Kinetic and Thermodinamic considerations on Spin trapping of inorganic sulfur radicals

PALERMO, JUAN CRUZ; SEMELAK, JONATHAN A; ESTRIN, DARÍO A.; BARI, SARA E:

Congreso; L Reunión de la Sociedad Argentina de Biofísica; 2022

Trapping of free radicals by using nitrones such as dimethyl-pirroline-N-oxide (DMPO) is awidespread practice for the detection of radicals by means of their radical adducts. As forsulfur redox chemistry, most studies focus on spin trapping of thiyl radicals, RS*, with lessattention on inorganic sulfur radicals, such as sulfanyl, HS*. However, this radical is partof 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-mediatedreduction of metmyoglobin, MbFeIII, we performed the addition of inorganic sulfide to theprotein in the presence of the spin traps DMPO, and phenyl-tert-butyl-nitrone (PBN), acyclic and a linear nitrone, respectively. We found that the presence of DMPO did notinterfere with the kinetics of the reduction reaction, while the reaction was hampered inthe presence of PBN.To assist the interpretation of the experimental results, we performed DFT calculationswith implicit solvent (Polarizable Continuum Model) to compare stability the aductintermediate. 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 freeenergy for all the species involved on those processes. The free energy changes for theaddition of HS* and the subsequent reactions evaluated is negative for all the evaluatedspin traps, suggesting that the thermodynamics regarding these processes are similar forDMPO and PBN.Taking into consideration that DMPO and PBN act as reversible traps of thiyl radicals atroom temperature with varying timescales, a similar behavior towards inorganic sulfurradicals can be proposed. Kinetic studies are still necessary to complete the interpretation.AcknowldegmentsJ.C.P and J.A.S gratefully acknowledge CONICET for their fellowships. D.A.E., is University of Buenos Airesand CONICET staff. S.E.B. is CONICET staff.We also acknowledge Dr. Carlos Brondino from Universidad del Litoral for the useful discussions andsuggestions on using PBN as a spin trap.