Model studies on the degradation of phenylalanine initiated by lipid hydroperoxides and their secondary and tertiary oxidation products

Article abstract of DOI:10.1021/jf801409w

The reaction of methyl 13-hydroperoxyoctadeca-9,11-dienoate (MeLOOH), methyl 13-hydroperoxyoctadeca-9,11,15-trienoate (MeLnOOH), methyl 13-hydroxyoctadeca-9,11-dienoate (MeLOH), methyl 13-oxooctadeca-9,11-dienoate (MeLCO), methyl 9,10-epoxy-13-hydroxy-11-octadecenoate (Me-LEPOH), and methyl 9,10-epoxy-13-oxo-11-octadecenoate (MeLEPCO) with phenylalanine was studied to determine the comparative reactivity of primary, secondary, and tertiary lipid oxidation products in the Strecker degradation of amino acids. All assayed lipids were able to degrade the amino acid to a high extent, although the lipid reactivity decreased slightly in the following order: MeLEPCO ≥ MeLCO > MeLEPOH ≥ MeLOH > MeLOOH ≈ MeLnOOH. These data confirmed the ability of many lipid oxidation products to degrade amino acids by a Strecker-type mechanism and suggested that, once the lipid oxidation is produced, a significant Strecker degradation of surrounding amino acids should be expected. The contribution of different competitive mechanisms to this degradation is proposed, among which the conversion of the different lipid oxidation products assayed into the most reactive MeLEPCO and the fractionation of long-chain primary and secondary lipid oxidation products into short-chain aldehydes are likely to play a major role.

Full text of DOI:10.1021/jf801409w

7970 J. Agric. Food Chem. 2008, 56, 7970–7975
Model Studies on the Degradation of Phenylalanine
Initiated by Lipid Hydroperoxides and Their
Secondary and Tertiary Oxidation Products
ROSARIO ZAMORA, EMERENCIANA GALLARDO, AND FRANCISCO J. HIDALGO*
Instituto de la Grasa, Consejo Superior de Investigaciones Cient´ıficas, Avenida Padre Garc´ıa Tejero 4,
41012 Seville, Spain
The reaction of methyl 13-hydroperoxyoctadeca-9,11-dienoate (MeLOOH), methyl 13-hydroperoxy-
octadeca-9,11,15-trienoate (MeLnOOH), methyl 13-hydroxyoctadeca-9,11-dienoate (MeLOH), methyl
13-oxooctadeca-9,11-dienoate (MeLCO), methyl 9,10-epoxy-13-hydroxy-11-octadecenoate (Me-
LEPOH), and methyl 9,10-epoxy-13-oxo-11-octadecenoate (MeLEPCO) with phenylalanine was
studied to determine the comparative reactivity of primary, secondary, and tertiary lipid oxidation
products in the Strecker degradation of amino acids. All assayed lipids were able to degrade the
amino acid to a high extent, although the lipid reactivity decreased slightly in the following order:
MeLEPCO g MeLCO > MeLEPOH g MeLOH > MeLOOH ≈ MeLnOOH. These data confirmed the
ability of many lipid oxidation products to degrade amino acids by a Strecker-type mechanism and
suggested that, once the lipid oxidation is produced, a significant Strecker degradation of surrounding
amino acids should be expected. The contribution of different competitive mechanisms to this
degradation is proposed, among which the conversion of the different lipid oxidation products assayed
into the most reactive MeLEPCO and the fractionation of long-chain primary and secondary lipid
oxidation products into short-chain aldehydes are likely to play a major role.
KEYWORDS: Carbonyl-amine reactions; hydroperoxides; hydroxydienes; ketodienes; lipid oxidation;
lipid oxidation products; Maillard reaction; Strecker aldehydes
INTRODUCTION
reaction took place in two steps. First, alkadienals and ketodienes
were oxidized to the corresponding epoxy derivatives (10) and,
then, the produced compounds reacted directly with the amino
acid as described above.
Strecker degradation of amino acids is a very important route
leading to final aroma compounds in the Maillard reaction (1, 2).
