Materials Dithiobis(succinimidyl propionate) was purchased from Sigma used as supplied. Casein kinase II and peptide (1) were purchased from New England BioLabs. Alkaline phosphatase was purchase from Sigma. Modification of Electrodes with peptide (1) Au wire electrodes (0.5 mm diameter, geometrical area ca. 0.16 cm 2 , roughness factor ca. 1.4) were used in the experiments. Prior to modification, the electrodes were soaked in concentrated HNO 3 for 5 min, rinsed with water. Then the electrodes

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... were rinsed again with boiling ethanol and water, dried with N 2 . The Au electrodes were reacted in 1 mM dithiobis(succinimidyl propionate) in dry DMSO for 1 hour, rinsed with DMSO and then briefly with water. The active ester modified electrodes were further reacted with peptide (1) (0.1 mM , in 0.1 M HEPES buffer, pH=7.4) for 2 h, washed with water. Phosphorylation and dephosphorylation of (1)-functionalized Electrodes The peptide (1)-functionalized peptide modified electrodes were incubated in different concentrations of CK solution (20 mM Tris-HCl, 50 mM KCl, 10 mM MgCl 2 pH = 7.5). The dephosphorylation of the peptide (1) was achieved by incubation the phosphorylated peptide (1)-modified electrode in alkaline phosphatase (25 units, borate buffer, pH=9) for 20 min.