Femtosecond Polarization Shaping of Free-Electron Laser Pulses.

We demonstrate the generation of extreme-ultraviolet (XUV) free-electron laser (FEL) pulses with time-dependent polarization. To achieve polarization modulation on a femtosecond timescale, we combine two mutually delayed counterrotating circularly polarized subpulses from two cross-polarized undulators. The polarization profile of the pulses is probed by angle-resolved photoemission and above-threshold ionization of helium; the results agree with solutions of the time-dependent Schrödinger equation. The stability limit of the scheme is mainly set by electron-beam energy fluctuations, however, at a level that will not compromise experiments in the XUV. Our results demonstrate the potential to improve the resolution and element selectivity of methods based on polarization shaping and may lead to the development of new coherent control schemes for probing and manipulating core electrons in matter.

Light-Induced Magnetization at the Nanoscale.

Triggering and switching magnetic moments is of key importance for applications ranging from spintronics to quantum information. A noninvasive ultrafast control at the nanoscale is, however, an open challenge. Here, we propose a novel laser-based scheme for generating atomic-scale charge current loops within femtoseconds. The associated orbital magnetic moments remain ferromagnetically aligned after the laser pulses have ceased and are localized within an area that is tunable via laser parameters and can be chosen to be well below the diffraction limit of the driving laser field. The scheme relies on tuning the phase, polarization, and intensities of two copropagating Gaussian and vortex laser pulses, allowing us to control the spatial extent, direction, and strength of the atomic-scale charge current loops induced in the irradiated sample upon photon absorption. In the experiment we used He atoms driven by an ultraviolet and infrared vortex-beam laser pulses to generate current-carrying Rydberg states and test for the generated magnetic moments via dichroic effects in photoemission. Ab initio quantum dynamic simulations and analysis confirm the proposed scenario and provide a quantitative estimate of the generated local moments.

Type S Non-Ribosomal Peptide Synthetases for the Rapid Generation of Tailormade Peptide Libraries.

Bacterial natural products in general, and non-ribosomally synthesized peptides in particular, are structurally diverse and provide us with a broad range of pharmaceutically relevant bioactivities. Yet, traditional natural product research suffers from rediscovering the same scaffolds and has been stigmatized as inefficient, time-, labour- and cost-intensive. Combinatorial chemistry, on the other hand, can produce new molecules in greater numbers, cheaper and in less time than traditional natural product discovery, but also fails to meet current medical needs due to the limited biologically relevant chemical space that can be addressed. Consequently, methods for the high throughput generation of new natural products would offer a new approach to identifying novel bioactive chemical entities for the hit to lead phase of drug discovery programs. As a follow-up to our previously published proof-of-principle study on generating bipartite type S non-ribosomal peptide synthetases (NRPSs), we now envisaged the de novo generation of non-ribosomal peptides (NRPs) on an unreached scale. Using synthetic zippers, we split NRPSs in up to three subunits and rapidly generated different bi- and tripartite NRPS libraries to produce 49 peptides, peptide derivatives, and de novo peptides at good titres up to 145 mg L-1 . A further advantage of type S NRPSs not only is the possibility to easily expand the created libraries by re-using previously created type S NRPS, but that functions of individual domains as well as domain-domain interactions can be studied and assigned rapidly.

Cooperation between a T†Domain and a Minimal C-Terminal Docking Domain to Enable Specific Assembly in a Multiprotein NRPS.

Non-ribosomal peptide synthetases (NRPS) produce natural products from amino acid building blocks. They often consist of multiple polypeptide chains which assemble in a specific linear order via specialized N- and C-terminal docking domains (N/C DDs). Typically, docking domains function independently from other domains in NRPS assembly. Thus, docking domain replacements enable the assembly of "designer" NRPS from proteins that normally do not interact. The multiprotein "peptide-antimicrobial-Xenorhabdus" (PAX) peptide-producing PaxS NRPS is assembled from the three proteins PaxA, PaxB and PaxC. Herein, we show that the small C DD of PaxA cooperates with its preceding thiolation (T1 ) domain to bind the N DD of PaxB with very high affinity, establishing a structural and thermodynamical basis for this unprecedented docking interaction, and we test its functional importance in vivo in a truncated PaxS assembly line. Similar docking interactions are apparently present in other NRPS systems.

