Selected papers

Ultrafast Tryptophan-to-Heme Electron Transfer in Myoglobins Revealed by UV 2D Spectroscopy

Science 339, 1586-1589 Cristina Consani, Gerald Auböck, Frank van Mourik, Majed Chergui

 

Tryptophan is commonly used to study protein structure and dynamics, such as protein folding, as a donor in fluorescence resonant energy transfer (FRET) studies. By using ultra-broadband ultrafast two-dimensional (2D) spectroscopy in the ultraviolet (UV) and transient absorption in the visible range, we have disentangled the excited state decay pathways of the tryptophan amino acid residues in ferric myoglobins (MbCN and metMb). Whereas the more distant tryptophan (Trp7) relaxes by energy transfer to the heme, Trp14 excitation predominantly decays by electron transfer to the heme. The excited Trp14→heme electron transfer occurs in <40 picoseconds with a quantum yield of more than 60%, over an edge-to-edge distance below ~10 angstroms, outcompeting the FRET process. Our results raise the question of whether such electron transfer pathways occur in a larger class of proteins.

Ultrabroadband femtosecond two-dimensional ultraviolet transient absorption

Optics Letters (ISSN: 0146-9592), vol. 37, p. 2337-2339

G. Auböck, C. Consani, F. van Mourik and M. Chergui

We present a broadband two-dimensional transient absorption setup for the UV around 300 nm with a time resolution of 150 fs. A narrowband, frequency tunable pump pulse and a broadband probe pulse are generated from the output of a noncollinear optical parametric amplifier operated at 20 kHz repetition rate and combined in a spectrally resolved transient absorption experiment. The high repetition rate and low noise of the setup allow us to acquire high quality two-dimensional data as a function of time delay with an unsurpassed frequency window of 10,000 and 8000 cm(-1) along the probe and pump axis, respectively. The performance of the setup is demonstrated on 2,5-Diphenyloxazol dissolved in cyclohexane.

A high-repetition rate scheme for synchrotron-based picosecond laser pump/x-ray probe experiments on chemical and biological systems in solution

Lima, Frederico A. ; Milne, Christopher J. ; Amarasinghe, Dimali C. V. ; Rittmann-Frank, Mercedes Hannelore ; der Veen, Van ; Renske, M. ; Reinhard, Marco ; Pham, Van Thai ; Karlsson, Lotta Susanne ; Johnson, Steve L. ; Grollman, Daniel ; Borca, Camelia ; Huthwelker, Thomas ; Janousch, Markus ; Van Mourik, Frank ; Abela, Rafael ; Chergui, Majed

Review of Scientific Instruments 82, p. 63111, 2011

We present the extension of time-resolved optical pump/x-ray absorption spectroscopy (XAS) probe experiments towards data collection at MHz repetition rates. The use of a high-power picosecond laser operating at an integer fraction of the repetition rate of the storage ring allows exploitation of up to two orders of magnitude more x-ray photons than in previous schemes based on the use of kHz lasers. Consequently, we demonstrate an order of magnitude increase in the signal-to-noise of time-resolved XAS of molecular systems in solution. This makes it possible to investigate highly dilute samples at concentrations approaching physiological conditions for biological systems. The simplicity and compactness of the scheme allows for straightforward implementation at any synchrotron beamline and for a wide range of x-ray probe techniques, such as time-resolved diffraction or x-ray emission studies.

Vibrational Relaxation and Intersystem Crossing of Binuclear Metal Complexes in Solution

 J. Am. Chem. Soc., 2011, 133 (2), pp 305–315

Renske M. van der Veen, Andrea Cannizzo, Frank van Mourik, Antonn Vlek, Jr., and Majed Chergui

The ultrafast vibrational− electronic relaxation upon excitation into the singlet 1A2u (dσ*→pσ) excited state of the d8−d8 binuclear complex [Pt2(P2O5H2)4]4− has been investigated in different solvents by femtosecond polychromatic fluorescence up-conversion and femtosecond broadband transient absorption (TA) spectroscopy. Both sets of data exhibit clear signatures of vibrational relaxation and wave packet oscillations of the Pt−Pt stretch vibration in the 1A2u state with a period of 224 fs, that decay on a 1−2 ps time scale, and of intersystem crossing (ISC) into the 3A2u state. The vibrational relaxation and ISC times exhibit a pronounced solvent dependence. We also extract from the TA measurements the spectral distribution of the wave packet at a given delay time, which reflects the distribution of Pt−Pt bond distances as a function of time, i.e., the structural dynamics of the system. We clearly establish the vibrational relaxation and coherence decay processes, and we demonstrate that PtPOP represents a clear example of a harmonic oscillator that does not comply with the optical Bloch description due to very efficient coherence transfer between vibronic levels. We conclude that a direct Pt−solvent energy dissipation channel accounts for the vibrational cooling in the singlet state. ISC from the 1A2u to the 3A2u state is induced by spin-vibronic coupling with a higher-lying triplet state and/or (transient) symmetry breaking in the 1A2u excited state. The particular structure, energetics, and symmetry of the molecule play a decisive role in determining the relatively slow rate of ISC, despite the large spin−orbit coupling strength of the Pt atoms.

