Gas-phase Chemical Kinetics Unveil Carbohydrate Structure: Using Mass Spectrometry and Gas-phase Chemical Kinetics Theory and Techniques to Solve Problems in Biochemistry / Dr. Chi-Kung Ni, Research Fellow and Deputy Director

(Left to right) Hou-Yu Lin, Shih-Pei Huang, Hsu Chen Hsu, Yu-Hsiang Kuo, Chi-Kung Ni, Chia Yen Liew, Jien-Lian Chen, Wei-Chien Weng, Hung-En, Liao

Dr. Chi-Kung Ni and co-workers recently developed a new mass spectrometry method for carbohydrate structural identification which they named “logically derived sequence (LODES) tandem mass spectrometry.” By applying theory and techniques frequently used in gas-phase chemical kinetics, Dr. Ni and colleagues found that the mass spectra of carbohydrates can be explained using a few simple rules. By using these rules, they successfully identified the complete structure of numerous oligosaccharides.

Nucleic acids, proteins, lipids, and carbohydrates are the four categories of molecules in creating life. Although the structural determination of nucleic acids, proteins, and lipids have been well established, the development of a simple and robust method for carbohydrate structural determination remains a conundrum in the field. A main obstacle hindering such a development is the numerous carbohydrate isomers. For example, even though they are constructed using only three common hexoses (i.e., glucose, galactose, and mannose) small oligosaccharides such as pentasaccharide and hexosaccharide have more than 106 and 107 isomers, respectively.

Mass spectrometry is highly sensitive and widely applied in determinaton of moleclar structure. In structural analysis through mass spectrometry, molecules (precursor ions) are dissociated into fragments, and the mass spectra of these fragments are recorded. Because different molecular structures result in different fragmentations, the mass spectra of the fragments can be integrated to reveal the structures of parent ions. However, most mass spectrometry methods can only determine a portion of a carbohydrate’s structure. The identification of a given carbohydrate from various isomers consequently remains difficult.

The novel mass spectrometry method Dr. Ni and co-workers developed involves successive collision-induced dissociation of underivatized oligosaccharides in a commercial ion-trap mass spectrometer. The key of LODES is the sequence in multistage tandem mass spectrometry, which is derived from the understanding of dissociation mechanisms using theory and techniques in gas-phase chemical kinetics. “This is an example to show that the tools developed in one field can be applied to solve problems in another field,” said Dr. Ni. “Changing the research field does not mean losing the advantage you once had, but an opportunity to explore the endless potential of your skill and knowledge.”

Dr. Ni has studied gas-phase kinetics for 25 years. His research topics include photodissociation of small organic molecules in molecular beams as well as molecular energy transfer in crossed molecular beams. He started considering carbohydrate structure determination in 2016. “If I had not studied gas-phase chemical kinetics in past years, I would not have come up with the idea to develop LODES,” said Dr. Ni. “The dissociation of carbohydrates in mass spectrometry is a standard problem in gas-phase chemical kinetics, though it is difficult and complicated. Most people who work on gas-phase chemical kinetics focus on small molecules. They are either unaware of this problem or perhaps they think carbohydrates are too large to be understood in detail like the typical small molecules studied in gas phase. I am lucky to notice this problem, and I happen to know the tools to solve it,” he added.

Fig. 1. Flow chart of the program for automatic carbohydrate structural determination.