Amine degradation by 4,5-epoxy-2-decenal in model systems

Article abstract of DOI:10.1021/jf052757l

The reactions of 4,5-epoxy-2-decenal with octylamine, benzylamine, and 2-phenylglycine methyl ester were studied to investigate if amines may suffer a Strecker type degradation by epoxyalkenals analogously to amino acids. In addition to other reactions, the studied amines were converted into their corresponding Strecker aldehydes (octanal, benzaldehyde, and methyl 2-oxo-2-phenylacetate, respectively) to an extent that depended on the pH, the temperature, the amount of epoxyalkenal, and the amine involved. Each amine exhibited an optimum pH for the reaction, but the corresponding Strecker aldehydes were produced to a significant extent within a broad pH range. In addition, the temperature mostly influenced the reaction rate, which was increased between 6.5 and 9.5 times when the reaction was carried out at 60°C than when it took place at 37°C. Furthermore, Strecker aldehyde formation was linearly correlated with the amount of the epoxyalkenal present in the reaction mixture. Nevertheless, the reaction yield mostly depended on the amine involved. Thus, octylamine only produced trace amounts of octanal, benzylamine was converted into benzaldehyde with a yield of 4.3%, and 2-phenylglycine methyl ester was converted into methyl 2-oxo-2-phenylacetate with a reaction yield of 49%. All of these results suggest that suitable amines can be degraded by epoxyalkenals to their corresponding Strecker aldehydes to a significant extent.

Full text of DOI:10.1021/jf052757l

2398 J. Agric. Food Chem. 2006, 54, 2398−2404
Amine Degradation by 4,5-Epoxy-2-decenal in Model Systems
ROSARIO ZAMORA, EMERENCIANA GALLARDO, AND FRANCISCO J. HIDALGO*
Instituto de la Grasa, Consejo Superior de Investigaciones Cient ´ı ficas,
Avenida Padre Garc ´ı a Tejero 4, 41012 Seville, Spain
The reactions of 4,5-epoxy-2-decenal with octylamine, benzylamine, and 2-phenylglycine methyl ester
were studied to investigate if amines may suffer a Strecker type degradation by epoxyalkenals
analogously to amino acids. In addition to other reactions, the studied amines were converted into
their corresponding Strecker aldehydes (octanal, benzaldehyde, and methyl 2-oxo-2-phenylacetate,
respectively) to an extent that depended on the pH, the temperature, the amount of epoxyalkenal,
and the amine involved. Each amine exhibited an optimum pH for the reaction, but the corresponding
Strecker aldehydes were produced to a significant extent within a broad pH range. In addition, the
temperature mostly influenced the reaction rate, which was increased between 6.5 and 9.5 times
when the reaction was carried out at 60 °C than when it took place at 37 °C. Furthermore, Strecker
aldehyde formation was linearly correlated with the amount of the epoxyalkenal present in the reaction
mixture. Nevertheless, the reaction yield mostly depended on the amine involved. Thus, octylamine
only produced trace amounts of octanal, benzylamine was converted into benzaldehyde with a yield
of 4.3%, and 2-phenylglycine methyl ester was converted into methyl 2-oxo-2-phenylacetate with a
reaction yield of 49%. All of these results suggest that suitable amines can be degraded by
epoxyalkenals to their corresponding Strecker aldehydes to a significant extent.
KEYWORDS: Alkylpyridines; amines; carbonyl-amine reactions; epoxyalkenals; flavors; lipid oxidation;
Maillard reaction; Strecker aldehydes
INTRODUCTION
4,5-Epoxy-2-alkenals have been shown to degrade amino
acids by a Strecker type mechanism (9). This reaction, which
has also been described for epoxyoxoene fatty esters (10) and
In contrast to the well-documented role of Strecker degrada-
tion of amino acids in flavor formation via Maillard reaction
4-hydroxy-2-alkenals (11), is produced through imine formation,
(
(
1, 2), Strecker degradation of amines is still an open question
3). However, it is an important subject because the conversion
which is then decarboxylated and hydrolyzed. This ability to
degrade amino acids points them out as potential candidates
for degrading amines. The amines selected for this study were
octylamine (2), as a representative aliphatic amine; benzylamine
of amines into carbonyl derivatives may be an alternative route
to the observed increase of carbonyl content in foods as a
consequence of processing and/or storage (4, 5). In an attempt
to clarify the conditions needed to produce the Strecker
degradation of amines, this study describes the reaction of 4,5-
epoxy-2-decenal (1) with different amines at several pH and
temperature conditions.
