Charge transfer complex formation constants of edta with methylviologen And its zwitterionic derivatives
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In the photochemical cycle of the labelled, tris (2,2'bipyridine) ruthenium(II), Ru(bpy)32+, the photo excited label is oxidized through electron transfer quenching by methylviologen (l,l'-dimethyl-4,4'-bipyridinium dication, MV ) producing the Ru(III) complex and MV+*. The resulting radical cation, MV+*, reacts with water to produce hydrogen peroxide which is detected electrochemically to produce the analytical signal. In the presence of EDTA, as a sacrificial donor, Ru(III) is reduced back to Ru(II) and the cycle is completed. The strong ground state interaction between viologen and EDTA results in a charge-transfer complex formation. This interaction competes with the excited-label quenching step and produces quite large background current. Replacing the dipositive viologen cation with the corresponding, net zero-charged zwitterionic derivatives is expected to reduce such background problem. Two derivatives were chosen, l,l'-dipropanesulfonate-4,4'-bipyridinium, PVS, and l,l'-dibutanesulfonate-4,4'bipyridinium, BVS to replace methyl viologen MV^"1". Better base line was obtained using both derivatives. Determination of the formation constants of the charge transfer complexes formed between EDTA and the three quenchers is very important. Such values are crucial for any theoretical or experimental study of the already complicated complex equilibrium involved in such biosencing scheme. Both Benesi-Hilebrand analysis and Nash treatment were applied to determine the formation constants spectrophotometrically and lower values were obtained for the zwitterionic derivatives.