Synthetic Zippers as an Enabling Tool for Engineering of Non-Ribosomal Peptide Synthetases*.

Non-ribosomal peptide synthetases (NRPSs) are the origin of a wide range of natural products, including many clinically used drugs. Efficient engineering of these often giant biosynthetic machineries to produce novel non-ribosomal peptides (NRPs) is an ongoing challenge. Here we describe a cloning and co-expression strategy to functionally combine NRPS fragments of Gram-negative and -positive origin, synthesising novel peptides at titres up to 220 mg L-1 . Extending from the recently introduced definition of eXchange Units (XUs), we inserted synthetic zippers (SZs) to split single protein NRPSs into independently expressed and translated polypeptide chains. These synthetic type of NRPS (type S) enables easier access to engineering, overcomes cloning limitations, and provides a simple and rapid approach to building peptide libraries via the combination of different NRPS subunits.

Topological light fields for highly non-linear charge quantum dynamics and high harmonic generation.

We study theoretically the electron quantum dynamics in atoms driven by intense IR laser pulses that are phase and/or polarization structured. The extremely non-linear electron dynamics causes high harmonic emission, which we calculate, analyze, and characterize. Results are presented for three different types of structured lasers: radially polarized and azimuthally polarized beams and optical skyrmions. We identify a topological index that is inherent to the driving pulse topology and is taken over by the high harmonics. All harmonics are found to have the same topological index. For vector IR pulses as driving fields, the far-field emitted beam tightens with a higher topological order and remains unchanged when the atom is driven by an optical skyrmion.

Imprinting photon orbital angular momentum during laser-assisted photoemission from quantum wells.

We study theoretically the transfer of the light field orbital angular momentum (OAM) to propagating electrons upon photoemission from quantum well states. Irradiation with a Laguerre-Gaussian mode laser pulse elevates the quantum well state into a laser-dressed Volkov state that can be detected in an angular and energy-resolved manner while varying the characteristics of the driving fields. We derive the photoemission cross section for this process using the S-matrix theory and illustrate how the OAM is embodied in the photoelectron angular pattern with the aid of numerical calculations. The results point to a new type of time-resolved spectroscopy, in which the electronic orbital motion is addressed exclusively, with the potential for a new insight in spin-orbitally or orbitally coupled systems.

Dynamic Double-Slit Experiment in a Single Atom.

A single-atom "double-slit" experiment is realized by photoionizing rubidium atoms using two independent low power lasers. The photoelectron wave of well-defined energy recedes to the continuum either from the 5P or 6P states in the same atom, resulting in two-path interference imaged in the far field using a photoelectron detector. Even though the lasers are independent and not phase locked, the transitions within the atom impart the phase relationship necessary for interference. The experiment is designed so that either 5P or 6P states are excited by one laser, before ionization by the second beam. The measurement cannot determine which excitation path is taken, resulting in interference in wave-vector space analogous to Young's double-slit studies. As the lasers are tunable in both frequency and intensity, the individual excitation-ionization pathways can be varied, allowing dynamic control of the interference term. Since the electron wave recedes in the Coulomb potential of the residual ion, a quantum model is used to capture the dynamics. Excellent agreement is found between theory and experiment.

NMR resonance assignments for a docking domain pair with an attached thiolation domain from the PAX peptide-producing NRPS from Xenorhabdus cabanillasii.

