Analysis of the microenvironment-pH around amino acids through mobility data of capillary zone electrophoresis in the low pH range

PIAGGIO, MARÍA VIRGINIA; PEIROTTI, MARTA BEATRIZ; DEIBER, JULIO ALCIDES

ELECTROPHORESIS

WILEY-VCH Verlag GmbH & Co.

Año: 2007 vol. 28 p. 3658 - 3673

0173-0835

Electrophorectic mobility data of 20 amino acids reported in the literature are analyzed and interpreted through simple physicochemical models, which are able to provide estimates of coupled quantities like hydrodynamic shape factor, equivalent hydrodynamic radius (size), net charge, actual pK value of ionizing groups, partial charges of ionizing groups, hydration number and pH near molecule (microenvironment-pH of the BGE). It is concluded that the modeling of the electrophoretic mobility of these analytes requires a careful consideration of hydrodynamic shape coupled to hydration. In the low range of pH studied here, distinctive hydrodynamic behaviors of amino acids are found. For instance, amino acids with basic polar and ionizing side chain remain with prolate shape for pH values varying from 1.99 to 3.2. It is evident that as the pH increases from low values, amino acids get higher hydrations as a consequence that each analyte total charge also increases with pH. This result is consistent with the monotonic increase of the hydrodynamic radius, which accounts both the analyte and the quite immobilized water molecules defining the electrophoretic kinematical unit. It is also found that the actual or effective pK of the _ _ carboxylic ionizing group of amino acids increases when pH is changed from 1.99 to 3.2. Several limitations concerning the simple modeling of the electrophoretic mobility of amino acids are presented for further research.