Electron Localization Dynamics in the Triplet Excited State of [RuII(bpy)3]2+ in Aqueous Solution

Chemistry-A European Journal 16, (20), 5889-5894 (2010)

Marc-Etienne Moret, Ivano Tavernelli, Majed Chergui and Ursula Röthlisberger

Solvent-induced electron localization and dynamics in the triplet metal-to-ligand charge-transfer (3MLCT) state of [Ru(bpy)3]2+ (bpy=2,2′-bipyridine) have been investigated by means of Car–Parrinello QM/MM simulations. In the gas phase, the unpaired electron is found to be delocalized over the three bipyridine ligands, but in solution the excited electron is, most of the time, located on a subset of ligands.

Relaxation Dynamics of Tryptophan in Water: A UV Fluorescence Up-Conversion and Molecular Dynamics Study

 Journal Of Physical Chemistry A, vol. 114, 2010, p. 9034-9042, 2010

Braem, O. ; Oskouei, A. Ajdarzadeh ; Tortschanoff, A. ; van Mourik, F. ; Madrid, M. ; Echave, J. ; Cannizzo, A. ; Chergui, M.

We report on an ultrafast experimental and simulations study of the early relaxation events of photoexcited tryptophan in water. Experimentally, we used fluorescence up-conversion in both polychromatic and single wavelength detection modes in the 300-480 nm range with polarization dependence. We report on the time evolution of the Stokes shift, bandwidth, and anisotropy from tens of femtoseconds to picoseconds. These observables contain signatures of the simultaneous occurrence of intramolecular and solvent-molecule interactions, which we disentangle with the help of nonequilibrium molecular dynamics simulations. We also observe a breakdown of the linear response approximation to describe our results.
Keyword(s): Resonance Energy-Transfer, Excited-State Dynamics, Particle Mesh Ewald, Echo Peak Shift, Hydration Dynamics, Solvation Dynamics, Femtosecond Resolution, Hybrid Simulations, Biological Water, Protein Surface

Vibrational coherences and relaxation in the high-spin state of aqueous [Fell(bpy)3]2+


Angewandte Chemie International Edition, vol. 48, num. 39, 7184-7187, 2009

Consani, C. ; Prémont-Schwarz, M. ; Cannizzo, A. ; El Nahhas, A ; van Mourik, F. ; Bressler, C. ; Chergui, M.

Femtosecond excitation of the singlet states of aqueous [FeII(bpy)3]2+ (bpy=2,2-bipyridine) leads to the formation of a vibrationally hot quintet state that exhibits wave-packet dynamics arising from a chelate-ring and bending mode. The vibrational relaxation involves at least two modes: the FeN stretching mode (see picture) and the coherently excited chelate-ring and bending mode, which relax on different time scales.

Structural determination of photochemically active diplatinum molecule by time-resolved EXAFS spectroscopy

Angewandte Chemie International Edition 48 (2009) 2711-2714

R. M. van der Veen, C. J. Milne, A. El Nahhas, F. A. Lima, V-T. Pham, J. Best, J.A. Weinstein, C. N. Borca, R. Abela, C. Bressler and M. Chergui

A large contraction of the PtPt bond in the triplet excited state of the photoreactive [Pt2(P2O5H2)4]4− ion is determined by time-resolved X-ray absorption spectroscopy (see picture). The strengthening of the PtPt interaction is accompanied by a weakening of the ligand coordination bonds, resulting in an elongation of the platinum–ligand bond that is determined for the first time

Femtosecond XANES Study of the Light-Induced Spin Crossover Dynamics in an Iron(II) Complex

Science, vol. 323, num. 5913, 2009, p. 489-492

Bressler, Ch ; Milne, C. ; Pham, V. T. ; ElNahhas, A. ; van der Veen, R. M. ; Gawelda, W. ; Johnson, S. ; Beaud, P. ; Grolimund, D. ; Kaiser, M. ; Borca, C. N. ; Ingold, G. ; Abela, R. ; Chergui, M.

