ESR Study of the Radical Cations 1,1′-Dimethylene and 1,1′-Trimethylene 2,2′-Bipyridinium: Charge-Transfer Complexes between Their Dications and the Donor Cysteine

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 (Cys²–) 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 ε_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.

Keywords

2,2′-Bipyridinium derivatives Charge-transfer complex Cysteine ESR spectroscopy

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