(
2
3), as an amine with a phenyl group at the R-position; and
-phenylglycine methyl ester (4), as an amine with both a phenyl
group and a methoxycarbonyl group at the R-position.
EXPERIMENTAL PROCEDURES
4
,5-Epoxy-2-alkenals are secondary products of lipid per-
Materials. 2,4-Decadienal, octylamine (2), benzylamine (3), (S)-
oxidation. They are produced in the decomposition of interme-
diate epoxyhydroperoxy fatty acids by a mechanism that is
common for the different polyunsaturated fatty acids. Thus,
when starting from n-6 polyunsaturated fatty acids, the epoxy-
alkenal obtained is 4,5-(E)-epoxy-2-(E)-decenal (1) by decom-
position of an intermediate 12,13-(E)-epoxy-9-hydroperoxy-10-
octadecenoic acid (6). Analogously, the 4,5-(E)-epoxy-2-(E)-
heptenal is the product of oxidation of the n-3 polyunsaturated
fatty acids (7). These epoxyalkenals have been detected in many
different food systems (8).
+
-2-phenylglycine methyl ester hydrochloride (4), 1-benzyl-1H-pyrrole
9), hexanal (11), octanal (16), benzaldehyde (17), methyl 2-oxo-2-
(
phenylacetate (18), and 2-pentylpyridin (20) were purchased from
Aldrich (Milwaukee, WI). All other chemicals were analytical grade
and were purchased from reliable commercial sources.
4,5-Epoxy-2-decenal (1) was prepared from 2,4-decadienal as
described previously (12). Briefly, 3-chloroperoxybenzoic acid (25
mmol) was dissolved in chloroform (175 mL) and washed with three
1
00 mL portions of buffer (0.2 M Na
2
4 2
HPO ‚12H O adjusted to pH 7.5
with 0.1 M citric acid monohydrate), followed by three 100 mL portions
of water, and dried with anhydrous sodium sulfate. This solution was
added slowly (25 mL every 10 min) to a solution of 2,4-decadienal
(
3.0 g, 19.7 mmol) in chloroform (29 mL), which was stirred at room
*
To whom correspondence should be addressed. Tel: +(34)954 611
5
50. Fax: +(34)954 616 790. E-mail: fhidalgo@ig.csic.es.
temperature. The reaction mixture was then stirred overnight and finally
1
0.1021/jf052757l CCC: $33.50
© 2006 American Chemical Society
Published on Web 02/23/2006

Amine Degradation by Epoxyalkenals
J. Agric. Food Chem., Vol. 54, No. 6, 2006 2399
Scheme 1. Reaction of 4,5-Epoxy-2-decenal with Primary Aminesa
a
For 2, 5, 8, 12, and 16: R1
phenyl.
3 2 7 2 3
) CH (CH ) ) ) ) ) CO CH , and
, and R2 H. For 3, 6, 9, 13, and 17: R1 phenyl, and R2 H. For 4, 7, 10, 14, and 18: R1
R2
)
washed with three 100 mL portions of buffer (0.2 M Na
2
HPO
4
‚12H
2
O
(8) and 1-benzyl-2-pentyl-1H-pyrrole (13) were characterized previously
(13, 14). Finally, other compounds were only tentatively identified in
this study on the basis of their mass spectra. Their spectra exhibited
the typical fragmentations of N-substituted 1H-pyrroles, N-substituted
2-(1-hydroxyhexyl)-1H-pyrroles, or N-substituted 2-pentyl-1H-pyrroles
(see, for example, ref 15 for a summary of mass spectroscopic
characteristics of these types of pyrrole derivatives). These compounds
were 1-(1-octyl-1H-pyrrol-2-yl)hexan-1-ol (5), 1-octyl-2-pentyl-1H-
pyrrole (12), 1-(1-benzyl-1H-pyrrol-2-yl)hexan-1-ol (6), methyl 2-(2-
(1-hydroxyhexyl)-1H-pyrrol-1-yl)-2-phenylacetate (7), methyl 2-phenyl-
2-(1H-pyrrol-1-yl)acetate (10), and methyl 2-(2-pentyl-1H-pyrrol-1-yl)-
2-phenylacetate (14).
