Unraveling Chirality with Photoelectron Circular Dichroism
Discover how PECD advances our study of chiral molecules in biology.
Dominik Stemer, Stephan Thuermer, Florian Trinter, Uwe Hergenhahn, Michele Pugini, Bruno Credidio, Sebastian Malerz, Iain Wilkinson, Laurent Nahon, Gerard Meijer, Ivan Powis, Bernd Winter
― 5 min read
Table of Contents
Photoelectron circular dichroism (PECD) is a method that lets us explore the world of Chiral Molecules, which are molecules that can't be superimposed on their mirror images, much like your left hand isn't the same as your right hand. This technique is especially useful for studying small chiral molecules like amino acids, which are crucial in biochemistry.
Chirality?
What isBefore diving into PECD, let's tackle chirality. In simple terms, chirality refers to objects that are mirror images but can't be perfectly aligned. Think of it this way: a right glove can't fit a left hand. Various substances in nature, including proteins, sugars, and DNA, show chirality. For living organisms, the majority of these chiral molecules exist in only one of their two forms. This preference for one form over the other is a puzzling aspect of biology.
Chiral Molecules in Biochemistry
When discussing biochemistry, understanding how chiral molecules behave in water is important. Since life is bathed in water, studying how these molecules act in aqueous environments is crucial. Amino acids, which are the building blocks of proteins, can change their shape depending on the acidity or basicity of their environment. This behavior is linked to their charge states, which can be cationic (positively charged), zwitterionic (neutral overall but with both a positive and negative charge), or anionic (negatively charged).
What is PECD?
PECD uses circularly polarized light to differentiate between the two forms of chiral molecules by measuring the way they emit electrons when illuminated. When light hits a chiral molecule, it can lead to a different pattern of electrons being released, depending on whether the light is left-handed or right-handed. PECD is sensitive and can detect subtle differences, which is why it's useful in studying biologically relevant molecules.
PECD and Aqueous Solutions
For a long time, it was unclear whether PECD could be used to study molecules in water. After all, water can change the molecular behavior of chiral compounds. However, recent advancements have shown that PECD can indeed be used to analyze chiral molecules in aqueous solutions. This is a significant leap for scientists as it means they can study the behavior of important biological molecules in conditions that more accurately mimic real life.
Alanine
The Case ofOne of the simplest chiral amino acids is alanine. Researchers have now shown that PECD can be effectively applied to study alanine in its aqueous forms. This tiny molecule has three carbons, each in different parts: a carboxylic acid group, a central carbon next to an amine group, and a methyl group. Each of these carbons exhibits a unique response when subjected to PECD measurements.
The research has revealed that the response of alanine changes depending on its charge state, which is influenced by the acidity or basicity of the surrounding water. This means that scientists can tailor their studies to focus on specific forms of alanine, depending on the conditions of the solution.
Measuring PECD
To measure PECD in alanine, researchers used a technique called liquid-jet photoelectron spectroscopy (LJ-PES). This method allowed them to examine how alanine responds to light that is circularly polarized. They effectively create a thin jet of alanine solution, and as the light hits this jet, they can study the electrons emitted in response.
During their experiments, they looked at alanine at different pH levels, which correspond to its various charge states. The results showed that the PECD effect was the largest when alanine was in its anionic form, which is the state that occurs in basic conditions. This outcome suggests that the interactions between alanine and water molecules significantly affect the observable PECD.
Water and Chiral Molecules
Water is not just a passive player in these experiments; it actively participates. The interactions between alanine and water can alter how alanine behaves on a molecular level. When pH changes, alanine's charge state changes, and this interaction with water can create a complex network of hydrogen bonds. As alanine's environment shifts, so does the surrounding water, possibly adopting a chiral arrangement around the chiral molecule.
Understanding these interactions is important for researchers who want to model how chiral molecules behave in biological conditions.
Challenges in Liquid-phase PECD
One of the main challenges with PECD in water is the scattering of the emitted electrons. In liquid solutions, electrons can collide with other molecules, which complicates the measurements. This background noise can obscure the clear signals scientists need to make accurate conclusions. Scientists had to develop methods to minimize these complications and improve the quality of their data.
The Future of PECD
The progress made in using PECD for studying chiral molecules like alanine in aqueous environments opens the door for many potential applications. It offers new ways to investigate how chiral molecules interact in biological systems, which could lead to better understanding in fields like drug design and molecular biology.
As this technique improves, there are hopes for simultaneous measurements and higher sensitivity that could greatly enhance the capability to study more complex biological molecules in their natural state.
Conclusion
Photoelectron circular dichroism has proven to be a powerful tool in chemistry, especially for studying chiral molecules in their natural aqueous environments. While there are still challenges to overcome, the advancements in this field offer exciting opportunities to deepen our understanding of the molecular basis of life itself. So, the next time you hear about chirality, just remember it’s not just about hands; it's about molecules, water, and a whole lot of chemistry!
Original Source
Title: Photoelectron Circular Dichroism of Aqueous-Phase Alanine
Abstract: Amino acids and other small chiral molecules play key roles in biochemistry. However, in order to understand how these molecules behave in vivo, it is necessary to study them under aqueous-phase conditions. Photoelectron circular dichroism (PECD) has emerged as an extremely sensitive probe of chiral molecules, but its suitability for application to aqueous solutions had not yet been proven. Here, we report on our PECD measurements of aqueous-phase alanine, the simplest chiral amino acid. We demonstrate that the PECD response of alanine in water is different for each of alanine's carbon atoms, and is sensitive to molecular structure changes (protonation states) related to the solution pH. For C~1s photoionization of alanine's carboxylic acid group, we report PECD of comparable magnitude to that observed in valence-band photoelectron spectroscopy of gas-phase alanine. We identify key differences between PECD experiments from liquids and gases, discuss how PECD may provide information regarding solution-specific phenomena -- for example the nature and chirality of the solvation shell surrounding chiral molecules in water -- and highlight liquid-phase PECD as a powerful new tool for the study of aqueous-phase chiral molecules of biological relevance.
Authors: Dominik Stemer, Stephan Thuermer, Florian Trinter, Uwe Hergenhahn, Michele Pugini, Bruno Credidio, Sebastian Malerz, Iain Wilkinson, Laurent Nahon, Gerard Meijer, Ivan Powis, Bernd Winter
Last Update: 2024-12-11 00:00:00
Language: English
Source URL: https://arxiv.org/abs/2412.08729
Source PDF: https://arxiv.org/pdf/2412.08729
Licence: https://creativecommons.org/licenses/by/4.0/
Changes: This summary was created with assistance from AI and may have inaccuracies. For accurate information, please refer to the original source documents linked here.
Thank you to arxiv for use of its open access interoperability.