Capillary electrophoresis detection of a complex sensitive to electric field

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