Strecker type degradation of phenylalanine by 4-hydroxy-2-nonenal in model systems

Article abstract of DOI:10.1021/jf052240+

The reaction of 4-hydroxy-2-nonenal, an oxidative stress product, with phenylalanine in acetonitrile-water (2:1, 1:1, and 1:2) at 37, 60, and 80°C was investigated to determine whether 4-hydroxy-2-alkenals degrade amino acids, analogously to 4,5-epoxy-2-alkenals, and to compare the reactivities of both hydroxyalkenals and epoxyalkenals for production of Strecker aldehydes. In addition to the formation of N-substituted 2-pentylpyrrole and 2-pentylfuran, the studied hydroxyalkenal also degraded phenylalanine to phenylacetaldehyde with a reaction yield of 17%. The reaction mechanism is suggested to be produced through the corresponding imine, which is then decarboxylated and hydrolyzed. This reaction also produced a conjugated amine, which both may be one of the origins of the produced 2-pentyl-1H -pyrrole and may contribute to the development of browning in these reactions. 4-Hydroxy-2-nonenal and 4,5-epoxy-2-decenal degraded phenylalanine in an analogous extent, which is likely a consequence of the similarity of the degradation mechanisms involved. These results suggest that different lipid oxidation products are able to degrade amino acids; therefore, the Strecker type degradation of amino acids produced by oxidized lipids may be quantitatively significant in foods.

Full text of DOI:10.1021/jf052240+

10254 J. Agric. Food Chem. 2005, 53, 10254−10259
Strecker Type Degradation of Phenylalanine by
-Hydroxy-2-nonenal in Model Systems
4
FRANCISCO J. HIDALGO, EMERENCIANA GALLARDO, AND ROSARIO ZAMORA*
Instituto de la Grasa, Consejo Superior de Investigaciones Cient ´ı ficas, Avenida Padre Garc ´ı a Tejero 4,
4
1012 Seville, Spain
The reaction of 4-hydroxy-2-nonenal, an oxidative stress product, with phenylalanine in acetonitrile-
water (2:1, 1:1, and 1:2) at 37, 60, and 80 °C was investigated to determine whether 4-hydroxy-2-
alkenals degrade amino acids, analogously to 4,5-epoxy-2-alkenals, and to compare the reactivities
of both hydroxyalkenals and epoxyalkenals for production of Strecker aldehydes. In addition to the
formation of N-substituted 2-pentylpyrrole and 2-pentylfuran, the studied hydroxyalkenal also degraded
phenylalanine to phenylacetaldehyde with a reaction yield of 17%. The reaction mechanism is
suggested to be produced through the corresponding imine, which is then decarboxylated and
hydrolyzed. This reaction also produced a conjugated amine, which both may be one of the origins
of the produced 2-pentyl-1H-pyrrole and may contribute to the development of browning in these
reactions. 4-Hydroxy-2-nonenal and 4,5-epoxy-2-decenal degraded phenylalanine in an analogous
extent, which is likely a consequence of the similarity of the degradation mechanisms involved. These
results suggest that different lipid oxidation products are able to degrade amino acids; therefore, the
Strecker type degradation of amino acids produced by oxidized lipids may be quantitatively significant
in foods.
KEYWORDS: Carbonyl-amine reactions; epoxyalkenals; flavor production; hydroxyalkenals; lipid
oxidation; Maillard reaction; nonenzymatic browning; phenylacetaldehyde; pyrroles; Strecker aldehydes
INTRODUCTION
their biological properties (8, 9). These compounds might also
contribute to the Strecker degradation of amino acids produced
by oxidized lipids because their chemical structures are similar
to those of 4,5-epoxy-2-alkenals.
Strecker degradation of amino acids is a very important route
leading to final aroma compounds in the Maillard reaction (1-
3
). It is produced by reaction of an R-dicarbonyl compound
The first objective of this study was to determine if
with the amino group of the amino acid, which results in the
formation of an R-amino carbonyl compound and the corre-
sponding Strecker aldehyde of the amino acid (2).
4
-hydroxy-2-alkenals are able to degrade amino acids, analo-
gously to 4,5-epoxy-2-alkenals, and to find out the mechanism
of the reaction. The second objective was to compare the
reactivities of hydroxyalkenals and epoxyalkenals for the
Strecker type degradation of amino acids by quantifying the
formation of the corresponding Strecker aldehyde in diverse
systems containing these lipid oxidation products. All of these
studies were carried out with 4-hydroxy-2-nonenal (1), which
is the most common hydroxyalkenal, and phenylalanine (8).
Compound 8 was employed because its aldehyde derivative
phenylacetaldehyde (9) has a high boiling point (195 °C), can
be easily determined by gas chromatography (GC), and is a
very powerful odorant (10).
