Spin Trapping of Inorganic Sulfur Radicals

PALERMO, JUAN CRUZ; SEMELAK, JONATHAN ALEXIS; ESTRIN, DARÍO ARIEL; BARI, SARA ELIZABETH

Rosario

Congreso; L Encuentro anual de la Sociedad Argentina de Biofísica; 2022

Sociedad Argentina de Biofísica

Trapping of free radicals by using nitrones such as dimethyl-pirroline-N-oxide (DMPO) is a widespread practice for the detection of radicals by means of their radical adducts. As for sulfur redox chemistry, most studies focus on spin trapping of thiyl radicals, RS*, with less attention on inorganic sulfur radicals, such as sulfanyl, HS*. However, this radical is part of the bio-relevant family of reactive sulfur species which are gaining increasing interest, for example, in reactive processes towards hemeproteins.To recognize the role of radicals as reaction intermediates in the sulfide-mediated reduction of metmyoglobin, MbFeIII, we performed the addition of inorganic sulfide to the protein in the presence of the spin traps DMPO, and phenyl-tert-butyl-nitrone (PBN), a cyclic and a linear nitrone, respectively. We found that the presence of DMPO did not interfere with the kinetics of the reduction reaction, while the reaction was hampered in the presence of PBN. To assist the interpretation of the experimental results, we performed DFT calculations with implicit solvent (Polarizable Continuum Model) to compare stability the aduct intermediate. We evaluated DMPO, PBN and ethylene-terbutyl-nitrone (EBN) comparatively, towards the addition of HS* radical, and subsequent possible reactions (e.g.: disproportionation, reaction with hydrosulfide anion) by calculating Gibbs free energy for all the species involved on those processes. The free energy changes for the addition of HS* and the subsequent reactions evaluated is negative for all the evaluated spin traps, suggesting that the thermodynamics regarding these processes are similar for DMPO and PBN. Taking into consideration that DMPO and PBN act as reversible traps of thiyl radicals at room temperature with varying timescales,1–3 we suggest that the main difference between them is due to kinetics. Thus, PBN reversibly traps radical reactive species generated in the onset of the reduction process, slowering it down.