Thus, it is the origin of significant flavor compounds such as
Strecker aldehydes and pyrazines, for example (3). However,
diverse studies have shown that Strecker aldehydes can also be
produced by the reaction of amino acids with lipid oxidation
products having two oxygenated functions including epoxyalk-
enals (4), unsaturated epoxyketo fatty esters (5), and hydroxy-
alkenals (6). These compounds react with the amino group of
the amino acid to form the corresponding imine, which, after
electronic rearrangement, decarboxylation, and hydrolysis,
produces the corresponding Strecker aldehyde. Alternatively,
this reaction can also take place without decarboxylation (7)
and, therefore, suitable amines can also be converted into their
corresponding aldehydes by this last mechanism (8).
Furthermore, recent studies have shown that Strecker deg-
radation of amino acids can also be produced by unoxidized
lipids. Thus, when unoxidized lipids were heated in the presence
of amino acids, the Strecker degradation of these last compounds
was produced to an extent that depended on the presence of
other compounds, that is, carbohydrates, which contributed to
the lipid oxidation (11). The Strecker degradation observed in
this last study was hypothesized to be produced by reaction of
the produced lipid oxidation products with the amino acid
following one of the mechanisms described in previous
studies (4-6). However, although alkadienals and ketodienes
are only a small part of the lipid oxidation products formed,
the yield of the reaction obtained with unoxidized lipids was
not much lower than the yield obtained with alkadienals or
ketodienes, therefore suggesting that other lipid oxidation
products, in addition to alkadienals and ketodienes, are likely
to be contributing to the Strecker degradation of the amino acids
observed.
Lipid oxidation products with only one oxygenated function
also degrade amino acids. However, the oxidized lipid has to
be oxidized as a step prior to its reaction with the amino acid.
Thus, alkadienals and ketodienes degraded phenylalanine to
phenylacetaldehyde when mixtures of the oxidized lipid and
the amino acid were heated in the presence of oxygen (9). This
In an attempt to uncover the routes by which unoxidized lipids
produce the Strecker degradation of amino acids, this study
analyzes the reaction of the primary lipid oxidation products
* Author to whom correspondence should be addressed [telephone
+(34) 954 611 550; fax +(34) 954 616 790; e-mail fhidalgo@ig.csic.es].
10.1021/jf801409w CCC: $40.75  2008 American Chemical Society
Published on Web 08/16/2008

Strecker Degradation Initiated by Lipid Hydroperoxides.
J. Agric. Food Chem., Vol. 56, No. 17, 2008 7971
ether (4:1) as the eluent. This compound was obtained chromatographi-
cally pure and exhibited the previously described 1H and 13C NMR
spectra (13).
Methyl 9,10-epoxy-13-oxo-11-octadecenoate (MeLEPCO) was pre-
pared by epoxidation of MeLCO with 3-chloroperoxybenzoic acid (16).
MeLEPCO was purified by column chromatography on silica gel using
hexane/diethyl ether (4:1) as the eluent. This compound was obtained
chromatographically pure and exhibited the previously described 1H
and 13C NMR spectra (13).
Methyl 9,10-epoxy-13-hydroxy-11-octadecenoate (MeLEPOH) was
prepared by reduction of MeLEPCO with sodium borohydride (13).
MeLEPOH was purified by column chromatography on silica gel using
hexane/diethyl ether (3:2) as the eluent. This compound was obtained
chromatographically pure and exhibited the previously described 1H
and 13C NMR spectra (13).
Lipid/Amino Acid Reaction Mixtures. Mixtures of 0-25 µmol of
lipid derivative and 25 µmol of phenylalanine in 0.5 mL of buffer were
introduced in Schott Duran test tubes (16 × 1.5 cm), which were closed
and heated at 180 °C. The atmosphere of the test tube was air, unless
otherwise indicated. The buffers employed for controlling the reaction
pH were 0.3 M sodium citrate buffer, pH 2.15-6.0; 0.3 M sodium
phosphate buffer, pH 6.0-8.0 and 11.0-12.0; or 0.3 M sodium borate
buffer, pH 8.0-10.0. At the end of the heating period, samples were
cooled, diluted with 1 mL of acetonitrile and 50 µL of internal standard
solution (54.8 mg of methyl heptanoate in 25 mL of methanol), and
analyzed by GC-MS.
