Capillary electrophoresis detection of a complex sensitive to electric field

This study describes the capillary electrophoresis detection of the weak interaction occurring between the negative form of 4-(2-hydroxyethyl)-1-piperazine-ethanesulfonic acid (HEPES -) and nicotinamide-adenine dinucleotide (NAD +). The ion-paired complex is formed probably by 1 molecule of each reactant and it is sensitive to the applied electric field, since it was found that the value of the apparent association constant decreases as a function of the field applied. The association constant at zero field was obtained by extrapolation of the values determined at different applied field strengths and was found to be 2.0 × 10 3 M -1. The peak convolution observed at the detection window was representative of a dissociation process that started at the beginning of the separation. The rate of the field-induced dissociation corresponded to 6.4 × 10 -10 mol.cm.L -1.V -1.s -1. Due to the complete absence of detectable signal shifts in 1H-NMR experiments, the interaction should be ascribed to the negative sulpho-moiety of HEPES - and to the positive charge of the NAD + pyrimidine ring. To our knowledge, this is the first example of a "secondary Wien effect" observed at low field.