In addition to R-dicarbonyl compounds derived from carbo-
hydrates, both short chain aldehydes and long chain oxidized
fatty acids having a 4,5-epoxy-1-oxo-2-pentene system have also
been shown to degrade amino acids by a Strecker type
mechanism (4, 5). These lipid oxidation products degrade the
amino acid to the corresponding Strecker aldehyde, and they
are transformed into a hydroxyl amino derivative.
Lipid oxidation products having two oxygenated functions,
such as the above cited epoxyalkenals and epoxyoxoene fatty
esters, are commonly produced during lipid oxidation because
hydroperoxides are converted to secondary oxidation products
and many of these last compounds are very easily oxidized (6,
EXPERIMENTAL PROCEDURES
7
). Therefore, in addition to epoxyalkenals and epoxyoxoene
Materials. All chemicals were purchased from Aldrich (Milwaukee,
WI), Sigma (St. Louis, MO), Fluka (Buchs, Switzerland), or Merck
Darmstadt, Germany). Compound 1 was prepared according to Gardner
et al. (11). 4,5-Epoxy-2-decenal was prepared from 2,4-decadienal as
described previously (12). 2-Pentyl-1-phenethyl-1H-pyrrole (6) was
prepared by reaction of 1 and 2-phenylethylamine (3), as described
previously (13).
fatty esters, other derivatives having hydroperoxy, hydroxy,
epoxy, and oxo functions are also produced. Among them,
4
(
-hydroxy-2-alkenals have been profusely studied because of
*
To whom correspondence should be addressed. Tel: +(34)954 611
5
50. Fax: +(34)954 616 790. E-mail: rzamora@ig.csic.es.
1
0.1021/jf052240+ CCC: $30.25
© 2005 American Chemical Society
Published on Web 11/24/2005

Strecker Type Degradation Produced by Hydroxyalkenals
J. Agric. Food Chem., Vol. 53, No. 26, 2005 10255
Synthesis of 1-Nonyl-2-pentyl-1H-pyrrole (5). A solution of 0.5
mmol of nonylamine (2) in 4.5 mL of tetrahydrofuran was treated with
0
.5 mmol of 1 and incubated for 16 h at 37 °C. After that, samples
were taken to dryness and compound 5 was purified by preparative
thin-layer chromatography (TLC) on aluminum oxide N-coated plates
1
using isooctane as the solvent. H NMR (CDCl
3
): δ 0.9 (m, 6H, CH
), 2.51 (dd, J ) 7.3 Hz, J )
), 3.76 (dd, J ) 7.3 Hz, J ) 7.7 Hz, 2H, CH ), 5.87
3
),
1
8
.3 (m, 16H, CH
.4 Hz, 2H, CH
2
), 1.5-1.7 (m, 4H, CH
2
2
2
(
)
m, 1H, H-3), 6.06 (t, J ) 3.1 Hz, H-4), and 6.57 (dd, J ) 1.9 Hz, J
2.7 Hz, 1H, H-5). Their C NMR pyrrole signals in CDCl appeared
3
13
at δ 104.95 (C-4), 106.42 (C-3), 119.54 (C-5), and 133.19 (C-2). GC-
+
+
MS m/z (relative intensity, ion structure): 263 (28, M ), 248 (2, M
+
+
-
1
methyl), 234 (4, M - ethyl), 220 (28, M - propyl), 207 (19,
-nonyl-2-methyl-1H-pyrrole), 206 (100, M - butyl), 192 (47), 178
5), 164 (10), 150 (18), 136 (16), 122 (19), 108 (23), 95 (32), 94 (54),
+
(
8
0 (25), 69 (6), and 55 (14).
Synthesis of 2-Pentyl-1H-pyrrole (11). A solution of 1 mmol of 1
and 10 mmol of ammonia (2 N in methanol) in tetrahydrofuran (9 mL)
was incubated at 60 °C for 21 h. Compound 11 was purified by
preparative TLC on aluminum oxide N-coated plates using isooctane-
1
acetone (8:2) as the solvent. H NMR (CDCl
3
): δ 0.90 (t, J ) 6.7 Hz,
), 1.66 (m, 2H, CH ), 2.60 (t, J )
), 5.92 (m, 1H, H-3), 6.14 (m, 1H, H-4), and 6.67
3
H, CH
3
), 1.25-1.33 (m, 4H, CH
2
2
7
.7 Hz, 2H, CH
2
13
(
(
(
ddd, J ) 1.4 Hz, J ) 2.7 Hz, and J ) 4.1 Hz, 1H, H-5). C NMR
CDCl ): δ 14.05 (CH ), 22.50 (CH ), 27.68 (CH ), 29.35 (CH ), 31.56
CH ), 104.80 (CH), 108.20 (CH), 115.95 (CH), and 132.89 (C). GC-
MS m/z (relative intensity, ion structure): 137 (19, M ), 94 (6, M
propyl), 81 (15, 2-methylpyrrole), 80 (100, M - butyl), and 53 (12).