GC-MS Analyses. GC-MS analyses were conducted with a Hewlett-
Packard 6890 GC Plus coupled with an Agilent 5973 MSD (mass
selective detector, quadrupole type). A fused-silica HP5-MS capillary
column (30 m × 0.25 i.d.; coating thickness, 0.25 µm) was used.
Working conditions were as follows: carrier gas helium (1 mL/min at
constant flow); injector, 250 °C; oven temperature programmed from
70 (1 min) to 240 at 5 °C/min and then to 325 at 10 °C/min; transfer
line to MSD, 280 °C; and ionization EI, 70 eV.
Determination of Phenylacetaldehyde Content. Quantification of
phenylacetaldehyde was carried out, as described previously (9), by
preparing standard curves of the aldehyde in the 1.55 mL of solution
prepared for GC-MS injection (see above). For each curve, eight
different concentration levels of the aldehyde were used. Phenylac-
etaldehyde content was directly proportional to the aldehyde/internal
standard area ratio (r ) 0.999, p < 0.0001). The coefficients of variation
were <10%. All data are mean values of, at least, two independent
experiments.
(the lipid hydroperoxides) with amino acids. Lipid hydroper-
oxides are very rapidly produced under the oxidizing conditions
in which amino acids are degraded. Therefore, the study of lipid
hydroperoxide/amino acid reaction mixtures may add to the
understanding of the Strecker aldehyde formation in complex
mixtures including unoxidized lipids, carbohydrates, and amino
acids, for example (11). These studies were carried out in model
systems of lipid hydroperoxides (derived from both linoleic and
linolenic acids as major n6 and n3 fatty acids, respectively) and
phenylalanine (as a model amino acid that produces the
important flavor phenylacetaldehyde). In addition, studies with
the hydroxydiene and the ketodiene derived from linoleic acid,
as major secondary oxidation products of linoleic acid hydro-
peroxide, and studies with epoxyhydroxy and epoxyketo
unsaturated fatty esters, as tertiary lipid oxidation products, have
also been included for comparison.
EXPERIMENTAL PROCEDURES
Materials. All chemicals were purchased from Aldrich (Milwaukee,
WI), Sigma (St. Louis, MO), Fluka (Buchs, Switzerland), or Merck
(Darmstadt, Germany) and were of analytical grade. In particular, the
following compounds were purchased to be employed in the identifica-
tion of lipid/phenylalanine reaction products by GC-MS: hexanal,
2-hexenal, benzaldehyde, 2-pentylfuran, phenylacetaldehyde, 2-octenal,
methyl octanoate, and 2,4-decadienal. 1-Phenethyl-1H-pyrrole and
2-pentyl-1-phenethyl-1H-pyrrole were prepared as described previously
(12). Methyl 13-hydroxyoctadeca-9,11-dienoate (MeLOH) and methyl
13-oxooctadeca-9,11-dienoate (MeLCO) were prepared as described
below. The techniques employed to confirm the purity of the prepared
compounds were mostly thin layer chromatography (TLC) and 1H and
13C nuclear magnetic resonance (NMR) spectroscopy. In addition, when
prepared compounds were sufficiently stable to be analyzed by gas
chromatography (GC), this technique was also employed.
Methyl 13-hydroperoxyoctadeca-9,11-dienoate (MeLOOH) and meth-
yl 13-hydroperoxyoctadeca-9,11,15-trienoate (MeLnOOH) were pre-
pared by oxidation of the corresponding fatty acids with lipoxygenase
following a previously described procedure (13). The obtained hydro-
peroxides were esterified with diazomethane and purified by column
chromatography on silica gel using hexane/diethyl ether (7:3) as the
eluent. MeLOOH and MeLnOOH were obtained chromatographically
pure. Additional confirmations of identity and purity were obtained by
1D and 2D NMR. 13C NMR (CDCl3, 75.4 MHz) of MeLOOH: δ 174.49
(C1), 133.96 (C9), 131.23 (C12), 130.11 (C11), 127.51 (C10), 86.85
(C13), 51.54 (OCH3), 34.06 (C2), 32.49 (C14), 31.72 (C16), 29.35,
29.03, 29.00, 28.87 (C4-C7), 27.69 (C8), 24.98 (C15), 24.85 (C3),
22.51 (C17), and 14.04 (C18). This spectrum was identical to that
previously described by Dussault et al. (14). 13C NMR (CDCl3, 75.4
MHz) of MeLnOOH: δ 174.53 (C1), 134.35, 134.11 (C9,C16), 130.37
(C12), 130.20 (C11), 127.50 (C10), 123.09 (C15), 86.21 (C13), 51.56
(OCH3), 34.07 (C2), 30.58 (C14), 29.34, 29.03, 29.00, 28.87 (C4-C7),
27.69 (C8), 24.85 (C3), 20.70 (C17), and 14.12 (C18).
