Ulrich J. Lorenz

A new tandem mass spectrometer for photofragment spectroscopy of cold, gas-phase molecular ions

We present here the design of a new tandem mass spectrometer that combines an electrospray ion source with a cryogenically cooled ion trap for spectroscopic studies of cold, gas-phase ions. The ability to generate large ions in the gas phase without fragmentation, cool them to approximately 10 K in an ion trap, and perform photofragment spectroscopy opens up new possibilities for spectroscopic characterization of large biomolecular ions. The incorporation of an ion funnel, together with a number of small enhancements, significantly improves the sensitivity, signal stability, and ease of use compared with the previous instrument built in our laboratory.

Spectroscopy of Protonated Peptides Assisted by Infrared Multiple Photon Excitation

We report here a new technique for spectroscopic studies of protonated, gas-phase biomolecules and demonstrate its utility by measuring highly resolved electronic and infrared spectra of peptides of up to 17 amino acids. After UV excitation of an aromatic chromophore of a protonated peptide, a CO(2) laser further excites the molecules, increasing their vibrational energy and hence their dissociation rate, allowing detection of the UV excitation by monitoring the resulting photofragments. We show that addition of the CO(2) laser excitation increases the fragmentation yield on the time scale of our experiments by as much as 2 orders of magnitude, significantly enhancing the sensitivity of UV photofragment spectroscopy. We also demonstrate that this approach can be applied in an IR-UV double-resonance scheme, allowing measurement of conformer-specific infrared spectra of protonated peptides.

Observing liquid flow in nanotubes by 4D electron microscopy

Watching lead flow at the nanoscale Microfluidic devices have recently become useful in commercial chemical synthesis. But what about fluid dynamics at the nanometer scale? Lorenz and Zewail used an electron microscope with nanosecond time resolution to capture images of molten lead flowing through a nanotube. They flash-melted the metal with a laser pulse to begin their flow measurements at a precise time point. The experiments offered insights into viscous friction as well as heat-transfer dynamics in a channel one-thousandth as wide as a strand of hair. Science, this issue p. 1496 Flash melting of lead in the core of a nanotube enables close tracking of nanometer-scale fluid dynamics. Nanofluidics involves the study of fluid transport in nanometer-scale structures. We report the direct observation of fluid dynamics in a single zinc oxide nanotube with the high spatial and temporal resolution of four-dimensional (4D) electron microscopy. The nanotube is filled with metallic lead, which we melt in situ with a temperature jump induced by a heating laser pulse. We then use a short electron pulse to create an image of the ensuing dynamics of the hot liquid. Single-shot images elucidate the mechanism of irreversible processes, whereas stroboscopic diffraction patterns provide the heating and cooling rates of single nanotubes. The temporal changes of the images enable studies of the viscous friction involved in the flow of liquid within the nanotube, as well as studies of mechanical processes such as those that result in the formation of extrusions.

Biomechanics of DNA structures visualized by 4D electron microscopy

We present a technique for in situ visualization of the biomechanics of DNA structural networks using 4D electron microscopy. Vibrational oscillations of the DNA structure are excited mechanically through a short burst of substrate vibrations triggered by a laser pulse. Subsequently, the motion is probed with electron pulses to observe the impulse response of the specimen in space and time. From the frequency and amplitude of the observed oscillations, we determine the normal modes and eigenfrequencies of the structures involved. Moreover, by selective “nano-cutting” at a given point in the network, it was possible to obtain Young’s modulus, and hence the stiffness, of the DNA filament at that position. This experimental approach enables nanoscale mechanics studies of macromolecules and should find applications in other domains of biological networks such as origamis.

Multiple isomers and protonation sites of the phenylalanine/serine dimer.

Our investigation of the phenylalanine/serine (Phe/Ser) protonated dimer suggests that the intermolecular interaction between the two amino acids is more complex than could have been anticipated from previous studies of similar systems. Isomer-specific infrared (IR) spectra, recorded at an internal temperature of ~10 K, demonstrate the presence of at least five isomers with nonzwitterionic structures. Moreover, isotopic substitution experiments provide evidence for different protonation sites among these isomers.

4D Cryo-Electron Microscopy of Proteins

Cryo-electron microscopy is a form of transmission electron microscopy that has been used to determine the 3D structure of biological specimens in the hydrated state and with high resolution. We report the development of 4D cryo-electron microscopy by integrating the fourth dimension, time, into this powerful technique. From time-resolved diffraction of amyloid fibrils in a thin layer of vitrified water at cryogenic temperatures, we were able to detect picometer movements of protein molecules on a nanosecond time scale. Potential future applications of 4D cryo-electron microscopy are numerous, and some are discussed here.

Planar multipole ion trap/time-of-flight mass spectrometer.

We present a novel, hybrid ion trap/time-of-flight mass spectrometer that is based on a planar multipole design. Compared with Paul trap/time-of-flight instruments, this design possesses the principal advantages of higher injection efficiency and more homogeneous extraction fields. We demonstrate the viability of the concept and describe the characterization of a first prototype. Ions can be injected into the trap with little mass discrimination and stored for several minutes. A resolution of over 1300 is achieved in reflectron mode, and the influence of the RF amplitude and pressure on the resolution is analyzed. We suggest several applications in which this new instrument could offer advantages over existing technology.

Structural Melting of an Amino Acid Dimer upon Intersystem Crossing

We present a spectroscopic investigation of the excited-state dynamics of the phenylalanine (Phe)/serine (Ser) protonated dimer in the gas phase. Using an ultraviolet (UV) laser pulse, we promote individual isomers to the S1 state and probe their fate with an infrared (IR) pulse. We find that the S1 state has a lifetime of ~70 ns and undergoes intersystem crossing (ISC) to the T1 state. Time-resolved IR spectra allow us to follow the structural evolution of the dimer. In the S1 state, the different isomers retain the hydrogen-bonding pattern of the ground state. Intersystem crossing triggers a sudden increase of the vibrational energy, so that the dimers can overcome isomerization barriers and explore large parts of the potential energy surface (PES). Their broad IR spectra largely resemble one another and indicate that the dimers adopt a molten structure.

A radio frequency/high voltage pulse generator for the operation of a planar multipole ion trap/time-of-flight mass spectrometer

We present a radio frequency (RF)/high voltage pulse generator designed to provide suitable waveforms for the operation of a planar multipole ion trap/time-of-flight mass spectrometer. Our generator supplies a RF signal to two pairs of trapping electrodes, allowing ions to be stored in between them. Subsequently, the RF is rapidly switched off and high voltage extraction pulses are applied to the trap electrodes in order to obtain a time-of-flight spectrum of the stored ions. The quenching of the RF and the extraction pulses are synchronized to the RF phase, ensuring well-defined ejection conditions.

Observing Liquid Flow in Nanotubes

Advances in microfabrication have led to the emergence of the field of nanofluidics, which studies fluid transport in nanometer-scale structures [1-2]. Nanoscale confinement may considerably modify the properties and dynamics of a liquid. For example, the flow of water through carbon nanotubes has been reported to be enhanced by several orders of magnitude [3-4]. However, the magnitude of the enhancement has remained a point of contention [5]. A particular obstacle to settling this controversy is the great challenge of studying flow in a single nanotube [6]. Here, we demonstrate how this can be achieved by using 4D Electron Microscopy, which combines the spatial resolution of a Transmission Electron Microscope with the (ultra)fast time resolution of modern laser systems [7]. We directly image the flow of liquid lead through individual ZnO nanotubes to capture a range of flow phenomena and to characterize the nanoscale viscous friction involved [8].