3
3
2
2
2
2
+
+
-
+
Reaction of Hydroxyalkenals or Epoxyalkenals with Amines or
Amino Acids. Mixtures of 50 µmol of the aldehyde and 50 µmol of
the amine or amino acid were incubated in 1 mL of acetonitrile-water
Figure 1. Total ion chromatograms of GC-MS analysis for the reaction
(2:1, 1:1, or 1:2) at 37, 60, or 80 °C for 21 h. The reaction was carried
of 1 with 2 after (A) 1 h of reaction in acetonitrile−water (2:1) at 37 °C
out in 4 mL microreaction vessels, which were closed with a cap. The
pH of samples was ∼5, and it did not change significantly upon
incubation. After incubation, 300 µL of ethanol was added, and 10 µL
of the resulting samples was diluted with 190 µL of acetonitrile-water
and (B) reduction with NaBH of the nonincubated reaction mixture. The
structures for the identified compounds are given in Scheme 1. Unidentified
peaks are marked with *.
4
(2:1, 1:1, or 1:2) and 25 µL of the internal standard solution [337 µg
1H and 13C NMR. 1H and 13C NMR spectra at 300 and 75.4 MHz,
of 3-(Z)-nonenol in 1 mL of methanol], and either studied by GC-MS
or 9 content determined by GC. Acetonitrile-water (2:1) was elected
as the solvent because it guarantees the solubility of the starting
aldehyde and amino acid as well as any possible intermediates and
products. In addition, it has been proposed to be a reasonable
compromise for modeling biochemical reactions, which could be
occurring in relatively low dielectric microenviroments rather than the
aqueous mileu (13).
respectively, were determined in a Bruker AC-300P (Karlsruhe,
4
Germany), with Me Si as an internal standard. Two-dimensional NMR
13
was used to assign the C NMR spectra.
Browning. Browning of samples was determined spectrophoto-
metrically using a Shimadzu UV-2401 PC UV-vis spectrophotometer.
Color differences (∆E) at the different periods of time were calculated
from the determined CIELAB L* a* b* values according to Hunter
(14):
The study of correlation between browning development and
disappearance of compound 12 was carried out in a reaction mixture
of 0.1 mmol of 1 and 0.1 mmol of 8 in 2 mL of acetonitrile-water
2
2
2 1/2
∆E ) [(L* - 100) + (a*) + (b*) ]
(
2:1) incubated for 2 h at 37 °C. After that time, 120 µL of 2 N KOH
in methanol was added, and the sample was incubated for 5 days at 37
C.
GC-MS Analyses. GC-MS analyses were conducted with a Hewlett-
by referring the determined values to an ideal colorless solution of L*
) 100 and a* ) b* ) 0 (15).
°
Statistical Analysis. Compound 9 determinations are expressed as
mean values ( standard deviations (SDs) of at least three determina-
tions. Statistical comparisons among different groups were made using
analysis of variance. When significant F values were obtained, group
differences were evaluated by the Student-Newman-Keuls test (16).
All statistical procedures were carried out using Primer of Biostatis-
tics: The Program (McGraw-Hill, Inc., New York). The significance
level is p < 0.05 unless otherwise indicated.
Packard 6890 GC Plus coupled with an Agilent 5973 MSD (Mass
Selective Detector-Quadrupole type). A fused-silica HP5-MS capillary
column (30 mm × 0.25 mm 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: from 70 (1 min)
to 240 °C at 5 °C/min and then to 325 °C at 10 °C/min; transfer line
to MSD, 280 °C; and ionization EI, 70 eV.
RESULTS
Determination of 9 Content by GC. GC analyses were conducted
with an Aligent 6890 GC Plus. Column and working conditions were
analogous to the above-described for the GC-MS analyses, and
compounds were detected with a flame ionization detector. Quantifica-
tion of 9 was carried out by preparing standard curves over a
concentration range of 5-90 nmol of 9 in the 225 µL of solution
prepared for GC injection (see above). For each curve, five different
concentration levels of the aldehyde were used. The compound 9 content
was directly proportional to the 9/internal standard area ratio (r > 0.99,
p < 0.0001). The coefficients of variation within this range were lower
than 5%.
