Abstract

In order to compare the acceptor properties of 1,1'-dimethylene and 1,1'-trimethylene 2,2'-bipyridinium dications opposite to a donor cysteine and the stabilities of their cation radicals, UV-visible spectrophotometric and ESR studies were carried out. The diquat dication (DQ²⁺) forms a red charge-transfer (CT) complex with cysteine (Cys^2-) of the ion-pair type. The complex presents two CT bands at 348 and 494 nm, respectively. By use of the Benesi-Hildebrand and other treatments (Scott, Foster, and Scatchard), the association constant for the ion association and ɛ_λ were determined. The results obtained by all the procedures are in fair agreement. The triquat dication (TQ²⁺) does not form a CT complex with cysteine (Cys^2-). The triquat radical (TQ·⁺) was generated by its reduction with alkaline sodium dithionite, and it was detected by electron spin resonance spectroscopy at room temperature. The ESR spectrum of the diquat radical (DQ·⁺) was reinvestigated. The corresponding hyperfine coupling and g factors of the radicals DQ·⁺ and TQ·⁺ were given.

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