Influence of pump laser fluence on ultrafast structural changes in myoglobin

High-intensity femtosecond pulses from an X-ray free-electron laser enable pump probe experiments for investigating electronic and nuclear changes during light-induced reactions. On time scales ranging from femtoseconds to milliseconds and for a variety of biological systems, time-resolved serial femtosecond crystallography (TR-SFX) has provided detailed structural data for light-induced isomerization, breakage or formation of chemical bonds and electron transfer. However, all ultra-fast TR-SFX studies to date have employed such high pump laser energies that several photons were nominally absorbed per chromophore. As multiphoton absorption may force the protein response into nonphysiological pathways, it is of great concern whether this experimental approach allows valid inferences to be drawn vis-a-vis biologically relevant single-photon-induced reactions. Here we describe ultrafast pump-probe SFX experiments on photodissociation of carboxymyoglobin, showing that different pump laser fluences yield markedly different results. In particular, the dynamics of structural changes and observed indicators of the mechanistically important coherent oscillations of the Fe-CO bond distance (predicted by recent quantum wavepacket dynamics) are seen to depend strongly on pump laser energy. Our results confirm both the feasibility and necessity of performing TR-SFX pump probe experiments in the linear photoexcitation regime. We consider this to be a starting point for reassessing design and interpretation of ultrafast TR-SFX pump probe experiments such that biologically relevant insight emerges

Intercom

»Intercom« heißt eine Anlage, die zur Übermittlung von Sprache mithilfe elektrischer Signale dient – ähnlich einem Telefon. Für die Verbindung muss jedoch kein Hörer abgenommen werden, weshalb die Sprechanlage oft eingesetzt wird, um eine unabhängige zusätzliche Gesprächsebene zu schaffen. Nach dem gleichen Prinzip gehen der Künstler Via Lewandowsky und der Dichter Durs Grünbein vor. Bilder von nahen und fernen Schauplätzen, die Lewandowsky auf verschiedenen Reisen aufgenommen hat, werden in einem zweiten Gang beschriftet. Die Fotografien zeigen ein breites Spektrum an Motiven, Objekten und Erinnerungsorten einer globalisierten Welt: Landschaften, Häuser, Städtebilder und Interieurs, die von der An-, mehr aber noch von der Abwesenheit der Menschen zeugen. Diese Spuren sowohl in der Zivilisation als auch in der entstellten Natur sind das Unheimliche an all diesen Orten. Intercom ist eine jahrelange Gemeinschaftsarbeit: Lewandowsky gibt ein Bild vor, wählt damit das Motiv aus, und Grünbein verfasst dazu einen freien Kommentar in kurzer Form, meist erzählerischer, auch halb dokumentarischer Prosa. Bild und Text ergänzen sich wie zwei Stimmen in der Wechselrede über die Sprechanlage.</p

Crystallography on a chip – without the chip: sheet-on-sheet sandwich

Crystallography chips are fixed-target supports consisting of a film (for example Kapton) or wafer (for example silicon) that is processed using semiconductor-microfabrication techniques to yield an array of wells or through-holes in which single microcrystals can be lodged for raster-scan probing. Although relatively expensive to fabricate, chips offer an efficient means of high-throughput sample presentation for serial diffraction data collection at synchrotron or X-ray free-electron laser (XFEL) sources. Truly efficient loading of a chip (one microcrystal per well and no wastage during loading) is nonetheless challenging. The wells or holes must match the microcrystal size of interest, requiring that a large stock of chips be maintained. Raster scanning requires special mechanical drives to step the chip rapidly and with micrometre precision from well to well. Here, a `chip-less' adaptation is described that essentially eliminates the challenges of loading and precision scanning, albeit with increased, yet still relatively frugal, sample usage. The device consists simply of two sheets of Mylar with the crystal solution sandwiched between them. This sheet-on-sheet (SOS) sandwich structure has been employed for serial femtosecond crystallography data collection with micrometre-sized crystals at an XFEL. The approach is also well suited to time-resolved pump–probe experiments, in particular for long time delays. The SOS sandwich enables measurements under XFEL beam conditions that would damage conventional chips, as documented here. The SOS sheets hermetically seal the sample, avoiding desiccation of the sample provided that the X-ray beam does not puncture the sheets. This is the case with a synchrotron beam but not with an XFEL beam. In the latter case, desiccation, setting radially outwards from each punched hole, sets lower limits on the speed and line spacing of the raster scan. It is shown that these constraints are easily accommodated