adjusted to pH 7.5 with 0.1 M citric acid monohydrate), followed by
three 100 mL portions of water, to remove 3-chlorobenzoic acid. The
organic solution was dried with anhydrous sodium sulfate and
concentrated under vacuum. The residue was fractionated by column
chromatography using hexane/acetone (95:5) as the eluent. 4,5-Epoxy-
2
-decenal was obtained chromatographically pure. Additional confirma-
1
13
tions of identity and purity were obtained by H and C NMR and gas
chromatography-mass spectrometry (GC-MS).
4,5-Epoxy-2-decenal/Amine Reaction Mixtures. A solution of 0-5
µmol of 4,5-epoxy-2-decenal (1) and 5 µmol of the amine (2-4) in
00 µL of acetonitrile-sodium citrate or sodium phosphate buffer
2:1) was heated at 37 or 60 °C and studied for either GC-MS or the
1
(
The GC-MS m/z (relative intensity, ion structure) of tentatively
+
+
content of the produced Strecker aldehyde (17 and 18) determined by
GC-flame ionization detection (FID). Samples were diluted with
chloroform (550 µL) for GC-MS analyses. For Strecker aldehyde
content determination, samples were diluted with 500 µL of chloroform,
and 50 µL of internal standard solution [337 µg of 3-(Z)-nonenol in 1
mL of methanol] was added. Either 50 mM sodium citrate buffer, pH
assigned compound 5 in the intact form was 279 (7, M ), 261 (13, M
+
+
- H
2
O), 250 (2, M - ethyl), 246 (1, M - H
2
O - methyl), 232 (5,
O - propyl), 209 (16, M
pentyl + 1), 208 (100, M - pentyl), 204 (7, M - H
+
+
+
M
- H
2
O - ethyl), 218 (44, M - H
2
-
+
+
2
O - butyl),
+
+
190 (4, M - H
2
O - pentyl), 178 (19, M - hydroxyhexyl), 162 (5,
- H O - heptyl), 148 (5), 134 (17), 120 (17), 106 (22), 96 (15),
+
M
2
2
.15-6.0, or 50 mM sodium phosphate buffer, pH 6.0-8.0, was
80 (18), 68 (10), 57 (15), 55 (16), 43 (30), and 41 (34). This compound
also appeared in the dehydrated form in the chromatogram. Its GC-
employed to control the reaction pH.
+
+
The products of 4,5-epoxy-2-decenal/amine reactions are formed by
different competing reactions. Therefore, other products, in addition
to Strecker aldehydes (16-18), were also produced (Scheme 1). These
compounds were N-substituted 1H-pyrroles (8-10), N-substituted 2-(1-
hydroxyhexyl)-1H-pyrroles (5-7), N-substituted 2-pentyl-1H-pyrroles
MS (relative intensity, ion structure) was 261 (37, M ), 246 (3, M -
+
+
methyl), 232 (11, M - ethyl), 219 (19, M - propyl + 1), 218 (100,
+
+
+
+
M
- propyl), 204 (15, M - butyl), 190 (8, M - pentyl), 178 (9,
+
+
M - hexenyl), 176 (8, M - hexyl), 162 (9, M - heptyl), 148 (10),
134 (29), 132 (15), 120 (32), 118 (24), 106 (45), 91 (8), 80 (13), 79
(13), 55 (10), 43 (25), and 41 (32).
