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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

Volume 48, Number 8 (Aug. 1994) Page 926-932

Sánchez-Palacios, Angela; Delgado, Rut

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 ɛλ 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 (Cys2-). 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.