Reaction between 1 and Primary Amines. The reaction
between hydroxyalkenals and amino acids is complex, and
diverse compounds are produced. In an attempt to facilitate the
characterization of the diverse compounds produced in these
reactions, the reaction between 1 and primary amines was
studied as a previous step. Figure 1A shows the total ion
chromatogram of GC-MS analysis obtained for a reaction of 1
and 2 in acetonitrile-water (2:1) after 1 h at 37 °C. Table 1
collects the retention indices of compounds described in this

10256 J. Agric. Food Chem., Vol. 53, No. 26, 2005
Hidalgo et al.
Table 1. Retention Indices of Compounds Described in This Studya
number
compound
retention index
1
2
3
4
5
6
7
9
1
2
3
4
5
6
7
9
1290/1316
1141/1150
1094
993
1945
1887
2172
1049
10
11
12
10
11
1132
1198
2118
unidentified
a
Structures for these compounds are given in Schemes 1 or 2.
Scheme 1. Reaction of 1 with Primary Aminesa
Figure 2. Total ion chromatogram of GC-MS analysis for the reaction of
1
with 3 in acetonitrile−water (2:1) after 1 h at 37 °C. The structures for
the identified compounds are given in Scheme 1. Unidentified peaks are
marked with *.
Figure 3. Total ion chromatogram of GC-MS analysis for the reaction of
1
with 8 in acetonitrile−water (2:1) after 24 h at 37 °C. The structures for
the identified compounds are given in Scheme 2. Unidentified peaks are
marked with *.
(Figure 1B). This compound was tentatively identified as
1
-(nonylamino)non-2-en-4-ol (7) on the basis of its mass spectra.
+
GC-MS m/z (relative intensity, ion structure): 283 (0.2, M ),
a
For 2, 5, and 7: R1
. For 3 and 6: R1
+
+
+
)
CH (CH )
3 2 8
)
2 2
PhCH CH .
282 (2, M - 1), 265 (2, M - H2O), 222 (2, M - H2O -
+
+
propyl), 212 (10, M - pentyl), 208 (7, M - H2O - butyl),
+
+
study. As observed, the reaction produced a major product,
which was isolated by preparative TLC and characterized by
182 (22), 170 (90, M - octyl), 152 (15, M - H2O - octyl),
+
144 (100, CH3(CH2)8NH3 ), 140 (22), 99 (42), 84 (20), 71 (27),
1
13
H and C NMR and MS, as 5. The reaction also produced as
69 (23), and 55 (30).
a minor product 2-pentylfuran (4), which was identified on the
basis of its retention time and mass spectra.
Analogously to the reaction between 1 and 2, the reaction of
the aldehyde with 3 also produced the corresponding pyrrole
as the major product. Figure 2 shows the total ion chromatogram
of GC-MS analysis obtained for a reaction of 1 and 3 in
acetonitrile-water (2:1) after 1 h at 37 °C. The major product
of the reaction was identified, on the basis of its retention time
and mass spectra, as 6. Analogously to the observed for 2, the
reaction also produced 4 as a minor product.
Reaction of 1 with 8. When a mixture of 8 and 1 was
incubated for 24 h in acetonitrile-water (2:1) at 37 °C, the
formation of different products was observed (Figure 3). The
number of the products formed in this reaction was higher than
those produced in the reaction between the hydroxyalkenal and
the primary amines, suggesting that other reactions than the
above-described for primary amines were also produced.
Previous studies have shown the formation of pyrrole
derivatives in the reaction between 1 and amines (17). It is
believed that they are produced by a reaction mechanism similar
to that described in Scheme 1. Thus, the reaction between the
hydroxyalkenal and the primary amine produces in a first step
an imine, which then cycles to the corresponding pyrrole 5. This
reaction may be accompanied by the cyclization of the hy-
droxyalkenal to produce the corresponding 4.
Although it has not been described, the reduction of the imine
produced between the hydroxyalkenal and the amine should
produce the secondary amine 7. Thus, when the mixture of 1
and 2 was reduced with sodium borohydride and studied by
GC-MS, the appearance of one major compound was observed

Strecker Type Degradation Produced by Hydroxyalkenals
J. Agric. Food Chem., Vol. 53, No. 26, 2005 10257
Scheme 2. Strecker Type Degradation of 8 by 1
As expected, the major reaction product was identified as 6.
This compound should be produced by a mechanism similar to
that described for primary amines, which is shown in Scheme
Figure 4. Correlation observed between the decrease in the area of
compound 12 produced as
a function of incubation time and the
development of browning in the reaction of 1 and 8. Once formed, 9
content remained constant during browning development. Total ion
chromatogram of GC-MS analysis for the reaction of 1 with 8 is given in
Figure 3. The structures for the identified compounds are given in Scheme
2. The reaction of the hydroxyalkenal and the amino acid should
produce the corresponding imine, which would evolve into the
corresponding pyrrole (13) by cyclization through an intermedi-
ate amine. The thermal decarboxylation of this acid, which is
produced in the injector port of the chromatograph (4), is the
origin of pyrrole 6. In addition, the cyclization of compound 1
should be responsible for the appearance of 4. This compound
was always produced in a major extent in the presence of amino
acids than in the presence of amines.