(12-14), hexanal (11), 2-pentylfuran (15), and 2-pentylpyridine (20).
Some of these compounds were commercial, and their formation in
the reaction mixtures was confirmed by comparison of their retention
indices and mass spectra with those of authentic compounds. This
occurred with 1-benzyl-1H-pyrrole (9), hexanal (11), 2-pentylfuran (15),
and 2-pentylpyridine (20). Other reaction products could be identified
by comparison with described compounds. Thus, 1-octyl-1H-pyrrole
The GC-MS m/z (relative intensity, ion structure) of tentatively
+
+
assigned compound 12 was 249 (26, M ), 234 (1, M - methyl), 220
+
+
+
(4, M - ethyl), 206 (26, M - propyl), 193 (17, M - butyl + 1),
+
+
+
192 (100, M - butyl), 178 (49, M - pentyl), 164 (10, M - hexyl),
+
150 (21, M - heptyl), 136 (18), 122 (24), 108 (25), 95 (33), 94 (60),
80 (27), 69 (9), 55 (15), 43 (22), and 41 (32).

2400 J. Agric. Food Chem., Vol. 54, No. 6, 2006
Zamora et al.
The GC-MS (relative intensity, ion structure) of tentatively assigned
Table 1. Retention Indices of Compounds Described in This Studya
+
+
+
compound 6 was 257 (6, M ), 239 (5, M - H
2
+
O), 228 (1, M
-
+
ethyl), 210 (1, M - H
2
O - ethyl), 196 (11, M - H
2
O - propyl),
compd
no.
retention
index
+
1
6
86 (36, M - butyl), 170 (5), 108 (9), 92 (9), 91 (100, benzyl), and
compd name
4,5-epoxy-2-decenal
1-(1-octyl-1H-pyrrol-2-yl)hexan-1-ol
5 (9). This compound also appeared in the dehydrated form in the
1
5
6
7
1366
chromatogram. Its GC-MS (relative intensity, ion structure) was 239
b
c
1976 /2069
+
+
+
(
21, M ), 210 (3, M - ethyl), 197 (9, M - propyl + 1), 196 (43,
b
c
1-(1-benzyl-1H-pyrrol-2-yl)hexan-1-ol
methyl 2-[2-(1-hydroxyhexyl)-1H-
1932 /2030
+
c
M - propyl), 118 (5), 104 (4), 92 (7), 91 (100, benzyl), and 65 (12).
2167
b
Tentatively assigned compound 7 only appeared in the chromatogram
in its dehydrated form. Its GC-MS (relative intensity, ion structure)
pyrrol-1-yl]-2-phenylacetate
8
9
1-octyl-1H-pyrrole
1-benzyl-1H-pyrrole
methyl 2-phenyl-2-(1H-pyrrol-1-yl)acetate
hexanal
1-octyl-2-pentyl-1H-pyrrole
1-benzyl-2-pentyl-1H-pyrrole
methyl 2-(2-pentyl-1H-pyrrol-1-yl)-
2-phenylacetate
1388
1341
1627
801
1837
1783
2028
+
+
+
was 297 (71, M ), 254 (48, M - propyl), 238 (28, M - CO
94 (41), 149 (37), 121 (100), 118 (32), 105 (53), 91 (52), and 77
34).
The GC-MS (relative intensity, ion structure) of tentatively assigned
2 3
CH ),
b
10
11
12
13
1
(
b
+
+
compound 10 was 215 (19, M ), 157 (13, M - CO
2
CH
3
+ 1), 156
14
+
b
(
(
100, M - CO
2
CH
3
), 153 (9), 128 (13), 121 (12), 77 (10), and 51
6).
15
2-pentylfuran
993
1
17
6
octanal
benzaldehyde
1006
960
The GC-MS (relative intensity, ion structure) of tentatively assigned
+
+
+
compound 14 was 285 (23, M ), 228 (18, M - butyl), 226 (24, M
CO CH ), 168 (100), 156 (14), 149 (18), 136 (16), 121 (60), 91
1
2
8
0
methyl 2-oxo-2-phenylacetate
2-pentylpyridin
1285
1191
-
(
2
3
28), 80 (19), and 77 (15).
