Detailed study of N-(3-pyrrol-1-yl-propyl)-4,4'-bipyridinium (PPB) electropolymerization

Although the electropolymerization of N-(3-pyrrol-1-yl-propyl)-4,4'-bipyridinium (PPB) was anticipated to proceed through the 2 and 4 positions of the pyrrole ring, the emergence of two new peaks at -0.14 and -0.56 V versus Ag/AgCl in addition to those expected at -0.89 and -1.60 V upon continuous potential scanning, indicated the existence of a competitive coupling reaction. Using cyclic voltammetry and electrochemical quartz crystal microbalance techniques, a plausible mechanism for the polymerization reaction is proposed. The proposed mechanism involves quaternization of the second pyridine nitrogen, resulting in polymerization through a bipyridinium-pyrrole bond formation. Consequently, four types of pyridines would be formed: type 1, pyridinium next to pyrrole; type 2, pyridinium next to type 1; type 3, pyridinium between propyl chain and nonquaternized pyridine; type 4, nonquaternized pyridine. The surface coverage values of the nonquaternized nitrogens, type 4, which had been assumed to be the sites that bind specifically to the maltose binding protein (MBP) domain of MBP enzyme fusions was found to be sufficiently high (lower limit ofca. 5 x 10 - 1 0 mol cm - 2 ) to ensure that the MBP fusions can easily bind.