Using Cyclodextrins to Encapsulate Oxygen-Centered and Carbon-Centered Radical Adducts: The Case of DMPO, PBN, and MNP Spin Traps

Abstract

We present electron spin resonance (ESR) experiments that describe the interaction of beta-cyclodextrin (beta-CD) with spin adducts of three spin traps: 5,5-dimethyl-1-pyrroline N-oxide (DMPO), N-tert-butyl-alpha-phenylnitrone (PBN), and 2-methyl-2-nitrosopropane (MNP). The focus was on spin adducts of oxygen-centered radicals trapped by DMPO and PBN and on carbon-centered radical adducts trapped by MNP. The radicals were generated by reaction with hydroxyl radicals and the spin adducts studied were DMPO/OH and PBN/OH, MNP/CH(2)COOH generated in CH(3)COOH, and MNP/CF(2)COOH in CF(2)HCOOH. Di-tert-butyl nitroxide ((CH(3))(3)C)(2)NO (DTBN) was also detected in experiments with MNP as the spin trap. A range of interactions of the spin adducts and DTBN with beta-CD was identified. The presence of beta-CD led to significant stabilization of DMPO/OH and PBN/OH but to a negligible effect on the (14)N hyperfine splitting of the adducts, a(N), indicating that the N-O group is outside the beta-CD cavity. An increase of a(N) was detected for DTBN and MNP/CH(2)COOH in CH(3)COOH in the presence of beta-CD, a result we assigned to bonding at the rim of the host. Experiments with methylated beta-CD (Me beta-CD) provided support for this conclusion. A different type of complexation was detected for DTBN and MNP/CF(2)COOH in CF(2)HCOOH: for specific host concentrations both "in" and "out" species were detected. We suggest that the hydrophobicity of the fluorinated adduct leads to insertion of the adduct inside the host cavity. Calculation of the association constant K(a) indicated the competition between DTBN and the adduct for inclusion in the host. For MNP as spin trap, the two nitroxide radicals (adduct and DTBN) have the same type of interaction with the host: at the rim in acetic acid, and inside the host cavity in CF(2)HCOOH. Experiments with DTBN in the absence of the spin trap and of adducts illuminated the effect of the local polarity and of the pH on the hyperfine splittings and indicated that the presence of acetic acid encourages rim complexation.

Characterization of inclusion complex of vitamin E compound with 2,6-di-O-methylated β-cyclodextrin as the solubility enhancer and its kinetic determination for radical scavenging ability

“…The upfield chemical shift indicates that the C(2)-H moiety is located close to the polar -OCH 3 groups of the CD portals and the downfield chemical shift indicates that the -CH 3 and C(3)-H moieties are encapsulated into the DM-b-CD cavity. Figure 2b depicts a plausible structure of the inclusion complex with DM-b-CD, which is in agreement with the optimized structure of the inclusion complex of native b-CD with DMPO-OH adduct using Gaussian 03 software [12].…”

Ceria loaded nanoreactors: a nontoxic superantioxidant system with high stability and efficacy

“…In the absence of CeNP (control), the time scans for the DMPO/ . OH adduct show a significant decrease with time, and a half-life (τ 1/2 ) of 960 s, which is similar to the reported value 48,58 . Surprisingly, in the presence of 2.5 mM and 1.5 mM free CeNP the relative intensity of the DMPO/ .…”

“…The characteristic DMPO/ . OH spin adduct has a half-life of around 15 min 57 , and its EPR spectrum consists of four lines with peak intensity ratios of 1:2:2:1 and hyperfine coupling constants a N and a H for 14 N and 1 H equal to 14.9 G (inset Figure 3A). Hydroxyl radicals were generated by a typical Fenton reaction involving Fe 2+ and H 2 O 2 , 42 and their scavenging activity by CeNP was estimated by analyzing the plots (time scans) for DMPO/ .…”

“…The half-life of hydroxyl radicals (HO • ) in solution at room temperature is quite low (100 µs), 55 but their formation and decay can be investigated by spin trapping EPR, in which the addition reaction of a free radical to a spin trap forms a more stable spin adduct with paramagnetic parameters that depend on the nature of the trapped radical. 56 The characteristic DMPO/ • OH spin adduct has a half-life of around 15 min, 57 and its EPR spectrum consists of four lines with peak intensity ratios of 1 : 2 : 2 : 1 and hyperfine coupling constants a N and a H for 14 N and 1 H equal to 14.9 G (inset Fig. 3A).…”

Atom transfer coupling reactions performed with benign reducing agents and radical traps

“…β‐CD as the reducing agent provided an interesting test, as β‐CD is known to encapsulate the nitroso radical trap used in these systems . There is even evidence that β‐CD may lead to higher amounts of the active monomeric form of MNP, which exists in an equilibrium with inactive aggregates .…”