Phthalimide-based Metal-Free Labelling of Peptide Nucleic Acids for Biosensor Applications

Peptide nucleic acids (PNAs), introduced in the last decade of 20th century [1], represent a class of artificially synthesized molecules with attracting features; especially their capability to strongly interact with deoxyribonucleic acid makes them very promising materials in many applications, from the development of innovative nucleic acid biosensors, and related diagnostic protocols, to their employment as tools in molecular biology and functional genomics. In the past our group already explored the field of PNA-labelling studying the behaviour of a transition metal-based electroactive marker (i.e. ferrocene) [2]; in the present work, aiming to improve biocompatibility, we propose two innovative, as far as we know, metal-free labels, based on the phthalimide unit (see figure). We have performed an extensive electrochemical characterization, from the isolated markers to their PNA conjugates (passing through the N-Boc-lysine conjugates), affording a neat rationalization of the relationship between molecular structure, redox activity and detection limit. More in detail, in order to clarify the mechanism of the electrochemical reduction of the species we have studied their behavior i) on two different electrode materials (glassy carbon and mercury electrode) and ii) in two different polar solvents (aprotic, DMF, and protic, H2O). In search of the method offering the best detection limit for the two decamers, we have found out that potentiometric stripping analysis (PSA), combined with a hanging mercury drop electrode, offers the best performance with water as solvent, the detection limit for the decamers being around 2·10-7 ÷ 2·10-8 mol dm-3. [1] P.E. Nielsen, M. Egholm, R.H. Berg, O. Buchardt, Science, 254 (1991) 1497. [2] C. Bandoli, E. Licandro, S. Maiorana, D. Risemini, C. Rigamonti, L. Falciola, M. Longhi, P.R. Mussini, J. Electroanal. Chem., 585 (2005) 197.