RESULTS
Reaction of Lipid Hydroperoxides with Phenylalanine. The
reaction between lipid hydroperoxides and phenylalanine is
complex, and numerous compounds were formed. These
compounds were produced both by hydroperoxide and, to a
lesser extent, by amino acid decomposition and also by reaction
of the oxidized lipid with the amino acid. Thus, the formation
of hexanal, benzaldehyde, 2-pentylfuran, 2-octenal, methyl
octanoate, 2,4-decadienal, methyl 9-oxononanoate, methyl 8-(2-
furyloctanoate), MeLOH, and MeLCO could be easily identified
in the total ion chromatograms of MeLOOH and phenylalanine
reaction mixtures on the basis of their retention indices and mass
spectra (Table 1). Some reaction mixtures also produced the
pyrrole derivatives 1-phenethyl-1H-pyrrole and 2-pentyl-1-
phenethyl-1H-pyrrole, which were previously shown to be
produced in the reaction between alkadienals, hydroxyalkenals,
or epoxyalkenals with phenylalanine (9, 12). The major reaction
product detected by GC-MS in samples heated for very short
time periods (5-10 min) was MeLOH. However, when these
samples were heated for longer periods, the major reaction
product was the Strecker aldehyde phenylacetaldehyde.
Methyl 13-hydroxyoctadeca-9,11-dienoate (MeLOH) was prepared
by reducing the 13-hydroperoxide of linoleic acid with sodium
borohydride and later esterification with diazomethane (13). MeLOH
was purified by column chromatography on silica gel using hexane/
diethyl ether (7:3) as the eluent. This compound was obtained
chromatographically pure. Additional confirmations of identity and
purity were obtained by 1D and 2D NMR. 13C NMR (CDCl3, 75.4
MHz) of MeLOH: δ 174.36 (C1), 135.84 (C12), 132.80 (C9), 127.71
(C10), 125.69 (C11), 72.87 (C13), 51.47 (OCH3), 37.21 (C14), 34.00
(C2), 31.72 (C16), 29.40 (C6), 28.99, 28.99, 28.89 (C4-C6), 27.61
(C8), 25.08 (C15), 24.82 (C3), 22.56 (C17), and 14.03 (C18). This
spectrum was identical, for example, to that previously collected by
Ha¨ma¨la¨inen and Kamal-Eldin (15).
Methyl 13-oxooctadeca-9,11-dienoate (MeLCO) was prepared by
oxidation of 13-hydroxyoctadeca-9,11-dienoic acid with chromium
trioxide and later esterification with diazomethane (13). MeLCO was
purified by column chromatography on silica gel using hexane/diethyl
Analogous results were also obtained when MeLnOOH/
phenylalanine reactions were analyzed by GC-MS, although
some of the produced lipid derivatives were different. Particu-

7972 J. Agric. Food Chem., Vol. 56, No. 17, 2008
Zamora et al.
Table 1. Retention Indices of Compounds Identified in Phenylalanine/
MeLOOH and Phenylalanine/MeLnOOH Mixtures
compd name
retention index
hexanal
2-hexenal
801
854
benzaldehyde
2-pentylfuran
960
993
phenylacetaldehyde
2-octenal
methyl octanoate
1053
1060
1125
1306
2,4-decadienal
methyl 8-oxooctanoatea
methyl 9-oxononanoatea
1-phenethyl-1H-pyrrole
methyl 8-(2-furyl)octanoate
1316
1418
1428
1612
a
2-pentyl-1-phenethyl-1H-pyrrole
methyl 13-hydroxyoctadeca-9,11-dienoate
methyl 13-oxooctadeca-9,11-dienoate
1885
2252/2263/2293
2421
a These compounds were only tentatively identified on the basis of their mass
spectra.
