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Figure 6.30. Carbon-centered radicals resulting from the abstraction of an α-hydrogen
atom, stabilized in the case of Gly but destabilized with other amino acids (adapted
from Abouelatta et al. [334] with the permission of the American Chemical Society).
See color insert.
substituted amino acids was relatively slow in comparison with a compound
derived from Gly. This difference in the reactivity of Gly from other aliphatic
amino acids was explained by considering captodative stabilization of the
radical formed (Fig. 6.30). For example, a nonbonding interaction between the
amide carbonyl and the side chain (R) in Ala destabilizes the coplanar con-
figuration in the radical, which does not have the full energetic favorability of
captodative stabilization. In contrast, the glycine radical can obtain resonance
stability through the coplanar alignment of six contiguous sp 2 -hybridized
atoms, necessary for maximization of the orbital overlap. Furthermore, steric
effects may also be a factor in the relative unreactivity of secondary and ter-
tiary amino acids [334].
The compounds derived from basic amino acids (Lys, Arg, and His) were
not as reactive with the [Fe IV (O)(N4Py)] 2+ species [334]. In contrast, the reac-
tivity of anilines was significantly higher with the ferryl species. The results
indicate a free base of amine may be required to participate in the electron
transfer-proton transfer (ET-PT) mechanism for the reaction of amines with
the [Fe IV (O)(N4Py)] 2+ species [334]. Decomposition rates of the ferryl species
for the compounds derived from Ser and Thr, containing alcohol functional
groups, were only about three to seven times faster than the control reaction.
Compounds derived from Asp, Gln, Glu, and Asn did not accelerate the
decomposition of ferryl species under pseudo-first-order reaction conditions,
having 10 equiv of the compound. Oxidation products of the compounds
derived from basic and polar amino models were not observed by nuclear
magnetic resonance (NMR) or ESMS analysis.
Sulfur-containing amino acids reacted very fast with the [Fe IV (O)(N4Py)] 2+
species. The reactivities of Cys and Met were approximately five and two
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