Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Stability reigns

07.12.2009
Water is life’s most essential nutrient, and one of its key roles is to regulate acidity concentrations, or pH levels, within our bodies.

But directly observing how water solvents participate in critical pH-dependent processes, such as protein assembly, is no easy matter. Scientists need a technique that can resolve the mutual interactions occurring between acids and millions of chemically similar surrounding water molecules.

Now, researchers led by Shik Shin from the RIKEN SPring-8 Center in Hyogo prefecture, Japan, have combined state-of-the-art technical engineering with x-rays to detect quantum electronic state changes during an aqueous acid–base reaction. The resulting data paint a surprising picture of water-induced stability that differs sharply from typical views of chemical dynamic

A delicate balance

The researchers used a molecule called acetic acid—best known as the main component of vinegar—as a model system to explore pH-dependent reactions. When acetic acid is dissolved in water, it can either remain intact or it can break its oxygen–hydrogen (O–H) bond and become a charged anion. At acidic pH, the neutral species dominates whereas at basic pH, the anion form prevails. Equilibrium always exists between the two states, however, regardless of the pH level.

During this chemical reaction, acetic acid moves through and interacts with a multitude of water molecules. Shin and colleagues found that the best way to understand how water influences the acid–base equilibrium was to probe the quantum electronic states that hold acetic acid together.

X-ray vision

The imaging abilities of x-rays are familiar to anyone who has visited a dentist’s clinic. However, x-ray generated by electrotons accelerated to near the speed of light in the RIKEN SPring-8 synchrotron also can be used to peer deep inside molecules to reveal their underlying chemical framework.

In x-ray emission spectroscopy (XES), a photon first excites an electron, which is tightly bound to the nucleus, to a higher, excited-energy state. This creates a ‘hole’—an absent electron—in the core level. This hole is quickly filled by a valence electron in the molecule, and, to conserve energy, a new x-ray photon is emitted.

The emitted x-rays are uniquely dependent on the chemical and physical environment of the excited atom—a characteristic that let the researchers distinguish the electronic signal of acetic acid from the surrounding water molecules. Following this signal at varying pH levels provided nearly instantaneous information on chemical interactions during the acid–base reaction.

Because the intensity of x-rays emitted by light elements such as carbon, nitrogen and oxygen is quite low, it was essential to use high-energy synchrotron light to produce detectable electronic signals. “Observing such kinds of weak x-ray emissions requires an intense excitation light source and precise energy tuning of the incident light,” says co-author Takashi Tokushima, a research scientist at RIKEN.

Engineering at the interface

To perform the experiment, the researchers developed a new liquid cell2 that could be bolted onto the vacuum chamber and connected to the beamline of the synchrotron light source. According to Tokushima, two challenges had to be overcome during the engineering process.

“First, we had to make a window as thin as possible to separate the vacuum chamber from the liquid cell,” says Tokushima. Once the team had determined the optimum window—a 150 nm thin silicon nitride film—they faced their second challenge: how to flow a liquid sample past it.

“The 150 nm window is very fragile,” states Tokushima. “Our liquid cell is designed to make liquid flow parallel to the window, with no pushing on its surface.” Otherwise, the window cracks—spilling water into the vacuum chamber.

Unexpected influence

Using their liquid cell, the scientists recorded x-ray emissions from a double-bonded oxygen atom located in acetic acid. The XES spectra showed significant differences between low and high pH levels—reflecting the change in chemical structures between the neutral and ionized species.

“Each peak structure mostly corresponds to a unique valence electronic state,” says Tokushima. “Thus, we can use XES to fingerprint the forms of acetic acid.”

At intermediate pH levels, chemists typically picture a dynamic acid–base equilibrium, in which the neutral and ionized forms of acetic acid rapidly change states. According to this model, the XES spectra of aqueous acetic acid at intermediate pH should be quite different than the acidic or basic spectra.

However, the researchers saw only gradual, systematic changes to the original XES spectra. In fact, the spectra from intermediate pH levels could be reproduced as ratios of either the pure neutral or ionic form. These results indicated that the two states of acetic acid did not interact with each other; instead, each state was static and stable, with populations determined by the solution pH.

Tokushima says that the unexpected perfect match between the ratio analysis of the XES spectra and a static acid–base equilibrium equation provides strong evidence that water molecules stabilize the different acetic acid states, possibly by forming a ‘shell’ of water around the acid molecules.

Fast company

The next plan for the researchers is to install a microfluidic mixer into their liquid cell to study chemical reactions involving two different reagents. Because the XES technique is extremely fast, the researchers hope to observe electronic structure changes with real-time precision. “In XES, the emission process occurs within the core hole lifetime—a few femtoseconds (1015 seconds),” says Tokushima. “That means it is possible to observe dynamics down to this timescale.”