X-ray absorption spectroscopy is a powerful probe of mol. structure, but it has previously been too slow to track the earliest dynamics after photoexcitation. We investigated the ultrafast formation of the lowest quintet state of aq. iron(II) tris(bipyridine) upon excitation of the singlet metal-to-ligand-charge-transfer (1MLCT) state by femtosecond optical pump/x-ray probe techniques based on x-ray absorption near-edge structure (XANES). By recording the intensity of a characteristic XANES feature as a function of laser pump/x-ray probe time delay, we find that the quintet state is populated in about 150 fs. The quintet state is further evidenced by its full XANES spectrum recorded at a 300-fs time delay. These results resolve a long-standing issue about the population mechanism of quintet states in iron(II)-based complexes, which we identify as a simple 1MLCT->3MLCT->5T cascade from the initially excited state. The time scale of the 3MLCT->5T relaxation corresponds to the period of the iron-nitrogen stretch vibration.

Solvent-assisted Photoacidity in “Real-Time”

 

Solvent-assisted Photoacidity in “Real-Time”

“Ultrafast solvent-assisted level crossing in 1-naphthol” scheduled for publication in Angewandte Chemie International edition (DOI: 10.1002/anie.201301931)

Upon absorption of light, molecules become excited to an electronic state. This excitation is often the trigger of a sequence of events where the molecular systems change their electronic excited states, en route to either the electronic ground state or to some photoproduct state. Such changes in electronic states are called non-adiabatic level crossing.  Non-adiabatic electronic transitions govern the underlying dynamics in many photophysical and photochemical processes. Prime examples are the photoinduced electron and proton transfer, which lead to a rearrangement of charges. Ultrafast non-adiabatic electronic transitions are part of the protection mechanisms of DNA against UV radiation damage. Many such non-adiabatic electronic transitions are strongly influenced by the nature and motions of the surrounding solvent molecules.

In a collaborative effort, researchers from the Ecole Polytechnique Fédérale de Lausanne, the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (Berlin) and Yale University have – for the first time – determined the time scale for ultrafast electronic excited state level crossing in a classic case: the solvent-driven level crossing in 1-naphthol. This solvent driven level crossing, only observed in media of high polarity (water, alcohols), has been postulated as the underlying mechanism for the much higher photoacidity of 1-naphthol compared to other photoacid molecules.

Photoacidity is the phenomenon by which the acidity of an organic molecule is greatly enhanced after absorption of light. Based on estimates, the pKa-value of 1-napthol changes from 8-9 for the electronic ground state to about 3 for the electronic excited state. However, when dissolved in water, a pKa-value of 0.5 was found in the electronic excited state.

In a combined experimental and theoretical study, it has become clear that this apparent discrepancy is indeed caused by a solvent-controlled level crossing between the lowest two singlet excited electronic states. This level crossing is of ultrafast nature: in the polar solvent dimethylsulfoxide, as evidenced by the time-dependent fluorescence anisotropy, it occurs with a 60 fs time constant!  Inspection of the transient behaviour of the IR-active C-O stretching mode shows that the hydrogen bond between 1-naphthol and dimethylsulfoxide molecules strongly increases, which is clearly not the case in weakly polar solvents, such as chloroform.

These findings will be of interest to any chemist, involved with experiments or theory of non-adiabatic electronic transitions. They offer a clear-cut example of solvent assisted non-adiabatic transitions, which are most probably the rule rather than the exception in liquid phase photochemistry. In addition, the results will propel many activities using state-of-the-art calculations of such dynamics. One aspect is that the electronic excited state structure calculations suggest a significant amount of excited state level mixing, controlled by the polarity of the surrounding solvent. A challenge will be to implement the dynamical nature of the solvent shells into a combined QM/MM description with which such an ultrafast nonadiabatic solvent-assisted level crossing can be described.

 

“Ultrafast solvent-assisted level crossing in 1-naphthol” scheduled for publication in Angewandte Chemie International edition (DOI: 10.1002/anie.201301931)

 

The absorption of light by large molecules is the trigger of a sequence of events where electronic excited states change, during what are called non-adiabatic transitions, en route to either the electronic ground state or to some photoproduct state. Non-adiabatic transitions are often strongly influenced by the nature and motions of the surrounding solvent molecules. In a recent paper, researchers of the LSU-EPFL, the Max-Born Institut in Berlin and Yale University determined the time scale for ultrafast electronic excited state level crossing in a classic case: the solvent-driven level crossing in 1-naphthol. This solvent driven level crossing has been postulated as the underlying mechanism for the much higher photoacidity of 1-naphthol compared to other photoacid molecules. They found that the level crossing occurs in about 60 femtoseconds! These findings offer a clear-cut example of solvent assisted non-adiabatic transitions, which are most probably the rule rather than the exception in liquid phase photochemistry.