Spin trapping of nitric oxide by aci anions of nitroalkanes

Abstract

In alkaline solutions, nitroalkanes (RCH 2NO 2) undergo deprotonation and rearrange to an aci anion (RHCNO 2 −), which may function as a spin trap. Using electron paramagnetic resonance (EPR) spectroscopy, we have investigated suitability of aci anions of a series of nitroalkanes (CH 3NO 2, CH 3CH 2NO 2, CH 3(CH 2) 2NO 2, and CH 3(CH 2) 3NO 2) to spin trap nitric oxide ( NO). Based on the observed EPR spectra, the general structure of the adducts, formed by addition of NO to RHCNO 2 −, was identified as nitronitroso dianion radicals of general formula [RC(NO)NO 2] 2− in strong base (0.5 M NaOH), and as a mono-anion radical [RCH(NO)NO 2] − in alkaline buffers, pH 10–13. The hyperfine splitting on 14N in the NO 2 moiety (11.2–12.48 G) is distinctly different from the splitting on 14N in the NO moiety of the adducts (5.23–6.5 G). The structure of the adducts was verified using 15N-labeled NO, which produced radicals, in which triplet due to splitting on 14N ( I=1) in 14NO/ aci nitro adducts was replaced by a doublet due to 15N ( I=1/2) in 15NO/ aci nitro adducts. EPR spectra of aci nitromethane/NO adduct recorded in NaOH and NaOD (0.5 M) showed that the hydrogen at α-carbon can be exchanged for deuterium, consistent with structures of the adducts being [CH(NO)NO 2] 2− and [CD(NO)NO 2] 2− , respectively. These results indicate that nitroalkanes could potentially be used as prototypes for development of NO-specific spin traps suitable for EPR analysis.