However, compounds 4 and 6 were not the unique reaction
products. Among the different compounds produced, some of
which could not be identified, the presence of 9 was significant
because it is the Strecker aldehyde derived from 8. The
formation of 9 should be produced by a mechanism analogous
to that described for epoxyalkenals (4), which is shown in
Scheme 2. As an alternative to the dehydration of the imine to
produce the pyrrole 13, its decarboxylation should produce the
intermediate imine 14, which is the origin of 9 and the
unsaturated amine 10. This amine, which appeared in the
chromatogram as a minor product, was tentatively identified as
2.
produced in the reaction because a linear relationship (r )
-0.97, p < 0.0001) was observed between the disappearance
of this peak and the development of browning observed as a
function of the incubation time (Figure 4). The development
of browning was followed during 5 days at 37 °C and pH 9.
Differently to compound 12, once 9 was produced, content of
9 remained constant during these 5 days.
Formation of 9 in the Reaction Mixtures of 1 and 8. The
effects of hydroxyalkenal concentration, reaction temperature,
and polarity of the solvent in the formation of 9 were studied
by employing diverse mixtures of 1 and 8 (0-50 µmol of
hydroxyalkenal and 50 µmol of the amino acid), dissolved in
acetonitrile-water (2:1, 1:1, or 1:2) and heated a 37, 60, or 80
°C. As expected and with independence of the temperature or
the polarity of the solvent employed, an increase in the
concentration of the hydroxyalkenal produced an increase in
the formation of 9 (Figure 5). However, this increase was not
linear, and the higher increases were observed in the range of
0-5 µmol of 1. On the contrary, phenylacetaldehyde production
was analogous when 25 or 50 µmol of 1 was employed.
Compound 9 was produced in the same extent at 37 or 60
°C (8.7 ( 0.4 µmol/50 µmol of 8) (Figure 5A,B). However,
when reaction mixtures were heated at 80 °C, lower values of
the Strecker aldehyde were determined (between 4.9 and 7.4
µmol of 9/50 µmol of 8 depending on the polarity of the
employed solvent) (Figure 5C). This may be related to the
volatility of acetonitrile (bp 82 °C) at this temperature that might
produce losses of 9. In fact, when acetonitrile-water (1:2) was
employed, the results were close to those obtained at 37 or 60
°C (7.4 ( 1.2 µmol/50 µmol of 8). However, when the solvent
was rich in acetonitrile [acetonitrile-water (2:1)], the amount
of 9 determined was significantly lower (4.9 ( 0.8 µmol/50
µmol of 8).
nona-1,3-dien-1-amine (10) on the basis of its mass spectrum.
+
GC-MS m/z (relative intensity, ion structure): 138 (2, M
1
-
+
+
+
), 124 (1, M - methyl), 110 (14, M - ethyl), 96 (31, M
+
-
propyl), 83 (100, penta-1,3-dien-1-amine), 82 (26, M
-
butyl), and 55 (13). In addition, its retention time was similar
to that of 2, as should be expected. Nevertheless, compound 10
was reactive, and at least one peak in the chromatogram is
related to it. Thus, the intramolecular addition of the amino
group to the γ,δ carbon-carbon double bond followed by
aromatization should produce 11. This compound, which
appeared as a minor product in the reaction mixture, was
identified on the basis of its retention time and mass spectrum.
Another important peak in the chromatogram is compound
12. Although its structure is unknown at present, its mass
spectrum suggests that it was produced by condensation of one
molecule of the amine 10 with one molecule of hydroxyalkenal
+
1
2
(
(
. GC-MS m/z (relative intensity, ion structure): 277 (16, M ),
+
+
20 (16, M - propyl), 206 (6, M - butyl), 202 (7), 177
15), 176 (100), 139 (21), 138 (44), 120 (34), 118 (16), 106
20), and 80 (70). Different attempts were carried out to isolate
Analogously to the reaction temperature, the polarity of the
solvent did not seem to influence the amount of 9 produced at
37 or 60 °C (Figure 5A,B). However, 9 was produced in a
this compound, but all of them were unsuccessful. In fact, this
compound seemed to be an intermediate in the browning

10258 J. Agric. Food Chem., Vol. 53, No. 26, 2005
Hidalgo et al.
Table 2. Formation of 9 in Mixtures of 1 or 4,5-Epoxy-2-decenal and
a
8
temperature
lipid
A/W ratiob
2:1
37
°
C
60
°C
1
166
±
±
±
±
8 a
188
178
170
160
±
±
±
±
24
20
6
1
:2
2:1
:2
178
174
248
22 a
18 a
12 b
4
,5-epoxy-2-decenal
1
12
a
Values are given in micromoles of 9 per millimole of 8. Means in the same
b
column with different letters are significantly different (p < 0.05). A/W
water.
