Tyramine electropolymerization revisited by DFT and experimental study

This work revisits tyramine electropolymerization on graphite electrodes through both theoretical and experimental investigation. Minimum energy structures of poly-tyramine oligomers were obtained with Monte Carlo Multiple Minimum conformational searches. Poly-tyramine octamer models were selected for electronic structure calculations based on the DFT hybrid functional B3LYP and the 6-31G(d,p) basis set, for the isolated, deprotonated and protonated forms with implicit solvation (IEFPCM). The theoretical vibrational and excitation (UV) spectra of the protonated octamer with IEFPCM implied solvation are in good agreement with available experimental data. The analysis of the excitation spectrum suggests expressive charge transfer from the electronic excitation of the polymer. The conformation of the model suggests that the preferred polymer structure has a helical backbone with ethylamine groups projected toward the bulk. This preferred conformation can be related to the low electrical conductivity of the polymer, sufficient in amplifying expected signals in electrochemical biosensors. Based on the analysis of the results, it is possible to propose a reaction mechanism which explains the greater yield obtained in cyclic voltammetry experiments conducted in acidic medium.