Before the start of this project, the chemical properties and the chirality of a biomolecule in its natural, aqueous environment had to be probed separately, applying different experimental techniques. As the central goal and result of this action, we have made it possible to address these two independent, and equally important properties at the same time and in a single measurement, using Photoelectron Circular Dichroism (PECD). This is shown in a landmark experiment on a prototypical biomolecule, alanine, probed in its neutral, protonated and deprotonated state in aqueous solutions. With that we have thus established PECD of liquid samples as a novel method to address the functioning of biologically important reactions depending on molecular chirality.
A broader look on liquid jet photoelectron spectroscopy (L-PES) is necessary however to appreciate the scientific progress achieved in this action. Investigating the chemical state of solvated molecules by LJ-PES uses a measurement of photoelectron kinetic energies, and to be dependable such energies must be calibrated. In this action we established a protocol, adapted from solid state PES, that allows to reliably fix the energy scale in any photoemission measurement of a liquid sample by relating it to the cutoff of zero-kinetic-energy electrons emitted from the sample as a result of their inelastic scattering. Using a second calibration measurement, we can refer experimental energies not only to the vacuum level, but also to the Fermi edge of the liquid sample (not a direct observable!). This allows for the first time to distinguish whether solvent energies measured in LJ-PES change because of electronic structure, or because of surface-structure effects, particularly arising from build-up of surface dipoles. We see these methods already being adopted by the physical chemistry / aqueous-solutions community, and we are convinced that this is a lasting result of the action that will become future textbook material.
Parallel to exploring the feasibility of detecting PECD from liquid (aqueous) phase, and aiming toward more routine LJ-PECD studies, we developed new detection and sample delivery schemes. Specifically, we have developed and commissioned the very first velocity map imaging (VMI) spectrometer prototype that is compatible with a liquid jet. This instrument will massively speed up future experiments due to its capability to record the full geometrically available solid angle for electron emission, while maintaining directional information. As a corollary to this instrument, we have constructed a device producing a flat liquid surface in vacuum. Beyond its use in this project, anticipated to play a key role in a next-version LJ-VMI spectrometer, such a flat jet has wide impact on more general topics in physical chemistry. One prominent aspect is the operation of a flat jet using two different solutions, providing a liquid–liquid interface, now accessible by a range of X-ray vacuum techniques.
Up to now, work accomplished within this action is covered in 32 refereed publications, and 49 conference contributions. Two PhD students have collected the data that will form considerable parts of their theses, and the PostDoc who was a main driver of the alanine PECD experiments has been appointed to an assistant professorship just at the end of the action. Two more scientists who were part of the AQUACHIRAL team have further pursued careers in scientific instrumentation development and scientific editing, respectively. Highlights of our work were covered in 8 press releases of our institute and the synchrotron radiation facility DESY, where we performed all our PECD measurements. For his lifetime achievements on liquid jet techniques, the PI of the action received the ‘Innovation Award on Synchrotron Radiation’ in 2024. Maybe most importantly, part of the team will use the methodological knowledge gained in this action to further develop LJ-PES from a forefront research endeavour into a lab technique with a broad applicability to chemical and biochemical samples.