) acetonitrile/
amounts of lipids occur, as in several vegetable products (18-
1). It is a complex process in which hydroperoxides are the
2
initial products, and then, they enter into numerous interlaced
reactions involving substrate degradation and interaction, which
results in myriad compounds of various molecular weights,
flavor thresholds, and biological significance (22). When this
oxidation takes place in the presence of amino compounds,
including amino phospholipids, amino acids, and proteins, the
formation of oxidized lipid/amino compound reaction products
is an unavoidable process, which should be considered as a last
step in the lipid oxidation (23-25) and that contributes to the
changes of colors, flavors, and antioxidative activities produced
in foods during processing and storage (4, 5, 26-28).
Lipid oxidation has been traditionally related to the production
of off-flavors in foods (3). On the other hand, the Maillard
reaction is considered very important to the production of
desirable food flavors. However, recent studies from this and
other laboratories have shown that this separation between lipid
oxidation and Maillard reaction is not so clear, and lipid
oxidation may produce analogous products to Maillard oxidation
by the same mechanisms (29, 30). In fact, this and the previous
Figure 5. Effect of 1 content on 9 formation in mixtures of 1 and 8
incubated at (A) 37, (B) 60, and (C) 80 C. The solvent employed was
acetonitrile water (2:1) ( ), acetonitrile water (1:1) ( ), or acetonitrile
water (1:2) ( ). Fifty micromoles of 8 was employed in all of the assays.
Values are means SD of at least three experiments.
(4, 5) studies have shown that Strecker aldehyde production is
°
not an exception and both carbohydrates and oxidized lipids
are able to degrade amino acids analogously. In addition, this
is not a property of only one type of lipid oxidation products,
like those having a 4,5-epoxy-1-oxo-2-pentene system previ-
ously described, but it has to be extended to hydroxyalkenals
and, more likely, to other lipid oxidation products having two
oxygenated functions. Therefore, Strecker type degradation of
amino acids produced by oxidized lipids may be quantitatively
significant in foods. Furthermore, the relative importance of both
carbohydrates and lipids in the formation of these compounds
should be determined. In this context, Strecker degradation of
amino acids produced by lipids may be very important at low
temperature because the obtained results (9 was produced in
the same extent at 37 or 60 °C) suggest that heating at high
temperature is not a requisite for this reaction when it is
produced by lipids.
−
4
0
−
O
−
±
lower extent at acetonitrile-water (2:1), followed by aceto-
nitrile-water (1:1) and by acetonitrile-water (1:2) when the
reaction temperature was 80 °C. As discussed above, these
results may be a consequence of volatility of acetonitrile at the
reaction temperature employed.
Comparative Formation of 9 in the Reactions of Hydroxy-
alkenals and Epoxyalkenals with 8. In an attempt to compare
the relative reactivities of hydroxyalkenals and epoxyalkenals
for producing Strecker type degradation of amino acids, the
formation of 9 was studied in the reaction of 1 and 4,5-epoxy-
2-decenal with 8. Table 2 collects the results obtained in diverse
mixtures of hydroxyalkenal and epoxyalkenal incubated for 21
Nevertheless, Strecker aldehydes are not the unique flavor
compounds produced in these reactions. If the previous studies
on epoxyalkenals indicated that this reaction is likely the origin
of 2-alkylpyridines and pyridine-containing long chain fatty
esters, some of which have been determined in processed foods
h at 37 or 60 °C in acetonitrile-water (2:1 or 1:2). As observed,
1
and 4,5-epoxy-2-decenal degraded 8 in an analogous propor-
tion (about 17% of initial 8 was converted into 9) and no
significant differences were observed among most of them. Only
the reaction between 4,5-epoxy-2-decenal and 8 in acetonitrile-
water (1:2) at 37 °C produced a significantly higher degradation
of 8 (conversion yield 25%).
(31-34), hydroxyalkenals seem to be related both to the origin
of other flavors, like 11, and to the development of browning.
Compound 11 has been found, for example, among flavor
components of roasted filberts (35), roasted chufa-tubers (36),
and dried squids (37).
