The binding site of zinc and indium metal to amino acid derivatized squarate complexes – Implications in inhibitor and mediator designs

Three novel metal squaric acid–peptide complexes, SQI–SQIII were prepared by addition of indium triflate or zinc chloride to the previously reported compounds [1], 3-(hydroxymethylamino)-4-( l-isoleucine methyl ester)-3-cyclobutene-1,2-dione (squarate 1), and 3-(hydroxymethylamino)-2-( l-isoleucine methyl ester)-4-thioxo-2-cyclobuten-1-one (squarate 2). The structures of SQI–SQIII were elucidated using NMR analysis. The electrochemical applications of two of these metal–squaric acid systems (SQI and SQII) were also investigated. Incorporation of SQII as a mediator, in the previously optimized Pt/p(HEMA)/p(pyrrole)/GOx electrode using the ionic liquid [bmim][BF 4] as the solvent medium, produced a biosensor with enhanced properties, namely a sensitivity of 175.9 mA/M d-glucose, working potential of +200 mV, large linear range (0–12 mM) and a detection limit of 1 × 10 −6 M. Three novel metal squaric acid–peptide complexes, SQI–SQIII were prepared by addition of indium triflate or zinc chloride to the previously reported compounds [1], 3-(hydroxymethylamino)-4-( l-isoleucine methyl ester)-3-cyclobutene-1,2-dione (squarate 1), and 3-(hydroxymethylamino)-2-( l-isoleucine methyl ester)-4-thioxo-2-cyclobuten-1-one (squarate 2). The structures of SQI–SQIII were elucidated using NMR analysis. The electrochemical applications of two of these metal–squaric acid systems (SQI and SQII) were also investigated. Incorporation of SQII as a mediator, in the previously optimized Pt/p(HEMA)/p(pyrrole)/GOx electrode using the ionic liquid [bmim][BF 4] as the solvent medium, produced a biosensor with enhanced properties, namely a sensitivity of 175.9 mA/M d-glucose, working potential of +200 mV, large linear range (0–12 mM) and a detection limit of 1 × 10 −6 M.