GC-MS Analyses. GC-MS analyses were conducted with a Hewlett-
a
b
Structures for these compounds are given in Scheme 1. These compounds
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.
Determination of Benzaldehyde (17) and Methyl 2-Oxo-2-
phenylacetate (18) Content by GC-FID. GC-FID 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 FID. Quantification of
benzaldehyde (17) and methyl 2-oxo-2-phenylacetate (18) was carried
out by preparing standard curves over a concentration range of 15-
75 nmol of benzaldehyde (17) or 50-1800 nmol of methyl 2-oxo-
-phenylacetate (18) in the 650 µL of solution prepared for GC-FID
injection (see above). For each curve, five different concentration levels
of the aldehyde were used. Benzaldehyde (17) and methyl 2-oxo-2-
phenylacetate (18) contents were directly proportional to the aldehyde/
internal standard area ratio (r > 0.99, p < 0.0001). The coefficients of
variation within this range were lower than 5%.
c
were tentatively assigned. This retention index corresponds to the dehydrated
compound.
and contribute by polymerization to the brown color and
fluorescence development in these reactions (18).
In addition to the formation of compounds 5 and 8, the
reaction between 4,5-epoxy-2-decenal (1) and octylamine (2)
may follow an alternative pathway to produce pyrroles and
furans with nine carbons (pathway b in Scheme 1) (15). The
mechanism of this reaction is not yet fully understood, but it
may be hypothesized to take place through the addition of the
amine to the epoxide producing an intermediate aminocarbonyl
derivative. This last compound should be the origin of 1-octyl-
1
2
2
-pentyl-1H-pyrrol (12) and 2-pentylfuran (15). Compound 15
appeared in the chromatogram, and a compound tentatively
assigned to the N-substituted 2-pentyl-1H-pyrrole 12 was also
present.
Other compounds, like 2-octenal and 2,4-decadienal, were
also produced in the reaction, and others could not be identified.
However, the detection of trace amounts of octanal (16) was
significant (see below). In addition, by increasing the pH from
RESULTS
4
to 8, the epoxyalkenal 1 disappeared completely and some
Reaction between 4,5-Epoxy-2-decenal (1) and Octylamine
2). The reaction between 4,5-epoxy-2-alkenals and primary
amines is very complex, and it has been the objective of different
studies. In fact, it seems to be the result of different competing
reactions. For this reason, when the reaction between 4,5-epoxy-
-decenal (1) and octylamine (2) was studied by GC-MS,
different compounds were detected (Table 1).
The major reaction product, at the three assayed pH values,
was the N-substituted pyrrole 8. According to previous studies
16, 17), this compound should be produced by the pathway a2
shown in Scheme 1. The reaction between the epoxyalkenal 1
and the amine 2 produces in the first step the imine A that it is
the precursor of a cyclic intermediate (pathway a). Depending
on the electronic rearrangement produced in this intermediate,
either the N-substituted pyrrole 8 (pathway a2) or the N-
substituted 2-(1-hydroxyhexyl)pyrrole 5 (pathway a1) may
evolve. Formation of pyrrole 8 should be accompanied by
production of hexanal (11), which was also identified in the
chromatogram.
changes in the relative proportions of the different products were
observed (data not shown).
(
Reaction between 4,5-Epoxy-2-decenal (1) and Benzyl-
amine (3). When the reaction was carried out between the
epoxyalkenal 1 and benzylamine (3), the formation of analogous
products to the above-described for the reaction between 4,5-
epoxy-2-decenal (1) and octylamine (2) was also observed. As
expected, pathway a produced 1-benzyl-1H-pyrrole (9) and
hexanal (11) as major products, and a compound tentatively
assigned as 1-(1-benzyl-1H-pyrrol-2-yl)hexan-1-ol (6) was also
produced to a lesser extent. In addition, the corresponding
products of pathway b were also produced. Thus, 1-benzyl-2-
pentyl-1H-pyrrole (13) and 2-pentylfuran (15) could be easily
identified in the chromatogram.