Figure 2. Effect of lipid concentration on phenylacetaldehyde (PA)
formation in the reaction of (A) MeLOOH (O) or MeLnOOH (4) with
phenylalanine (Phe) and (B) MeLOOH (O), MeLOH (]), or MeLCO (0)
with phenylalanine (Phe) at 180 °C for 1 h. Samples were heated in 0.3
M sodium citrate buffer. The chemical structures of MeLOOH, MeLOH,
and MeLCO are given in Figure 5. MeLnOOH is the corresponding
analogous hydroperoxide of methyl linolenate.
Previous studies in the Strecker degradation initiated by
different lipid oxidation products were carried out at pH 3.
Although the Strecker degradation initiated by lipid hydroper-
oxides seemed to be produced at pH 2.15 to a higher extent
than at pH 3, a pH of 3 was selected for the rest of this study
to obtain results comparative to those previously obtained with
the different secondary and tertiary lipid oxidation products
assayed.
The yield of the Strecker degradation of the amino acid
increased with the amount of hydroperoxide added, therefore
confirming that, in hydroperoxide/phenylalanine systems, the
Strecker degradation of the amino acid was mostly a
consequence of the presence of the oxidized lipid (Figure
2A). However, the amount of the produced phenylacetalde-
hyde did not increase linearly with the amount of hydrop-
eroxide present, and the highest increases in phenylacetal-
dehyde were observed when small amounts of hydroperoxide
were added. This behavior was different from the linear
increases observed for epoxyalkenals (4), epoxyketo fatty
esters (5), alkadienals, and ketodienes (9) and similar to that
observed for hydroxyalkenals (6). The yield of phenylac-
etaldehyde as a function of hydroperoxide concentration could
be described using a Boltzmann fit (r2 > 0.97) for both
MeLOOH and MeLnOOH.
The yield of the Strecker degradation also depended on the
incubation time, and the amount of the Strecker aldehyde increased
linearly (r > 0.98, p < 0.02) for the first 30 min until achieving a
maximum, which seemed to increase slowly afterward (Figure 3A).
This behavior was similar to that observed previously for other
lipid oxidation products (4-6, 9).
Comparative Reactivity of Primary, Secondary, and
Tertiary Lipid Oxidation Products in the Formation of
Phenylacetaldehyde in Phenylalanine/Oxidized Lipid Reac-
tion Mixtures. The comparative reactivity of different lipid
oxidation products was studied in an attempt to understand the
mechanism by which lipid hydroperoxides are able to produce
the Strecker degradation of the amino acids. Figure 2B shows
Figure 1. Effect of pH on phenylacetaldehyde (PA) formation in the
reaction of MeLOOH (O) or MeLnOOH (4) with phenylalanine (Phe) at
180 °C for 1 h. The buffers employed were 0.3 M sodium citrate for pH
2.15-6, 0.3 M sodium phosphate for pH 6-8 and 11-12, and 0.3 M
sodium borate for pH 8-10. The chemical structure for MeLOOH is given
in Figure 5. MeLnOOH is the corresponding analogous hydroperoxide of
methyl linolenate.
larly, a major product of the decomposition of the employed
MeLnOOH was 2-hexenal, which was also identified in the total
ion chromatograms of MeLnOOH/phenylalanine mixtures on
the basis of its retention index and mass spectrum (Table 1).
Nevertheless, and analogously to the observed MeLOOH/
phenylalanine mixtures, the major reaction product in most
heated MeLnOOH/phenylalanine reactions was the Strecker
aldehyde phenylacetaldehyde.