DISCUSSION
Lipid oxidation is a major cause of deterioration in foods
and feeds, both in those containing substantial amounts of fats,
such as lard or edible oils, and in those where only minor
Compound 1 and 4,5-epoxy-2-decenal have been found to
degrade amino acids in an analogous extent. This may be a
consequence of both the similarity of the degradation mecha-

Strecker Type Degradation Produced by Hydroxyalkenals
J. Agric. Food Chem., Vol. 53, No. 26, 2005 10259
nisms produced by both aldehydes and the similar chain length
of the assayed aldehydes. These results are in agreement with
the results found in a previous study comparing the reactivities
of epoxyalkenals and epoxyoxoene fatty esters (5) and confirm
that the chain length of the oxidized lipid plays an important
role in the reaction yield. According to the results obtained in
this and in the previous study, oxidized lipids derived from n-6
fatty acids (1, 4,5-epoxy-2-decenal, methyl 9,10-epoxy-13-oxo-
(17) Sayre, L. M.; Arora, P. K.; Iyer, R. S.; Salomon, R. G. Pyrrole
formation from 4-hydroxynonenal and primary amines. Chem.
Res. Toxicol. 1993, 6, 19-22.
(
18) Erickson, M. C. Lipid oxidation of muscle foods. In Food Lipids.
Chemistry, Nutrition, and Biotechnology; Akoh, C. C., Min, D.
B., Eds.; Dekker: New York, 1998; pp 297-332.
(
19) German, J. B. Food Processing and lipid oxidation. AdV. Exp.
Med. Biol. 1999, 459, 23-50.
(
20) Min, D. B.; Boff, J. M. Lipid oxidation of edible oil. In Food
Lipids. Chemistry, Nutrition, and Biotechnology, 2nd ed.; Akoh,
C. C., Min, D. B., Eds.; Dekker: New York, 2002; pp 335-
363.
1
1-octadecenoate, and methyl 12,13-epoxy-9-oxo-11-octa-
decenoate) have been found to produce the Strecker type
degradation of 8 in a higher extent than oxidized lipids derived
from n-3 fatty acids (4,5-epoxy-2-heptenal). Additional studies
should confirm these results and might reveal a different
contribution of different types of fatty acids to flavor formation
by the Strecker type mechanism.
(
(
(
21) Baron, C. P.; Andersen, H. J. Myoglobin-induced lipid oxidation.
A review. J. Agric. Food Chem. 2002, 50, 3887-3897.
22) Nawar, W. W. Lipids. In Food Chemistry, 3rd ed.; Fennema,
O. R., Ed.; Dekker: New York, 1996; pp 225-319.
23) Hidalgo, F. J.; Zamora, R. Methyl linoleate oxidation in the
presence of bovine serum albumin. J. Agric. Food Chem. 2002,
50, 5463-5467.
ACKNOWLEDGMENT
(
24) Zamora, R.; Hidalgo, F. J. Comparative methyl linoleate and
methyl linolenate oxidation in the presence of bovine serum
albumin at several lipid/protein ratios. J. Agric. Food Chem.
We are indebted to Jos e´ L. Navarro for technical assistance.
2
003, 51, 4661-4667.
LITERATURE CITED
(25) Zamora, R.; Hidalgo, F. J. Phosphatidylethanolamine modifica-
tion by oxidative stress product 4,5(E)-epoxy-2(E)-heptenal.
Chem. Res. Toxicol. 2003, 16, 1632-1641.
(
1) Ho, C.-T. Thermal generation of Maillard aromas. In The
Maillard Reaction: Consequences for the Chemical and Life
Sciences; Ikan, R., Ed.; John Wiley & Sons: Chichester, United
Kingdom, 1996; pp 27-53.
(
26) Zamora, R.; Olmo, C.; Navarro, J. L.; Hidalgo, F. J. Contribution
of phospholipid pyrrolization to the color reversion produced
during deodorization of poorly degummed vegetable oils. J.
Agric. Food Chem. 2004, 52, 4166-4171.
(
2) Yaylayan, V. A. Recent advances in the chemistry of Strecker
degradation and Amadori rearrangement: Implications to aroma
and color formation. Food Sci. Technol. Res. 2003, 9, 1-6.
3) Reineccius, G. FlaVor Chemistry and Technology, 2nd ed.; Taylor
(
27) Hidalgo, F. J.; Nogales, F.; Zamora, R. Changes produced in
the antioxidative activity of phospholipids as a consequence of
their oxidation. J. Agric. Food Chem. 2005, 53, 659-662.
28) Zamora, R.; Nogales, F.; Hidalgo, F. J. Phospholipid oxidation
and nonenzymatic browning development in phosphatidyl-
ethanolamine/ribose/lysine model systems. Eur. Food Res.
Technol. 2005, 220, 459-465.
(
(
&
Francis: Boca Raton, FL, 2006.
(
4) Hidalgo, F. J.; Zamora, R. Strecker-type degradation produced
by the lipid oxidation products 4,5-epoxy-2-alkenals. J. Agric.
Food Chem. 2004, 52, 7126-7131.