Nevertheless, the reaction between 4,5-epoxy-2-decenal (1)
and benzylamine (3) also produced other products that were
not so clearly observed in the above-described reaction between
the epoxyalkenal 1 and the octylamine (2). These compounds
were benzaldehyde (17) and 2-pentylpyridine (20). Previous
studies have shown that 2-pentylpyridine is produced when 4,5-
epoxy-2-decenal degraded amino acids (9). Analogously, the
Strecker type degradation of amines produced by epoxyalkenals
2
(
The product of pathway a1 (pyrrole 5) was tentatively
identified in the chromatogram on the basis of its mass spectrum.
Like other hydroxyalkylpyrroles, compound 5 should be unstable

Amine Degradation by Epoxyalkenals
J. Agric. Food Chem., Vol. 54, No. 6, 2006 2401
should also produce this pyridine in addition to the correspond-
ing Strecker aldehyde. Pathway c in Scheme 1 shows the
proposed mechanism for the Strecker type degradation of amines
produced by 4,5-epoxy-2-decenal (1). Thus, the imine A should
first be converted into the imine B, which after hydrolysis would
produce the hydroxyl amino derivative 19 and the aldehyde 17.
The internal cyclation reaction and aromatization of compound
1
9 should be the origin of the pyridine 20. This last reaction
was produced in more or less extension depending on the
reaction conditions.
Differently to pyrrole formation produced by pathways a and
b, the Strecker degradation of amines produced by pathway c
seemed to be very dependent on the reaction conditions, and
benzaldehyde (17) content seemed to decrease when the pH
was increased (see below).
Reaction between 4,5-Epoxy-2-decenal (1) and 2-Phenylg-
lycine Methyl Ester (4). The introduction of a methoxycarbonyl
group at the R-position of the amino group favored the Strecker
type degradation of the amine. Therefore, the proportions among
the products of pathways a, b, and c in the reaction of 4,5-
epoxy-2-decenal (1) with 2-phenylglycine methyl ester (4) were
very different to the above-described reactions between 4,5-
epoxy-2-decenal (1) and octylamine (2) or benzylamine (3).
Thus, although the products of pathway a [the corresponding
N-substituted 1H-pyrrole (10), the corresponding N-substituted
Figure 1. Effect of pH on Strecker aldehyde formation in the reaction of
4
,5-epoxy-2-decenal (1) with (A) benzylamine (3) and (B) 2-phenylglycine
methyl ester (4), in acetonitrile/buffer (2:1) after 21 h at 37 C. The
employed buffers were 50 mM sodium citrate buffer for pH 2.15 6 (
and 50 mM sodium phosphate buffer for pH 6 8 ( ). The Strecker
°
−
0)
−
O
aldehydes determined were benzaldehyde (17) (A) and methyl 2-oxo-2-
phenylacetate (18) (B).
2
-(1-hydroxyhexyl)-1H-pyrrole (7), and hexanal (11)] or path-
way b [the corresponding N-substituted 2-pentyl-1H-pyrrole (14)
and 2-pentylfuran (15)] could be easily identified in the
chromatogram, the Strecker aldehyde 18 produced by pathway
c was the major product of the reaction.
Analogously to the above-described for the reaction between
the 4,5-epoxy-2-decenal (1) and the benzylamine (3), the
proportion among the different products of pathways a, b, and
c in the reaction between 4,5-epoxy-2-decenal (1) with 2-phen-
ylglycine methyl ester (4) seemed also to depend on the reaction
pH.
Effect of pH on Strecker Aldehyde Formation in the
Reaction of 4,5-Epoxy-2-decenal (1) and Primary Amines.
The reaction of 4,5-epoxy-2-decenal (1) with primary amines
produced the Strecker type degradation of the amine to an extent
that depended on the involved amine and the reaction pH.
