Intermediates caught in the act: tracing insulin amyloid fibril formation in time by combined optical spectroscopy, light scattering, mass spectrometry and microscopy.

Interest in the topic of amyloid formation by peptides and proteins has increased dramatically in recent years, transforming it from a puzzling phenomenon associated with a small number of diseases into a major subject of study in disciplines ranging from material science to biology and medicine. The tendency of numerous (also non-pathogenic) proteins such as insulin to self-assemble into amyloid-like fibrils is well known. While fibrils are usually easily detected, the observation of transient intermediates is a big challenge in general. They are the key and the 'holy grail' for a molecular understanding of mechanisms in this context. Here we show that intermediates, i.e. oligomers, can be detected and their hydrodynamic radius RH as well as their overall conformation and structure can be monitored and the aggregation dynamics as well as structure formation can be detected in time with a suitable combination of experimental techniques. We have observed transient intermediates that resemble large oligomers held together in solution by non-covalent forces. The oligomers appear to convert into building blocks for mature fibrils with largely beta-sheet conformation resembling key players in a mechanism, which is termed 'nucleated conformation conversion' in the literature. Structural transformations of oligomers in time towards dominant beta-sheet conformations have been observed for the first time. The structures can even be observed in liquid phase AFM experiments. With this approach we have successfully shed new light into the aggregation and fibrilization process of insulin being possibly a model system for other amyloid systems.

Hydrogen bond dynamics of superheated water and methanol by ultrafast IR-pump and EUV-photoelectron probe spectroscopy.

Supercritical water and methanol have recently drawn much attention in the field of green chemistry. It is crucial to an understanding of supercritical solvents to know their dynamics and to what extent hydrogen (H) bonds persist in these fluids. Here, we show that with femtosecond infrared (IR) laser pulses water and methanol can be heated to temperatures near and above their critical temperature Tc and their molecular dynamics can be studied via ultrafast photoelectron spectroscopy at liquid jet interfaces with high harmonics radiation. As opposed to previous studies, the main focus here is the comparison between the hydrogen bonded systems of methanol and water and their interpretation by theory. Superheated water initially forms a dense hot phase with spectral features resembling those of monomers in gas phase water. On longer timescales, this phase was found to build hot aggregates, whose size increases as a function of time. In contrast, methanol heated to temperatures near Tc initially forms a broad distribution of aggregate sizes and some gas. These experimental features are also found and analyzed in extended molecular dynamics simulations. Additionally, the simulations enabled us to relate the origin of the different behavior of these two hydrogen-bonded liquids to the nature of the intermolecular potentials. The combined experimental and theoretical approach delivers new insights into both superheated phases and may contribute to understand their different chemical reactivities.

Ultrafast soft X-ray photoelectron spectroscopy at liquid water microjets.

Since the pioneering work of Kai Siegbahn, electron spectroscopy for chemical analysis (ESCA) has been developed into an indispensable analytical technique for surface science. The value of this powerful method of photoelectron spectroscopy (PES, also termed photoemission spectroscopy) and Siegbahn's contributions were recognized in the 1981 Nobel Prize in Physics. The need for high vacuum, however, originally prohibited PES of volatile liquids, and only allowed for investigation of low-vapor-pressure molecules attached to a surface (or close to a surface) or liquid films of low volatility. Only with the invention of liquid beams of volatile liquids compatible with high-vacuum conditions was PES from liquid surfaces under vacuum made feasible. Because of the ubiquity of water interfaces in nature, the liquid water-vacuum interface became a most attractive research topic, particularly over the past 10 years. PES studies of these important aqueous interfaces remained significantly challenging because of the need to develop high-pressure PES methods. For decades, ESCA or PES (termed XPS, for X-ray photoelectron spectroscopy, in the case of soft X-ray photons) was restricted to conventional laboratory X-ray sources or beamlines in synchrotron facilities. This approach enabled frequency domain measurements, but with poor time resolution. Indirect access to time-resolved processes in the condensed phase was only achieved if line-widths could be analyzed or if processes could be related to a fast clock, that is, reference processes that are fast enough and are also well understood in the condensed phase. Just recently, the emergence of high harmonic light sources, providing short-wavelength radiation in ultrashort light pulses, added the dimension of time to the classical ESCA or XPS technique and opened the door to (soft) X-ray photoelectron spectroscopy with ultrahigh time resolution. The combination of high harmonic light sources (providing radiation with laserlike beam qualities) and liquid microjet technology recently enabled the first liquid interface PES experiments in the IR/UV-pump and extreme ultraviolet-probe (EUV-probe) configuration. In this Account, we highlight features of the technology and a number of recent applications, including extreme states of matter and the discovery and detection of short-lived transients of the solvated electron in water. Properties of the EUV radiation, such as its controllable polarization and features of the liquid microjet, will enable unique experiments in the near future. PES measures electron binding energies and angular distributions of photoelectrons, which comprise unique information about electron orbitals and their involvement in chemical bonding. One of the future goals is to use this information to trace molecular orbitals, over time, in chemical reactions or biological transformations.