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Spin Delocalization in the Radical Cations of Porphyrin Molecular Wires: A New Perspective on EPR Approaches.

ABSTRACT: The spin delocalization in the radical cations of a series of ethyne-linked oligoporphyrins was investigated using EPR spectroscopy. The room-temperature spectral envelope for these oligomers deviates significantly from the benchmark N^-0.5 trend in line width expected for a completely delocalized spin density, in contrast to the butadiyne-linked analogues measured previously. Here, we show, using DFT calculations and complementary low-temperature ENDOR measurements, that this deviation is primarily driven by a more pronounced inequivalence of the ¹⁴N spins within individual subunits for the ethyne-linked oligoporphyrins. Once this ¹⁴N inequivalence is taken into consideration, the room-temperature and ENDOR spectra for both butadiyne-linked and ethyne-linked oligomers, up to N = 5, can be simulated by similar static delocalization patterns. This work highlights the importance of EPR in exploring such spin delocalization phenomena while also demonstrating that the N^-0.5 trend should not be interpreted in isolation but only in combination with careful simulation and theoretical modeling.

SUBMITTER: Moise G

PROVIDER: S-EPMC6778911 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Recently, we showed that peroxynitrite (ONOO(-)) reacts directly and rapidly with aromatic and aliphatic boronic acids (k ? 10(6) M(-1)s(-1)). Product analyses and substrate consumption data indicated that ONOO(-) reacts stoichiometrically with boronates, yielding the corresponding phenols as the major product (?85-90%), and the remaining products (10-15%) were proposed to originate from free radical intermediates (phenyl and phenoxyl radicals). Here, we investigated in detail the minor, free radical pathway of boronate reaction with ONOO(-). The electron paramagnetic resonance (EPR) spin-trapping technique was used to characterize the free radical intermediates formed from the reaction between boronates and ONOO(-). Using 2-methyl-2-nitrosopropane (MNP) and 5-diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide (DEPMPO) spin traps, phenyl radicals were trapped and detected. Although phenoxyl radicals were not detected, the positive effects of molecular oxygen, and inhibitory effects of hydrogen atom donors (acetonitrile, and 2-propanol) and general radical scavengers (GSH, NADH, ascorbic acid, and tyrosine) on the formation of phenoxyl radical-derived nitrated product, suggest that the phenoxyl radical was formed as the secondary species. We propose that the initial step of the reaction involves the addition of ONOO(-) to the boron atom in boronates. The anionic intermediate undergoes both heterolytic (major pathway) and homolytic (minor pathway) cleavage of the peroxy (O-O) bond to form phenol and nitrite as a major product (via a nonradical mechanism), or a radical pair PhB(OH)(2)O(•-)···(•)NO(2) as a minor product. It is conceivable that phenyl radicals are formed by the fragmentation of the PhB(OH)(2)O(•-) radical anion. According to the DFT quantum mechanical calculations, the energy barrier for the dissociation of PhB(OH)(2)O(•-) radical anion to form phenyl radical is only a few kcal/mol, suggesting rapid and spontaneous fragmentation of the PhB(OH)(2)O(•-) radical anion in aqueous media. Biological implications of the minor free radical pathway are discussed in the context of ONOO(-) detection, using the boronate probes.

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Spin Delocalization in the Radical Cations of Porphyrin Molecular Wires: A New Perspective on EPR Approaches.

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