Protamine-selective electrode

Soon after the initial development of the heparin sensor, an electrode for the detection of the polycation protamine was proposed [38] based on a polymeric membrane doped with the cation exchanger tetrakis-(4-chlorophenyl)borate. Protamine is a polypeptide and usually administered as a heparin antidote. Protamine is a polycation with an average charge of +20 and is rich in arginine (Fig. 4.8). The response function of protamine-selective electrodes is similar to the heparin response function (Fig. 4.9b). 

Due to their response mechanism the polyion-selective electrodes are not sensitive to the small fragments of polyionic macromolecules. Thus, if an enzyme cleaves the polyionic molecule these sensors can be used for detection of enzyme activity. Polycation protamine is rich in arginine residues that make it a suitable substrate for protease-sensitive electrochemical assays. Real-time detection of trypsine activity was demonstrated with the protamine-selective electrode as a detector [38],... 

FIGURE 4.11 Amplitude-time behavior of potential during calibration in 0.1 M Nad solution containing 50 mM TRIS (pH 7.40) with a pulsed galvanostatic sensor (a) and a classical potentiometric protamine-selective electrode (b) [54], Logarithmic protamine concentrations (mg l-1) are indicated on the traces. The first and last samples did not contain protamine. 

FIGURE 4.9 Potentiometric response of heparin (a) and protamine (b) selective electrodes after different 5min and 24h equilibration times, respectively. Sample solutions contained 0.1 M NaCl and 50mM TRIS (pH 7.40) [39]. 

Another important bioanalytical application of voltammetric ISEs is the detection of polyions (see also above). A technique using cyclic voltammetry on micropipette electrodes filled with the organic electrolyte solutions in 1,2-dichloroethane was successfully applied for the detection of protamine [65] in saline solution and heparin in undiluted sheep plasma samples [66]. Protamine transport was facilitated with dino-nylnaphthalenesulfonic acid (DNNS). As a heparin-selective component the tetrakis-(4-chlorophenyl)borate salt of trimethyloctadecyl ammonium was used. 

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