Fast Reactivity of Cyclic Nitrone-Calix4pyrrole Conjugate with Superoxide Radical Anion: Theoretical and Experimental Studies

摘要

Nitrone spin traps have been employed as probes for the identification of transient radical species in chemical and biological systems using electron paramagnetic resonance (EPR) spectroscopy, and have found pharmacological activity against oxidative stress-mediated diseases. Since superoxide radical anion (O2^•−) is a major precursor to most reactive oxygen species and that calix[4]pyrroles have shown to exhibit high affinity to anions, cyclic nitrone conjugate of calix[4]pyrrole (CalixMPO) was designed, synthesized, and characterized. Computational studies at the PCM/B3LYP/6-31+G(d,p)//B3LYP/6-31G(d) suggest a pendant-type linkage between the calix[4]pyrrole and the nitrone to be the most efficient design for spin trapping of O2^•−, giving exoergic reaction enthalpies (ΔH298K,aq) and free energies (ΔG298K,aq) of -16.9 and -2.1 kcal/mol, respectively. ¹H NMR study revealed solvent-dependent conformational changes in CalixMPO leading to changes in electronic properties of the nitronyl group upon H-bonding with the pyrrole groups as also confirmed by calculations. CalixMPO spin trapping of O2^•− exhibited distinctive EPR spectra. Kinetic analysis of O2^•− adduct formation and decay in polar aprotic solvents using UV-vis stopped-flow and EPR method gave larger trapping rate constant for CalixMPO and longer half-life for its O2^•− adduct compared to the commonly used nitrones. The unusually high reactivity of CalixMPO to O2^•− was rationalized to be due to the synergy between α- and electrostatic effects by the calix[4]pyrrole moiety on the O2^•− and nitrone, respectively. This work demonstrates for the first time the application of an anion receptor for the detection of one the most important radical intermediates in biological and chemical systems (i.e., O2^•−).