Effect of Reaction Conditions in the Formation of Phe-
nylacetaldehyde in Phenylalanine/Lipid Hydroperoxide Re-
action Mixtures. The yield of the Strecker-type degradation
of the amino acid depended on the reaction conditions. Figure
1 collects the amount of phenylacetaldehyde produced after
heating for 1 h at 180 °C equimolecular mixtures of MeLOOH
(or MeLnOOH) and phenylalanine as a function of pH. As
observed, the amount of Strecker aldehyde produced decreased
exponentially (r2 > 0.96) as a function of reaction pH, and the
reaction yield decreased from 12 to 0.25% when the reaction
pH increased from 2.15 to 12. This behavior was similar to
that observed for the different secondary and tertiary lipid
oxidation products analyzed previously (4-6, 9) and suggests
that this reaction is always favored at acid pH values indepen-
dent of the involved lipid. In addition, both hydroperoxides
exhibited very similar behaviors, therefore suggesting that n6
and n3 fatty acids have an analogous reactivity for the Strecker
degradation of the amino acids.

Strecker Degradation Initiated by Lipid Hydroperoxides.
J. Agric. Food Chem., Vol. 56, No. 17, 2008 7973
changed when reaction mixtures were heated under nitrogen.
Thus, the amount of phenylacetaldehyde produced by Me-
LEPCO did not change, but the amounts of phenylacetaldehyde
produced by the other lipid oxidation products were much
reduced. In the absence of oxygen, the reactivity of the assayed
lipids was as follows: MeLEPCO . MeLEPOH > MeLOH ≈
MeLCO > MeLOOH.
DISCUSSION
Until very recently, lipids have not been considered as
contributors to the formation of Strecker aldehydes during food
processing. However, oxidized lipids exhibit a behavior very
similar to that of carbohydrates for carbonyl-amine reactions,
and analogous products and reaction mechanisms are frequently
found in both carbohydrate/amino acid and oxidized lipid/amino
acid reaction mixtures (17). Furthermore, recent studies have
shown that lipid oxidation and Maillard reaction are so
interrelated that they should be considered simultaneously
to understand their consequences in foods when lipids,
carbohydrates, and amino acids or proteins are simultaneously
present (18).
The results obtained in the present study confirm the ability
of lipid oxidation products to produce the Strecker degradation
of amino acids and suggest that the reactivity of oxidized lipids
for this reaction is much more general than previously believed
because hydroperoxides, hydroxydienes, ketodienes, unsaturated
epoxyhydroxy derivatives, and unsaturated epoxyketo deriva-
tives, in addition to different short-chain aldehydes (4, 6, 9),
are able to degrade phenylalanine to phenylacetaldehyde to a
great extent.
Figure 3. Time course of phenylacetaldehyde (PA) formation in the
reaction of (A) MeLOOH (O) or MeLnOOH (4) with phenylalanine (Phe)
and (B) MeLOOH (O), MeLOH (]), or MeLCO (0) with phenylalanine
(Phe) at 180 °C. Samples were heated in 0.3 M sodium citrate buffer.
The chemical structures of MeLOOH, MeLOH, and MeLCO are given in
Figure 5. MeLnOOH is the corresponding analogous hydroperoxide of
methyl linolenate.
Lipid oxidation is a very complex process in which, by
decomposition of the lipid hydroperoxides, many different
compounds are formed (19). In fact, lipid oxidation can take
place at different reaction rates in separate positions of the same
food product (20), therefore producing distinct lipid oxidation
products that can coexist simultaneously. In addition, many of
these produced compounds are only intermediates in the
formation of more stable products. Furthermore, some of the
formed products can also react with the surrounding amino
compounds to produce new compounds. Because of the many
lipid oxidation products simultaneously present, it is frequently
very difficult to identify which of these lipid derivatives reacts
with the amino compound. Thus, in the reactions analyzed in
the present study, the action of different compounds by diverse
mechanisms is likely to be contributing to the formation of the
Strecker aldehyde.
One possible reaction pathway is an interconversion among
the different lipid oxidation products to achieve the lipid
oxidation product that would react with the amino acid. This
reaction pathway is schematically shown in Figure 5. The
decomposition of the hydroperoxides would produce hydroxy-
dienes and ketodienes, as major degradation products, following,
for example, a Russell mechanism (21). However, these
compounds are not final products. In fact, previous studies have
shown that ketodienes are oxidized to unsaturated epoxyketo
derivatives, and these last products are the compounds respon-
sible for the amino acid degradation (9). Furthermore, under
the reaction conditions employed, MeLOH can be either
oxidized to MeLCO or epoxidized to MeLEPOH. The formation
of MeLCO was observed in MeLOH/phenylalanine reaction
mixtures (data not shown). However, MeLEPOH, like Me-
LEPCO, is more reactive than the corresponding dienes and
could not be detected by GC-MS under the assayed conditions.
