https://doi.org/10.15255/KUI.2004.009
Published: Kem. Ind. 54 (6) (2005) 295–302
Paper reference number: KUI-09/2004
Paper type: Review
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Mass Spectrometric Determination of Gas Phase Structures of Amino Acids

M. Rožman and D. Srzić

Abstract

In the past two decades mass spectrometry became an important tool in the structural investigations of biomolecules (amino acids). Although, the primary focus of mass spectrometry is on compound identification and sequence information (primary structure), some mass spectrometry based methods as ion chromatography, hydrogen/deuterium exchange, and kinetic method are able to determine secondary gas phase structure of the amino acids. For example, it is possible to distinguish the zwitterionic and charge solvated structures of the sodiated amino acid, figure 1. Ion chromatography The ion chromatography (also ion mobility) of a gas-phase ion is the quantity that describes how quickly the ion moves through a drift cell filled with a buffer gas under the influence of a weak electric field, figure 2. Basic setup for the ion mobility experiments is shown in figure 4. Measuring the mobility (collision cross section) of an ion yields information about its structure i. e. whether it is compact or extended. Based on this, some conclusions about gas phase structure of alkali cationized amino acids, are obtained. On the other hand some collision cross section data for ionic systems, obtained from the ion mobility experiments are often not conclusive when the shape of charge solvation is very similar to zwitterionic structures. Hydrogen/deuterium exchange In the typical hydrogen/deuterium experiment a cationized amino acid is exposed to gas phase of the deuterating reagent and the rate of the exchange is monitored by the mass spectrometer. The generally accepted mechanism for the hydrogen/deuterium exchange consists of three steps, scheme 1. The information obtained from site specific reaction rate coefficients can give information about the number of exchangeable hydrogens, location of the ionization site, location of the reactive groups, structure hydrogen bonding and shape of the cationized amino acid (zwitterionic versus charge solvated). On the basis of such information, results for protonated and sodiated proline are given in figures 5 and 6. Kinetic method The kinetic method is a procedure for the determination of thermochemical properties based on the rates of competitive dissociations of mass selected cluster ions. It is widely used for thermochemical and ion structural determinations. For example the kinetic method is used for determination of the most stable conformation of sodiated amino acids. Relative alkali metal ion affinities of amino acid and the corresponding methyl ester are determined. Based on the difference of sodium affinity between zwitterionic and charge solvated structures of the gas phase sodiated amino acids, some structural conclusions can be obtained.

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Keywords

gas phase structure, amino acids, mass spectrometry, ion chromatography, kinetic method, hydrogen/deuterium exchange