Cupric ions chelation activities of the plants did not
show any significant correlation with any of the three assays
(Table 1). Phenolic compounds have been reported
to be chelators of free metal ions. However, a poor correlation
of cupric ion chelating activity with TPC of all plants
was observed and this might indicate that phenolic compounds
might not be the main chelators of copper ions.
In a complex mixture, organic acids, amino acids and sugars
can be sequesters of transition metal ions. In addition,
the ability for phenolic compounds to chelate metal ions
depend on the availability of properly oriented functional
groups (van Acker et al., 1996). A sample high in polyphenols
might not chelate metal if the polyphenols present did
not have suitable groups that could chelate the cations.
Bidentate ligands are more powerful scavengers of metal
cations than monondentate ligands, for example, catechol
binds ferric ions tightly whereas phenol does not (Hider,
Liu, & Khodr, 2001). When a phenolic group is conjugated
with a carbohydrate group, as in naturally-occurring phenolic
glycosides, it can no longer bind metals (Hider
et al., 2001). So far, no clear relationship had been reported
between free radical scavenging activity, ferric ions reducing
abilities with metal ion chelation activities.