(5) Zamora, R.; Gallardo, E.; Navarro, J. L.; Hidalgo, F. J. Strecker-
(
29) Zamora, R.; Hidalgo, F. J. Coordinate contribution of lipid
oxidation and Maillard reaction to the nonenzymatic food
browning. Crit. ReV. Food Sci. Nutr. 2005, 45, 49-59.
30) Hidalgo, F. J.; Zamora, R. Interplay between the Maillard
Reaction and lipid peroxidation in biochemical systems. Ann.
N. Y. Acad. Sci. 2005, 1043, 319-326.
type degradation of phenylalanine by methyl 9,10-epoxy-13-oxo-
1
1-octadecenoate and methyl 12,13-epoxy-9-oxo-11-octa-
decenoate. J. Agric. Food Chem. 2005, 53, 4583-4588.
6) Gardner, H. W. Oxygen radical chemistry of polyunsaturated
fatty acids. Free Radical Biol. Med. 1989, 7, 65-86.
7) Frankel, E. N. Lipid Oxidation, 2nd ed.; The Oily Press:
Bridgwater, United Kingdom, 2005.
(
(
(
(
(
(
(
31) Schieberle, P. Odour-active compounds in moderately roasted
sesame. Food Chem. 1996, 55, 145-152.
8) Petersen, D. R.; Doorn, J. A. Reactions of 4-hydroxynonenal
with proteins and cellular targets. Free Radical Biol. Med. 2004,
32) Kim, Y.-S.; Ho, C.-T. Formation of pentylpyridines in an oil
medium. J. Agric. Food Chem. 1998, 46, 644-647.
3
7, 937-945.
33) Zhou, A.; Boatright, W. L. Precursors for formation of 2-pentyl
pyridine in processing of soybean protein isolates. J. Food Sci.
(
9) Liu, Q.; Raina, A. K.; Smith, M. A.; Sayre, L. M.; Perry, G.
Hydroxynonenal, toxic carbonyls, and Alzheimer disease. Mol.
Aspects Med. 2003, 24, 305-313.
2
000, 65, 1155-1159.
(
34) Fukami, K.; Ishiyama, S.; Yaguramaki, H.; Masuzawa, T.;
Nabeta, Y.; Endo, K.; Shimoda, M. Identification of distinctive
volatile compounds in fish sauce. J. Agric. Food Chem. 2002,
(
10) Carunchia Whetstine, M. E.; Cadwallader, K. R.; Drake, M. A.
Characterization of aroma compounds responsible for the rosy/
floral flavour in cheddar cheese. J. Agric. Food Chem. 2005,
5
0, 5412-5416.
5
3, 3126-3132.
(
(
(
35) Kinlin, T. E.; Muralidhara, R.; Pittet, A. O.; Sanderson, A.;
Walradt, J. P. Volatile components of roasted filberts. J. Agric.
Food Chem. 1972, 20, 1021-1028.
(
11) Gardner, H. W.; Bartelt, R. J.; Weisleder, D. A facile synthesis
of 4-hydroxy-2(E)-nonenal. Lipids 1992, 27, 686-689.
(
12) Zamora, R.; Hidalgo, F. J. Linoleic acid oxidation in the presence
of amino compounds produces pyrroles by carbonylamine
reactions. Biochim. Biophys. Acta 1995, 1258, 319-327.
13) Zamora, R.; Hidalgo, F. J. 2-Alkylpyrrole formation from 4,5-
epoxy-2-alkenals. Chem. Res. Toxicol. 2005, 18, 342-348.
14) Hunter, R. S. The Measurement of Appearance; Hunter Associ-
ates Laboratory: Fairfax, VA, 1973.
36) Soliman, M. M.; Osman, F.; El-Sawy, A. A. Volatile components
of roasted chufa-tubers. Agric. Biol. Chem. 1982, 46, 2843-
2
845.
(
(
(
37) Kawai, T.; Ishida, Y.; Kakiuchi, H.; Ikeda, N.; Higashida, T.;
Nakamura, S. J. Agric. Food Chem. 1991, 39, 770-777.
Received for review September 12, 2005. Revised manuscript received
October 21, 2005. Accepted October 30, 2005. This study was supported
in part by the European Union (FEDER funds) and the Plan Nacional
de I + D of the Ministerio de Educaci o´ n y Ciencia of Spain (Project
AGL2003-02280).
15) Hidalgo, F. J.; Nogales, F.; Zamora, R. Effect of the pyrrole
polymerization mechanism on the antioxidative activity of
nonenzymatic browning reactions. J. Agric. Food Chem. 2003,
5
1, 5703-5708.
(16) Snedecor, G. W.; Cochran, W. G. Statistical Methods, 7th ed.;
Iowa State University Press: Ames, IA, 1980.
JF052240+