In any case, the final compound seems to be the MeLEPCO,
Figure 4. Reactivity of different lipid oxidation products in the Strecker
degradation of phenylalanine to produce phenylacetaldehyde (PA). Binary
reaction mixtures of the lipid and the amino acid were heated in 0.3 M
sodium citrate buffer for 1 h at 180 °C in the presence of air (slashed
bars) or nitrogen (horizontally striped bars). The chemical structures of
the different lipid oxidation products are given in Figure 5.
the effect of lipid concentration in the formation of Strecker
aldehydes in mixtures of phenylalanine with MeLOOH, MeLOH,
and MeLCO. The three assayed lipids exhibited similar behav-
iors, although the amount of phenylacetaldehyde seemed to
increase linearly (r > 0.97, p < 0.001) as a function of the
lipid concentration when mixtures of phenylalanine with either
MeLOH or MeLCO were assayed, and it was better described
using a Boltzmann fit (r2 ) 0.97) when the oxidized lipid was
MeLOOH.
Similar results were obtained when the effect of reaction time
was studied (Figure 3B). Thus, the three assayed lipids degraded
analogously to the amino acid, although MeLOOH and MeLOH
seemed to produce higher amounts of phenylacetaldehyde at
shorter reaction times.
When the reactivities of different primary, secondary, and
tertiary lipid oxidation products for this reaction were compared
after 1 h of heating at 180 °C, a small increase in the reactivity
of the assayed lipids was observed: MeLOOH < MeLOH e
MeLEPOH < MeLCO e MeLEPCO (Figure 4). This reactivity

7974 J. Agric. Food Chem., Vol. 56, No. 17, 2008
Zamora et al.
Figure 5. Proposed interconversion, under oxidative conditions, among the different lipid oxidation products included in this study to produce MeLEPCO
as a major compound responsible for the observed Strecker degradation of phenylalanine.
which would react with the amino acid by the previously
described mechanism (5). This hypothesis is supported by the
relative reactivities of the different oxidized lipids assayed
(Figure 4). The highest reactivity was observed for the
MeEPCO, and this reactivity decreased as a function of the
number of lipid intermediates needed for the formation of
MeLEPCO. In addition, when oxygen was not present, the
oxidative steps in the lipid interconversion were not favored
and the yield of phenylacetaldehyde was much reduced except
for MeLEPCO.
However, this is not likely the only mechanism involved
because all assayed lipids degraded phenylalanine even under
nonoxidative conditions (Figure 4). Thus, other mechanisms
can be assumed to take part to some extent, including those
that involve lipid fractionation. These last mechanisms would
be related to the fractionation of the oxidized lipids to produce
compounds able to react directly with the amino acid. Thus,
the formation of hydroxyalkenals from hydroxydienes has been
described (22), and hydroxyalkenals are able to produce the
Strecker degradation of the amino acid (6). The major reactivity
of MeLOOH and MeLOH compared with MeLCO at short
reaction times (Figure 3B) supports the contribution of alterna-
tive mechanisms like this one to the Strecker degradation of
amino acids.
degradation of amino acids as a function of the reaction
conditions. However, the results obtained in the present study
suggest that, once the lipid oxidation is produced, a significant
Strecker degradation of surrounding amino acids should be
expected, therefore explaining the significant contribution of
lipids to the Strecker aldehydes formed in complex mixtures
of unoxidized lipids, carbohydrates, and amino acids (11).
Furthermore, the much lower reactivity of unoxidized lipids
compared with lipid hydroperoxides in binary mixtures of
lipids and amino acids (11) suggests that the limiting step
of the Strecker degradation of amino acids by unoxidized
lipids is the formation of the lipid oxidation products.
ACKNOWLEDGMENT
We are indebted to Jose´ L. Navarro for technical assistance.
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Received for review May 6, 2008. Revised manuscript received July 8,
2008. Accepted July 10, 2008. This study was supported in part by the
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