Potentiometric stripping analysis of bioactive peptides at carbon electrodes down to subnanomolar concentrations

The bioactive peptides bombesin, neurotensin and luteinizing hormone releasing hormone (LH-RH) can be adsorbed and accumulated at carbon paste electrodes and determined at low solution concentrations by potentiometric stripping analysis (PSA), The determination is based on the oxidation peaks of tryptophan (at about 0.7 V, against Ag/AgCl reference electrode) and/or of tyrosine (at about 0.55 V). Neurotensin and bombesin containing only tyrosine or tryptophan, respectively, produce single peaks at the corresponding potentials, while LH-RH, which contains both tyrosine and tryptophan residues, produces two well-resolved peaks. The coupling of the effective adsorptive preconcentration step with the advanced PSA operation allows peptide measurements down to subnanomolar and nanomolar concentrations. Analogous voltammetric runs produce no measurable signals at these levels. With 10 min accumulation, the PSA detection limit for bombesin is 2x10(-10) M (370 pg). The peptides can be immobilized at the electrode by strong adsorption forces. If the electrode with the immobilized peptide is washed and transferred into the blank electrolyte to perform PSA, interferences of a number of low molecular mass substances (which are not attached to the electrode or are removed from it by washing) can be avoided. The peptides can be analyzed in the presence of an excess of compounds frequently present in the protein and peptide samples. In addition, large (75-fold) excess of the monomeric tyrosine and tryptophan show no interference.