Journal of Agricultural and Food Chemistry
Article
All of these results confirm that polyphenols can produce the
Strecker degradation of amino acids in the absence of added
oxidants. In addition, analogous reactions can also be produced
in amines, therefore demonstrating that the presence of the
carboxylic group might be desirable but not essential. The only
requisite for polyphenols to produce Strecker-type degradations
is the presence of two hydroxyl groups in ortho or para
positions. On the other hand, the presence of two hydroxyl
groups in meta position in another part of the molecule can
inhibit the Strecker-degrading ability of the hydroxyl groups in
ortho or para positions.
AUTHOR INFORMATION
■
Corresponding Author
*(F.J.H.) Phone: +34 954 611 550. Fax: +34 954 616 790. E-
Figure 8. Proposed pathway for the reaction of phenylalanine with
hydroquinone and benzoquinone.
Funding
This study was supported in part by the European Union
(FEDER funds) and the Plan Nacional de I + D of the
hydroxyphenyl)aldimine. After hydrolysis of the latter aldimine,
phenylacetaldehyde is produced.
́
Ministerio de Economıa y Competitividad of Spain (Project
AGL2012-35627).
These pathways explain the relative activities of simple di-
and trihydroxy derivatives, which are related to their relative
facility to be converted into quinones. In addition, the
electronic effects of other substituents in the ring will also
play a role in the overall reaction yield. On the contrary, it does
not explain the low yields of phenylacetaldehyde obtained with
complex phenols. These last results are likely a consequence of
two opposite effects. On the one hand, the above results
demonstrate that o- and p-diphenols are easily transformed into
quinones, and these derivatives are able to convert amino acids
and amines into their corresponding carbonyl derivatives. On
the other, m-diphenols have been shown to be very efficient
carbonyl scavengers.10,24,25 The obtained results seem to
suggest that the carbonyl-scavenging ability of the A-ring is
higher than the Strecker-inducer capacity of the B-ring. Only
myricetin, which has a pyrogallol moiety in the B-ring, seemed
to increase slightly the phenylacetaldehyde produced in
comparison to control, although this increase was not
significant.
A further proof of this pathway was obtained when the
reaction between benzoquinone and phenylglycine methyl ester
was studied. The reaction not only produced compounds
identical to those of the hydroquinone/phenylglycine methyl
ester reaction mixtures but significant amounts of hydro-
quinone were found, therefore confirming the existence of
redox processes in these mixtures that would facilitate the
conversion of benzoquinone into hydroquinone and vice versa.
For the same reason, the appearance of benzoic acid as an
oxidation product of benzaldehyde is not surprising. The only
differences between benzoquinone and hydroquinone for these
reactions are the reaction yield, which was higher for
benzoquinone (Table 1 and Figure 3), and the lower reaction
time needed for benzoquinone to produce phenylacetaldehyde
(Figure 4). Both results are in agreement with the conversion of
hydroquinone into benzoquinone as a previous step for the
Strecker degradation. Nevertheless, Ea for phenylacetaldehyde
formation in benzoquinone/phenylalanine reaction mixtures
was only slightly lower than Ea for phenylacetaldehyde
formation in hydroquinone/phenylalanine reaction mixtures,
therefore indicating that conversion of hydroquinone into
benzoquinone is not an energetically difficult process, although
it does not seem to be very favored kinetically.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are indebted to Jose
́
L. Navarro for technical assistance.
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