Frank van Mourik

Vibrational Coherences and Relaxation in the High‐Spin State of Aqueous [FeII(bpy)3]2+

Dizzy cooling: 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.

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

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.

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

Fretting About Electrons The FRET technique (Fluorescence Resonance Energy Transfer) is widely used to probe structural dynamics in large macromolecules such as proteins. Essentially, the technique relies on photoexciting a donor chromophore and then watching for signs of energy transfer to an acceptor chromophore elsewhere in the framework. Consani et al. (p. 1586, published online 7 February; see the Perspective by Winkler) now show, using a sophisticated type of broadband time-resolved ultraviolet spectroscopy, that in myoglobin, an excited tryptophan residue relaxes by electron, rather than energy, transfer. The distinction is generally difficult to observe but has strong bearing on the applicability of FRET in this and analogous systems. Relaxation in a photoexcited protein by electron transfer may limit the generality of a common energy transfer–based probe. [Also see Perspective by Winkler] 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.

A setup for ultrafast time-resolved x-ray absorption spectroscopy

We present a setup which allows the measurement of time-resolved x-ray absorption spectra with picosecond temporal resolution on liquid samples at the Advanced Light Source at Lawrence Berkeley National Laboratories. The temporal resolution is limited by the pulse width of the synchrotron source. We characterize the different sources of noise that limit the experiment and present a single-pulse detection scheme.

Pigment organization and energy transfer in green bacteria. 2. Circular and linear dichroism spectra of protein-containing and protein-free chlorosomes isolated from Chloroflexus aurantiacus strain Ok-70-fl

We have measured the circular dichroism (CD) and linear dichroism (LD) spectra of chlorosomes isolated from Chloroflexus aurantiacus strain Ok-70-fl obtained by two different isolation procedures. The gel-electrophoretic filtration procedure yields chlorosomes that are essentially free of BChl a 790 and proteins, while isolation by sucrose density gradient centrifugation yields the conventional chlorosome preparations. The LD spectra of the two kinds of preparation were very similar. In both cases the Q y LD signals correspond to an average angle between the BChl- c -Q y transition and the long axis of the chlorosome of approx. 15 ± 10°. In contrast to the LD spectra, the CD spectra of different preparations (membranes, BChl- a -free chlorosomes, BChl- a -containing chlorosomes) show pronounced differences both in the ellipticity as well as in the shape of the spectra and the number of maxima. However, these differences are not caused by the isolation procedure or the detergents used. We show that even freshly prepared membranes (of different, parallel grown batch cultures) give rise to very different CD spectra. The set of different CD spectra we obtained could be simulated well by linear combinations of two basic spectra. This strongly suggests that the variations in the CD spectra are caused by a variation in the relative amounts of two different species, two different types of chlorosome, or possibly by two different types of pigment aggregate within the chlorosomes.

LOW-INTENSITY PUMP-PROBE SPECTROSCOPY ON THE B800 TO B850 TRANSFER IN THE LIGHT HARVESTING 2 COMPLEX OF RHODOBACTER SPHAEROIDES

Abstract The isolated LH2 (B800–850) complex of Rb. sphaeroides has been studied at 77 K using low-intensity one-colour pump-probe spectroscopy. Delta absorbance transients were measured at several wavelengths within the B800 band. In the red part of the band, the excited B800 population decays mono-exponentially with a lifetime of about 1.2 ± 0.1 ps. More to the blue much faster rates are found which are ascribed to downhill energy-transfer among the B800 pigments. To our surprise, increasing the average intensity from 4 W/cm 2 to higher excitation densities significantly slows the decay of B800 ∗ . Since this effect is permanent we conclude that it is due to some form of photodamage. We propose that this observation explains the slower decay reported by others.

Vibrational relaxation and intersystem crossing of binuclear metal complexes in solution.

The ultrafast vibrational-electronic relaxation upon excitation into the singlet (1)A(2u) (dσ*→pσ) excited state of the d(8)-d(8) binuclear complex [Pt(2)(P(2)O(5)H(2))(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 (1)A(2u) state with a period of 224 fs, that decay on a 1-2 ps time scale, and of intersystem crossing (ISC) into the (3)A(2u) 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 (1)A(2u) to the (3)A(2u) state is induced by spin-vibronic coupling with a higher-lying triplet state and/or (transient) symmetry breaking in the (1)A(2u) 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.

Multiphoton-Excited Luminescent Lanthanide Bioprobes: Two- and Three-Photon Cross Sections of Dipicolinate Derivatives and Binuclear Helicates

Multiphoton excited luminescent properties of water-soluble Eu(III) and Tb(III) complexes with derivatives of dipicolinic acid functionalized with a polyoxyethylene pendant arm and terminal groups, [Eu(L(OMe))(3)](3-), [Eu(L(NH2))(3)](3-), and [Tb(L(OH))(3)](3-), as well as of binuclear helicates with overall composition [Ln(2)(L(CX))(3)] (X = 2, 5) are investigated. Characteristic emission from the (5)D(0) and (5)D(4) excited levels of Eu(III) and Tb(III), respectively, upon approximately 800 nm excitation results from three-photon absorption (3PA) for [Eu(L(OMe))(3)](3-), [Eu(L(NH2))(3)](3-), [Tb(L(OH))(3)](3-), and [Ln(2)(L(C2))(3)], while luminescence from [Eu(2)(L(C5))(3)] is induced by two-photon absorption (2PA) owing to its 1PA spectrum extending further into the visible. The 3PA cross sections have been determined and are the first ones reported for lanthanide complexes: (i) those of Eu(III) and Tb(III) bimetallic helicates [Ln(2)(L(C2))(3)] are 20 times larger compared to the corresponding values for tris(dipicolinates); (ii) derivatization of dipicolinic acid for Tb(III) complexes has almost no influence on the 3PA cross section; however, for Eu(III) complexes a approximately 2 times decrease is observed. The feasibility of [Eu(2)(L(C5))(3)] as multiphoton luminescence bioprobe is demonstrated by two-photon scanning microscopy imaging experiments on HeLa cells incubated with this bimetallic helicate.

Exciton interactions in the light-harvesting antenna of photosynthetic bacteria studied with triplet-singlet spectroscopy and singlet-triplet annihilation on the B820 subunit form of Rhodospirillum rubrum

The pigment-protein complex, B820, isolated from the long-wavelength antenna (LH-1) of the photosynthetic purple bacterium Rhodospirillum rubrum was studied with polarized nanosecond laser spectroscopy. The polarized triplet (T)-singlet (S) spectrum was obtained at 77 K. The spectrum is significantly different from the T-S spectrum of monomeric BChl a , and can be explained by assuming that upon excitation the absorption band at 825 nm, which is due to a dimeric pair of BChl a molecules, disappears from the absorption spectrum, and is replaced by a monomer absorption band peaking at 809 nm. From the energy-dependence of the triplet yield, the high polarization of the bleaching, and the absence of singlet-triplet quenching we conclude that the B820 complex contains only one dimer of interacting BChl a molecules.

(Sub)-Picosecond Spectral Evolution of Fluorescence Studied with a Synchroscan Streak-Camera System and Target Analysis

Summary A synchroscan streak camera in combination with a spectrograph can simultaneously record temporal dynamics and wavelength of fl uorescence representable as an image with time and wavelength along the axes. The instrument response width is about 1% of the time range (of typically 200 ps to 2 ns). The spectral window of 250 nm may lie between 250 and 850 nm. Such spectrotemporal measurements using low excitation intensities