Non-ribosomal peptide synthetases (NRPSs) are large multienzyme machineries. They synthesize numerous important natural products starting from amino acids. For peptide synthesis functionally specialized NRPS modules interact in a defined manner. Individual modules are either located on a single or on multiple different polypeptide chains. The "peptide-antimicrobial-Xenorhabdus" (PAX) peptide producing NRPS PaxS from Xenorhabdus bacteria consists of the three proteins PaxA, PaxB and PaxC. Different docking domains (DDs) located at the N-termini of PaxB and PaxC and at the C-termini of PaxA and BaxB mediate specific non-covalent interactions between them. The N-terminal docking domains precede condensation domains while the C-terminal docking domains follow thiolation domains. The binding specificity of individual DDs is important for the correct assembly of multi-protein NRPS systems. In many multi-protein NRPS systems the docking domains are sufficient to mediate the necessary interactions between individual protein chains. However, it remains unclear if this is a general feature for all types of structurally different docking domains or if the neighboring domains in some cases support the function of the docking domains. Here, we report the 1H, 13C and 15 N NMR resonance assignments for a C-terminal di-domain construct containing a thiolation (T) domain followed by a C-terminal docking domain (CDD) from PaxA and for its binding partner - the N-terminal docking domain (NDD) from PaxB from the Gram-negative entomopathogenic bacterium Xenorhabdus cabanillasii JM26 in their free states and for a 1:1 complex formed by the two proteins. These NMR resonance assignments will facilitate further structural and dynamic studies of this protein complex.

A New Docking Domain Type in the Peptide-Antimicrobial-Xenorhabdus Peptide Producing Nonribosomal Peptide Synthetase from Xenorhabdus bovienii.

Nonribosomal peptide synthetases (NRPSs) produce a wide variety of different natural products from amino acid precursors. In contrast to single protein NRPS, the NRPS of the bacterium Xenorhabdus bovienii producing the peptide-antimicrobial-Xenorhabdus (PAX) peptide consists of three individual proteins (PaxA/B/C), which interact with each other noncovalently in a linear fashion. The specific interactions between the three different proteins in this NRPS system are mediated by short C- and N-terminal docking domains (C/NDDs). Here, we investigate the structural basis for the specific interaction between the CDD from the protein PaxB and the NDD from PaxC. The isolated DD peptides feature transient α-helical conformations in the absence of the respective DD partner. Isothermal titration calorimetry (ITC) and nuclear magnetic resonance (NMR) titration experiments showed that the two isolated DDs bind to each other and form a structurally well-defined complex with a dissociation constant in the micromolar range as is typical for many DD interactions. Artificial linking of this DD pair via a flexible glycine-serine (GS) linker enabled us to solve the structure of the DD complex by NMR spectroscopy. In the complex, the two DDs interact with each other by forming a three helix bundle arranged in an overall coiled-coil motif. Key interacting residues were identified in mutagenesis experiments. Overall, our structure of the PaxB CDD/PaxC NDD complex represents an architecturally new type of DD interaction motif.

All-optical generation and ultrafast tuning of non-linear spin Hall current.

Spin Hall effect, one of the cornerstones in spintronics refers to the emergence of an imbalance in the spin density transverse to a charge flow in a sample under voltage bias. This study points to a novel way for an ultrafast generation and tuning of a unidirectional nonlinear spin Hall current by means of subpicosecond laser pulses of optical vortices. When interacting with matter, the optical orbital angular momentum (OAM) carried by the vortex and quantified by its topological charge is transferred to the charge carriers. The residual spin-orbital coupling in the sample together with confinement effects allow exploiting the absorbed optical OAM for spatio-temporally controlling the spin channels. Both the non-linear spin Hall current and the dynamical spin Hall angle increase for a higher optical topological charge. The reason is the transfer of a higher amount of OAM and the enhancement of the effective spin-orbit interaction strength. No bias voltage is needed. We demonstrate that the spin Hall current can be all-optically generated in an open circuit geometry for ring-structured samples. These results follow from a full-fledged propagation of the spin-dependent quantum dynamics on a time-space grid coupled to the phononic environment. The findings point to a versatile and controllable tool for the ultrafast generation of spin accumulations with a variety of applications such as a source for ultrafast spin transfer torque and charge and spin current pulse emitter.