Postulations for the Migration Behavior of Amino Acids as Cations in Capillary Zone Electrophoresis

Amino acids (AAs) in their cationic form at pH 2.2 and usual ionic strength show a non-intuitive migration order in CZE. This is explained by setting up four postulates. The central points in these postulates are the influence of the AA side chain on the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>p</mml:mi> <mml:msub><mml:mi>K</mml:mi> <mml:mi>a</mml:mi></mml:msub> </mml:mrow> <mml:annotation>${\rm p}K_{\rm a}$</mml:annotation></mml:semantics> </mml:math> value and the adoption of a defined, preferred conformation to build up the different <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>p</mml:mi> <mml:msub><mml:mi>K</mml:mi> <mml:mi>a</mml:mi></mml:msub> </mml:mrow> <mml:annotation>${\rm p}K_{\rm a}$</mml:annotation></mml:semantics> </mml:math> values. This conformation then also influences the hydrodynamic radius. The rotational orientation of an AA in the electric field aligns it, which also affects the hydrodynamic radius. Overall a special electrophoretical hydrodynamic radius is postulated and distinguished from the hydrodynamic radius, which is determined by the translational diffusion constant. With the help of the four postulates, the migration order could be explained. Glutamic acid has a special feature in this study: due to its observed higher mobility than the smaller and even higher charged aspartic acid, the hypothesis is that it would deprotonate first at the C5 and not at the C1 carboxylic group as all other AAs. This has the consequence of a more streamlined conformation and by that a faster migration in capillary electrophoresis.