the heme iron motion by the evolution of the iron-histidine Raman band intensity after NO photolysis. Unexpectedly, we found that the heme response and iron motion do not follow the kinetics of NO rebinding. Whereas NO dissociation induces quasi-instantaneous iron motion and heme doming (<0.6 ps), the reverse process results in a much slower picosecond movement of the iron toward the planar heme configuration after NO binding. The time constant for this primary domed-to-planar heme transition varies among proteins (approximately 30 ps for myoglobin and its H64V mutant, approximately 15 ps for hemoglobin, approximately 7 ps for dehaloperoxidase, and approximately 6 ps for cytochrome c) and depends upon constraints exerted by the protein structure on the heme cofactor. This observed phenomenon constitutes the primary structural transition in heme proteins induced by NO binding.

Ultrafast dynamics of ligands within heme proteins

Physiological bond formation and bond breaking events between proteins and ligands and their immediate consequences are difficult to synchronize and study in general. However, diatomic ligands can be photodissociated from heme, and thus in heme proteins ligand release and rebinding dynamics and trajectories have been studied on timescales of the internal vibrations of the protein that drive many biochemical reactions, and longer. The rapidly expanding number of characterized heme proteins involved in a large variety of functions allows comparative dynamics-structure-function studies. In this review, an overview is given of recent progress in this field, and in particular on initial sensing processes in signaling proteins, and on ligand and electron transfer dynamics in oxidases and cytochromes.

Primary protein response after ligand photodissociation in carbonmonoxy myoglobin

Time-resolved UV resonance Raman (UVRR) spectroscopic studies of WT and mutant myoglobin were performed to reveal the dynamics of protein motion after ligand dissociation. After dissociation of carbon monoxide (CO) from the heme, UVRR bands of Tyr showed a decrease in intensity with a time constant of 2 ps. The intensity decrease was followed by intensity recovery with a time constant of 8 ps. On the other hand, UVRR bands of Trp residues located in the A helix showed an intensity decrease that was completed within the instrument response time. The intensity decrease was followed by an intensity recovery with a time constant of approximately 50 ps and lasted up to 1 ns. The time-resolved UVRR study of the myoglobin mutants demonstrated that the hydrophobicity of environments around Trp-14 decreased, whereas that around Trp-7 barely changed in the primary protein response. The present data indicate that displacement of the E helix toward the heme occurs within the instrument response time and that movement of the FG corner takes place with a time constant of 2 ps. The finding that the instantaneous motion of the E helix strongly suggests a mechanism in which protein structural changes are propagated from the heme to the A helix through the E helix motion.

Subpicosecond UV spectroscopy of carbonmonoxy-myoglobin: absorption and circular dichroism studies

A thorough absorption and circular dichroism study is performed in carbonmonoxy-myoglobin with a sub-picosecond visible pump, ultraviolet probe experiment. Differential absorption in the 220-360 nm range shows that the time-resolved response mainly comes from the heme and that aromatic amino acids do not contribute significantly. Time-resolved CD at 260 nm shows no dynamics and confirms this result. On the contrary, a strong CD dynamics is observed at 230 nm. This signal could originate from transient deformation of the alpha-helices in the protein.

Time-resolved circular dichroism in carbonmonoxy-myoglobin: The central role of the proximal histidine

A calculation of the circular dichroism (CD) spectra of carbonmonoxy- and deoxy-myoglobin is carried out in relation to a time-resolved CD experiment. This calculation allows us to assign a dominant role to the proximal histidine in the definition of the electronic normal modes and to interpret the transient CD structure observed in a strain of the proximal histidine. This strain builds up in 10 ps and relaxes in 50 ps as the protein evolves towards its deoxy form.

Calculation of the circular dichroism spectra of carbon monoxy- and deoxy myoglobin: Interpretation of a time-resolved circular dichroism experiment

A calculation of the circular dichroism (CD) spectra of carbon monoxy- and deoxy myoglobin is carried out in relation with a time-resolved CD experiment. The calculation is based on the polarizability theory and the parameters are adjusted to fit the experimental absorption and CD spectra. By performing the calculation for intermediate configurations of the protein, we are able to propose an explanation of the CD structure observed on a sub-100 ps time scale. The role of the proximal histidine is, in particular, clearly demonstrated in the first step of the myoglobin relaxation from its liganded to it deliganded form.