About the Researcher

Shik Shin

Shik Shin obtained his doctorate of science in 1983 from the University of Tokyo in Japan. He then joined the Research Institute of Measurement at Tohoku University as a research associate, and was later promoted to associate professor. In 1991, he returned to the University of Tokyo as an associate professor in the Institute for Solid State Physics, where he later obtained the position of professor. Shin joined RIKEN in 2008 as team leader of the Excitation Order Research Team in the Quantum Order Research Group.

Journal information

1. Horikawa, Y., Tokushima, T., Harada, Y., Takahashi, O., Chainani, A., Senba, Y., Ohashi, H., Hiraya, A., & Shin, S. Identification of valence electronic states of aqueous acetic acid in acid–base equilibrium using site-selective X-ray emission spectroscopy. Physical Chemistry Chemical Physics 11, 8676–8679 (2009).

2. Tokushima, T., Horikawa, Y., Harada, Y., Takahashi, O., Hiraya, A., & Shin, S. Selective observation of the two oxygen atoms at different sites in the carboxyl group (–COOH) of liquid acetic acid. Physical Chemistry Chemical Physics 11, 1679–1682 (2009).

Saeko Okada | Research asia research news
Further information:
http://www.rikenresearch.riken.jp/eng/hom/6127
http://www.researchsea.com

More articles from Life Sciences:

nachricht Transforming plant cells from generalists to specialists
07.12.2016 | Duke University

nachricht What happens in the cell nucleus after fertilization
06.12.2016 | Helmholtz Zentrum München - Deutsches Forschungszentrum für Gesundheit und Umwelt

All articles from Life Sciences >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Significantly more productivity in USP lasers

In recent years, lasers with ultrashort pulses (USP) down to the femtosecond range have become established on an industrial scale. They could advance some applications with the much-lauded “cold ablation” – if that meant they would then achieve more throughput. A new generation of process engineering that will address this issue in particular will be discussed at the “4th UKP Workshop – Ultrafast Laser Technology” in April 2017.

Even back in the 1990s, scientists were comparing materials processing with nanosecond, picosecond and femtosesecond pulses. The result was surprising:...

Im Focus: Shape matters when light meets atom

Mapping the interaction of a single atom with a single photon may inform design of quantum devices

Have you ever wondered how you see the world? Vision is about photons of light, which are packets of energy, interacting with the atoms or molecules in what...

Im Focus: Novel silicon etching technique crafts 3-D gradient refractive index micro-optics

A multi-institutional research collaboration has created a novel approach for fabricating three-dimensional micro-optics through the shape-defined formation of porous silicon (PSi), with broad impacts in integrated optoelectronics, imaging, and photovoltaics.

Working with colleagues at Stanford and The Dow Chemical Company, researchers at the University of Illinois at Urbana-Champaign fabricated 3-D birefringent...

Im Focus: Quantum Particles Form Droplets

In experiments with magnetic atoms conducted at extremely low temperatures, scientists have demonstrated a unique phase of matter: The atoms form a new type of quantum liquid or quantum droplet state. These so called quantum droplets may preserve their form in absence of external confinement because of quantum effects. The joint team of experimental physicists from Innsbruck and theoretical physicists from Hannover report on their findings in the journal Physical Review X.

“Our Quantum droplets are in the gas phase but they still drop like a rock,” explains experimental physicist Francesca Ferlaino when talking about the...

Im Focus: MADMAX: Max Planck Institute for Physics takes up axion research

The Max Planck Institute for Physics (MPP) is opening up a new research field. A workshop from November 21 - 22, 2016 will mark the start of activities for an innovative axion experiment. Axions are still only purely hypothetical particles. Their detection could solve two fundamental problems in particle physics: What dark matter consists of and why it has not yet been possible to directly observe a CP violation for the strong interaction.

The “MADMAX” project is the MPP’s commitment to axion research. Axions are so far only a theoretical prediction and are difficult to detect: on the one hand,...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

ICTM Conference 2017: Production technology for turbomachine manufacturing of the future

16.11.2016 | Event News

Innovation Day Laser Technology – Laser Additive Manufacturing

01.11.2016 | Event News

#IC2S2: When Social Science meets Computer Science - GESIS will host the IC2S2 conference 2017

14.10.2016 | Event News

 
Latest News

Predicting unpredictability: Information theory offers new way to read ice cores

07.12.2016 | Earth Sciences

Sea ice hit record lows in November

07.12.2016 | Earth Sciences

New material could lead to erasable and rewriteable optical chips

07.12.2016 | Materials Sciences

VideoLinks
B2B-VideoLinks
More VideoLinks >>>