• H Mohan

      Articles written in Journal of Chemical Sciences

    • Quenching of nucleotide-derived radicals bybisbenzimidazole derivative Hoechst-33258 in aqueous solution

      H Mohan A Adhikary V Jain J P Mittal

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      The pulse radiolysis technique has been employed to investigate the reaction of DNA-minor-groove ligand bisbenzimidazole Hoechst 33258 with pyrimidine and purine nucleotide-derived radicals. Formation of an N-centred Hoechst-33258 radical is observed. Bimolecular rate constants and the yields of Hoechst-33258 radical have been evaluated. While the rate constant for the reaction of pyrimidine-derived radicals with Hoechst-33258 remained the same (1–2) × 109 dm3 mol−1 s−1, the yields of the Hoechst-33258 radical varied from 25% (5′-cytidine monophosphate) to 75% (5′-guanosine monophosphate) under anoxic conditions. The rate constant values for the reaction of purine-derived radicals with Hoechst-33258, under oxic and anoxic conditions, remained the same whereas with pyrimidine-derived radicals, the rate constant value under oxic conditions was about two orders of magnitude lower than under anoxic conditions. The difference in the yields of Hoechst-33258 radical with various nucleotide-derived radicals suggest the formation of different types of radicals and that the reaction mainly occurs by electron transfer from Hoechst-33258 to the nucleotide radicals.

    • Redox chemistry ofo- andm-hydroxycinnamic acids: A pulse radiolysis study

      P Yadav H Mohan B S M Rao J P Mittal

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      Radiation chemical reactions ofOH, O•−, N3and eaqt- witho- and m-hydroxycinnamic acids were studied. The second-orderrateconstantsforthereaction ofOH with ortho and meta isomers in buffer solution at pH7 are 3.9±0.2 × 109 and 4.4 ± 0.3 × 109 dm3 mol-1 s-1 respectively. At pH 3 the rate with the ortho isomer was halved (1.6 ± 0.4 × 109 dm3 mol-1 s-1) but it was unaffected in the case of meta isomer (k = 4.2±0.6 × 109dm3mol-1 s-1). The rate constant in the reaction of N3 with the ortho isomer is lower by an order of magnitude (k = 4.9 ± 0.4 × 108 dm3 mol-1s-1). The rates of the reaction of eaqt- with ortho and meta isomers were found to be diffusion controlled. The transient absorption spectrum measured in theOH witho-hydroxycinnamic acid exhibited an absorption maximum at 360 nm and in meta isomer the spectrum was blue-shifted (330 nm) with a shoulder at 390 nm. A peak at 420 nm was observed in the reaction of Obb−with theo-isomer whereas the meta isomer has a maximum at 390 and a broad shoulder at 450 nm. In the reaction of the absorption peaks were centred at 370–380 nm in both the isomers. The underlying reaction mechanism is discussed.

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