Inhibition of Free Radical Degradation in Medical Grade Hyaluronic Acid - Hyaluronic Acid for Biomedical and Pharmaceutical Applications - page 112

The formed · OH radicals were demonstrated to disappear after 1
h by electron paramagnetic resonance [30]. Further, a propagation
phase can occur, which is summarised in the reactions below
( Equations 5.8-59 ).

Polymers with -CH groups, which include HA, are readily degraded
by · OH radicals. The · OH radical abstracts the H · radical from the
HA molecule to produce a HA-derived C-macroradical the so called
alkyl radical (A · ):

HA + · OH → A · + H 2 O

(5.7)

Under aerobic conditions, during a phase known as propagation, a
dioxygen molecule reacts with the alkyl radical to form peroxy-type
radicals (AOO · ):

A · + O 2 → AOO ·

(5.8)

Peroxyl radicals, by random trapping of H · radicals from adjacent
HA macromolecules, form hydroperoxyls, and novel HA-derived
C-macroradicals are generated:

AOO · + HA → AOOH + A · (5.9)

Due to a continual process of propagation reactions biopolymer
fragments of lower mass are formed, which leads to the decrease of
the HA solution dynamic viscosity. The radical process involving the
four steps (initiation, propagation, transfer, and termination) can
be stopped by the addition of a free radical scavenger. When such a
scavenger is admixed into the HA solution before applying WBOS,
the scavenger may be tested as a preventive antioxidant (against the
production of · OH radicals), while, when the substance is added
during the propagation phase of the HA degradation, the substance
is examined as a chain-breaking antioxidant (against the production
of peroxy-type radicals AOO · ).