1. Introduction
X-ray science has evolved dramatically in the 21st century with the use of large X-ray facilities. Recently developed X-ray free electron lasers (XFELs) such as LCLS, SACLA, PAL-XFEL, European XFEL, can generate ultrashort X-ray pulses with unprecedented brilliance and coherence that is a billion times higher than that of the best conventional X-ray radiation sources. This has been a breakthrough for many X-ray related techniques on a broad range of scientific disciplines and brought us to investigate many interesting new sciences which was previously not possible. Our group utilize X-ray scattering, X-ray spectroscopy, and X-ray photon correlation spectroscopy to study the structural origin of water (ice) anomalies, catalytic reactions, and protein dynamics.
2. Research Topics
a. Revealing the mystery of water
Water is the most important liquid for our existence and plays an essential role in physics, chemistry, biology and geoscience. Water has numerous physical and chemical properties which are very different from other liquids, which are often referred to as anomalous. As an explanation for these anomalous experimental observations, a hypothetical liquid-liquid transition (LLT) and a liquid-liquid critical point (LLCP) has been proposed deep in the supercooled regime, but never been experimentally observed. Recently, with a new method of rapid cooling down to 227 K and ultrafast probing with static WAXS, the first experimental evidence of the existence of the Widom line which strongly indicates the existence of LLCP has been reported. With an external perturbation, we aim to have a direct search of the LLCP using SAXS and WAXS. And we also aim to reveal the existence of fragile-to-strong transition of liquid water.
Related publications:
K. H. Kim, A. Späh, H. Pathak, F. Perakis, D. Mariedahl, K. Amann-Winkel, J. A. Sellberg, J. H. Lee, S. Kim, J. Park, K. H. Nam, T. Katayama, and A. Nilsson*, “Maxima in the Thermodynamic Response and Correlation Functions of Deeply Supercooled Water”, Science, 358, 1589-1593 (2017).
K. H. Kim, H. Pathak, A. Späh, F. Perakis, D. Mariedahl, J. A. Sellberg, T. Katayama, Y. Harada, H. Ogasawara, L. G. M. Pettersson, and A. Nilsson*, “Temperature-Independent Nuclear Quantum Effects on the Structure of Water”, Phys. Rev. Lett.119, 075502 (2017).
b. Reaction mechanism of organometallic catalyst in (supercooled) solution
Homogeneous catalyst is widely used in many research fields and industries. Even though it is one of the most intense research subject and has many practical applications, the entire reaction mechanism is generally not well known since the intermediate species would only be populated during short time and the overall concentration would be very low even though this short-lived intermediates plays an important role in the reaction mechanism. With the recent X-ray techniques and also with ultrashort and bright X-ray beam from XFEL, it is possible to study the reaction mechanism of homogeneous catalyst by monitoring important but short-lived intermediates. X-ray spectroscopy is very sensitive to the electronic structure of the metal complex and X-ray scattering is sensitive to the structure of intermediates.
Another major challenge in understanding solution-phase chemistry arises from the presence of numerous solvent molecules surrounding solute molecules. Solvent serves as an energy source for activating a reaction as well as a heat bath to stabilize the products or sometimes, one or few solvent molecules actively participate to the reaction. As a result, the reaction can be significantly influenced by properties of solvent. The effect become even more significant in hydrogen bonding solvents such as water and also become more significant if the system is supercooled. It is known that many properties related with hydrogen bonding become strongly anomalous as the temperature decreases and this can cause strong influence on the structure of solutes and also on the chemical reactions. With a new method of rapidly cooling, it is possible to study the strongly enhanced solute-solvent interactions, and thermodynamic properties of the chemical reactions.
Related publications:
K. H. Kim, J. G. Kim, S. Nozawa, T. Sato, K. Y. Oang, T. W. Kim, H. Ki, J. Jo, S. Park, C. Song, T. Sato, K. Ogawa, T. Togashi, K. Tono, M. Yabashi, T. Ishikawa, J. Kim, R. Ryoo, J. Kim, H. Ihee*, S.-i. Adachi*, "Direct observation of bond formation in solution with femtosecond X-ray scattering", Nature, 518, 385-389 (2015)
K. H. Kim, T. Ok, K. Lee, H.-S. Lee, K. T. Chang, H. Ihee*, J.-H. Sohn*, "Quantitative Catalyst-Substrate Association Relationships between Metathesis Molybdenum or Ruthenium Carbene Complexes and Their Substrates", J. Am. Chem. Soc., 132, 12027-12033 (2010)
c. Ground-state protein dynamics
Proteins with their function are not static but dynamic even when they are in a ground state. It is known that the protein motions arise from the complex interplay of thermal motions of proteins and their environments and each motion has a broad range of time scales, temperature scales and size scales. It is also known that there is hierarchy of dynamic motions depending on the temperature related with solvent, sidechain, and backbone motion. Understanding how a protein moves at the ground state gives insight into how it works. With the ultrafast X-ray lasers which provide bright and coherent beams, X-ray photon correlation spectroscopy (XPCS) become a unique probe-probe type technique which can directly provide the information about the dynamics of the system in a true ground state on a molecular level. XPCS can measure the dynamics with the timescale of femtoseconds to seconds also with the length scale of Å (local motion) to ~100 nm (global motion) depending on the interested momentum transfer.
Related publications:
Fivos Perakis*, Gaia Camisasca, Thomas J. Lane, Alexander Späh, Kjartan Thor Wikfeldt, Jonas A. Sellberg, Felix Lehmkühler, Harshad Pathak, Kyung Hwan Kim, Katrin Amann-Winkel, Simon Schreck, Sanghoon Song, Takahiro Sato, Marcin Sikorski, Andre Eilert, Trevor McQueen, Hirohito Ogasawara, Dennis Nordlund, Wojciech Roseker, Jake Koralek, Silke Nelson, Philip Hart, Roberto Alonso-Mori, Yiping Feng, Diling Zhu, Aymeric Robert, Gerhard Grübel, Lars G. M. Pettersson & Anders Nilsson*, “Coherent X-rays reveal the influence of cage effects on ultrafast water dynamics”, Nat. Commun., 9, 1917 (2018).
3. Methods
a. Supercooled droplet & Amorphous (glassy) ice
It is known that many properties of water related with hydrogen bonding become strongly anomalous as the temperature decreases and this can also cause very strong influence on the solute structure and the reaction. With a new method of rapidly cooling of liquid that has allowed us to venture into no-man’s land (cooling down to 227 K without freezing), it is possible to study the liquid-liquid critical point (LLCP) of water itself, strongly enhanced solute-solvent interaction, and thermodynamic properties of reaction.
Amorphous ice can be prepared in three different ways. i) Amorphous solid water (ASW) is obtained by vapor deposition on a very cold surface ii) hyper-quenched glassy water (HGW) is obtained with very fast cooling rates and has been used for cryoEM iii) high density amorphous ice (HDA) is obtained by pressure induced amorphization of hexagonal ice. Amorphous (glassy) ice is very useful sample for studying the anomalous properties of water itself. And it can be used as very good medium for many other studies like cryoEM since it can embed protein molecules or cells without any damage.