Quantum-chemistry-aided identification, synthesis and experimental validation of model systems for conformationally controlled reaction studies: separation of the conformers of 2,3-dibromobuta-1,3-diene in the gas phase

The Diels-Alder cycloaddition, in which a diene reacts with a dienophile to form a cyclic compound, counts among the most important tools in organic synthesis. Achieving a precise understanding of its mechanistic details on the quantum level requires new experimental and theoretical methods. Here, we present an experimental approach that separates different diene conformers in a molecular beam as a prerequisite for the investigation of their individual cycloaddition reaction kinetics and dynamics under single-collision conditions in the gas phase. A low- and high-level quantum-chemistry-based screening of more than one hundred dienes identified 2,3-dibromobutadiene (DBB) as an optimal candidate for efficient separation of itsgaucheand s-transconformers by electrostatic deflection. A preparation method for DBB was developed which enabled the generation of dense molecular beams of this compound. The theoretical predictions of the molecular properties of DBB were validated by the successful separation of the conformers in the molecular beam. A marked difference in photofragment ion yields of the two conformers upon femtosecond-laser pulse ionization was observed, pointing at a pronounced conformer-specific fragmentation dynamics of ionized DBB. Our work sets the stage for a rigorous examination of mechanistic models of cycloaddition reactions under controlled conditions in the gas phase

Optical Funnel to Guide and Focus Virus Particles for X-Ray Diffractive Imaging

Photophoretic forces are induced when light causes a net momentum exchange between a particle and a surrounding gas. Such forces have been shown to be a robust means for trapping and guiding particles in air over long distances. Here, we apply the concept of an optical funnel for the delivery of bioparticles to the focus of an x-ray free-electron laser (XFEL) for femtosecond x-ray diffractive imaging. We provide the experimental demonstration of transversely compressing a high-speed beam of aerosolized viruses via photophoretic forces in a low-pressure gas environment. Relative temperature gradients induced on the viruses by the laser are estimated via particle-velocimetry measurements. The results demonstrate the potential for an optical funnel to improve particle-delivery efficiency in XFEL imaging and spectroscopy

Femtosecond Single-Particle Diffractive Imaging of 3D DNA-Origami Molecular Scaffolds with XFEL Pulses

Single-particle diffractive imaging is one of the key foundational goals behind the establishment of X-ray Free-Electron Laser (XFEL) facilities. Outrunning radiation damage, extremely intense femtosecond XFEL pulses open up the possibility of imaging uncrystallized aperiodic single-particles frozen in time at room-temperature at the timescales of atomic and electronic motions and thus enabling the capturing of complete energy landscape of molecules both at ground and excited states with sufficiently large data. Despite the current sample-delivery and background scattering challenges, there has been a steady progress in XFEL-single-particle imaging (XFEL-SPI), especially with large symmetric viruses. As a step towards XFEL imaging of single-macromolecules and small-proteins, here we report the coherent diffractive imaging of 3D DNA-origami molecular scaffolds using the soft-X-ray pulses at the European X-ray Free-Electron Laser (EuXFEL). Asymmetric and symmetric 3D DNA-origami scaffold structures were nebulized and delivered to the XFEL beam using an aerodynamic lens stack. The aerosolized DNA-origami structures were intact, and tens-of-thousands of diffraction patterns with expected size and shape matching the simulations of corresponding cryo-EM structures were collected, which were 3D mergeable in the reciprocal space with the expand-maximize-compression (EMC) algorithm. This first demonstration of imaging intact electrospray-aerosolized 3D DNA-origami structures is an important first step towards exploring the application of DNA-origami in XFEL diffractive imaging, especially as, but not-limited-to, molecular scaffolds for small proteins, as references in holographic single-particle imaging or as carriers of strongly-scattering nanoparticle references, and also in time-resolved diffractive imaging of photonic/photoactivatable DNA-origami machines

UV-induced dissociation of CH2BrI probed by intense femtosecond XUV pulses

The ultraviolet (UV)-induced dissociation and photofragmentation of gas-phase CH2BrI molecules induced by intense femtosecond extreme ultraviolet (XUV) pulses at three different photon energies are studied by multi-mass ion imaging. Using a UV-pump-XUV-probe scheme, charge transfer between highly charged iodine ions and neutral CH2Br radicals produced by C-I bond cleavage is investigated. In earlier charge-transfer studies, the center of mass of the molecules was located along the axis of the bond cleaved by the pump pulse. In the present case of CH2BrI, this is not the case, thus inducing a rotation of the fragment. We discuss the influence of the rotation on the charge transfer process using a classical over-the-barrier model. Our modeling suggests that, despite the fact that the dissociation is slower due to the rotational excitation, the critical interatomic distance for charge transfer is reached faster. Furthermore, we suggest that charge transfer during molecular fragmentation may be modulated in a complex way

Megahertz single-particle imaging at the European XFEL

The emergence of high repetition-rate X-ray free-electron lasers (XFELs) powered by superconducting accelerator technology enables the measurement of significantly more experimental data per day than was previously possible. The European XFEL is expected to provide 27,000 pulses per second, over two orders of magnitude more than any other XFEL. The increased pulse rate is a key enabling factor for single-particle X-ray diffractive imaging, which relies on averaging the weak diffraction signal from single biological particles. Taking full advantage of this new capability requires that all experimental steps, from sample preparation and delivery to the acquisition of diffraction patterns, are compatible with the increased pulse repetition rate. Here, we show that single-particle imaging can be performed using X-ray pulses at megahertz repetition rates. The results obtained pave the way towards exploiting high repetition-rate X-ray free-electron lasers for single-particle imaging at their full repetition rate