Determination of Caffeine in Various Coffee Types by Capillary Electrophoresis Through the Anionic Complex with 3,4-Dimethoxycinnamate - Caffeine: Chemistry, Analysis, Function and Effects - page 184

noise, which obviously impacts the signal-to-noise ratio and consequently, the
limit of detection.
The basic difference between a classic conductivity measurement and C 4 Dis
that the second approach is not based on Faradaic contact of the electrodes
with the solution to be evaluated. There is a dielectric layer—more specifically,
the silica capillary wall—that prevents this contact, which is beneficial because
electrode corrosion is also prevented and the high electric potential inside the
capillary does not destroy the electronics of the conductivity meter.

d n 0 t 2 n g | 9

10.3 The Formation of p-Complexes of Caffeine and
Chlorogenic Acids and Analogues
Caffeine is produced by many plants as a pesticide, as a substance that
discourages insect feeding and also as an inhibitor for the germination of
competitors (Baumann and Gabriel 1984; Nathanson 1984). Caffeine can
permeate the membrane of the cells of the plant that provides its synthesis, and
remains inside the cell as a complex with the chlorogenic acid (Waldhauser and
Baumann 1996). A chlorogenic acid is an ester of trans-cinnamic acid and
quinic acid, 5-O-caffeoylquinic acid being the most common (Clifford 1999).
The first observation of the caffeine-chlorogenate complex has been
described at the beginning of last century (Gorter 1908). Six decades after
that, the behavior of solutions of caffeine and chlorogenic acids as well as with
some analogues was studied (Horman and Viani 1972), and they concluded
that the structure is an 1 : 1 p-complex, which is formed between the flat
molecule of caffeine and the flat region of the chlorogenic acid, i.e., the
cinnamic acid portion of this molecule. In this study, based on NMR spectra,
the authors calculated the association constant of caffeine and some
chlorogenic acid analogues, which inspired the approach used to develop a
capillary electrophoresis method published recently (Nogueira and do Lago
2007).

10.4 The Ionic p-Complex with Cinnamic Acid
Derivatives as A Mobile Version of Caffeine and
the Resulting Electrophoretic Method
Taking into account the possibility of forming a p-complex with caffeine as
shown on the previous section, one can easily envision an electrophoretic
method based on the formation of an anionic or cationic structure, which can
be manipulated by applying a convenient electric field. Obviously the size of
such a structure is bigger than caffeine alone, but the effective charge makes
the mobility of the complex different from zero (Equation (10.2)).
The thermodynamic data suggest that more than one of the complexes
studied could be used (Horman and Viani 1972). However, no kinetic data
were available. Taking into account the concept of effective mobility (Equation