Figure 1 shows the amount of Strecker aldehyde produced as
a function of pH when starting from benzylamine (3) (Figure
Figure 2. Effect of epoxyalkenal concentration on Strecker aldehyde
formation in the reaction of 4,5-epoxy-2-decenal (1) with (A) benzylamine
(3) and (B) 2-phenylglycine methyl ester (4), in acetonitrile/50 mM sodium
1
A) or 2-phenylglycine methyl ester (4) (Figure 1B) at 37 °C.
citrate buffer (2:1) after 21 h at 37 ( ) or 60 C ( ). The reaction pH
0
°
O
Benzaldehyde (17), which is the Strecker aldehyde derived from
benzylamine (3), was produced to a higher extent at pH 2.5-4
values were 4 (A) and 6 (B). The Strecker aldehydes determined were
benzaldehyde (17) (A) and methyl 2-oxo-2-phenylacetate (18) (B).
(
2
Figure 1A). A higher pH decreased linearly (r ) -0.97, p )
.38 × 10 ) the amount of aldehyde produced, and benzalde-
-
4
with 2-phenylglycine methyl ester (4). These pH values
produced the highest amounts of benzaldehyde (17) and methyl
hyde was produced similarly in both citrate and phosphate
buffers.
2
-oxo-2-phenylacetate (18), respectively.
The effect of pH on methyl 2-oxo-2-phenylacetate (18)
formation (Figure 1B) was different to the above-described for
the formation of benzaldehyde (17). Thus, degradation of
Effect of 4,5-Epoxy-2-decenal (1) Concentration on Streck-
er Aldehyde Production. The effect of the concentration of
the epoxyalkenal on the Strecker type degradation of primary
amines is shown in Figure 2. The amount of Strecker aldehyde
produced increased linearly with the amount of epoxyalkenal
present in the reaction mixture for the two tested amines at the
two assayed temperatures. Thus, the amount of benzaldehyde
2
-phenylglycine methyl ester (4) increased from pH 2.15 to 6.0
when using sodium citrate buffer. However, when sodium citrate
was changed by sodium phosphate, methyl 2-oxo-2-phenyl-
acetate (18) was produced to a much lesser extent and continued
increasing from pH 6 to 7.
These results suggested that pH played an important role in
the Strecker type degradation of amines. In addition, the
optimum pH seemed to be specific for each amine. The pH
values selected for studying Strecker aldehyde production in
the rest of this study were pH 4 for the reaction with
benzylamine (3) and pH 6 (sodium citrate buffer) for the reaction
(
17) produced correlated linearly with the amount of 4,5-epoxy-
-
4
2-decenal (1) added at both 37 (r ) 0.99, p ) 2.86 × 10 )
-4
and 60 °C (r ) 0.99, p ) 3.17 × 10 ) (Figure 2A). In addition,
the amount of methyl 2-oxo-2-phenylacetate (18) produced was
also linearly correlated with the amount of 4,5-epoxy-2-decenal
(1) present in the reaction mixture at both 37 (r ) 0.9997, p <
0.0001) and 60 °C (r ) 0.9998, p < 0.0001) (Figure 2B).

2402 J. Agric. Food Chem., Vol. 54, No. 6, 2006
Zamora et al.
µmol methyl 2-oxo-2-phenylacetate/100 µmol 2-phenylglycine
methyl ester h, respectively.
Methyl 2-oxo-2-phenylacetate (18) formation at 60 °C also
followed zero-order reaction kinetics (r ) 0.999, p < 0.0001)
during the first 4 h and was also described by eq 2. After that,
methyl 2-oxo-2-phenylacetate concentration increased much
more slowly until a maximum value was achieved. The values
for MO0 and k at 60 °C were 2.03 µmol methyl 2-oxo-2-
phenylacetate/100 µmol 2-phenylglycine methyl ester and 10.53
µmol methyl 2-oxo-2-phenylacetate/100 µmol 2-phenylglycine
methyl ester h, respectively.
DISCUSSION
Strecker degradation of amino acids is one of the most
important reactions leading to final aroma compounds in the
Maillard reaction (1, 2). However, the results obtained in this
study suggest that amines can also be degraded in a similar
way, in accordance with previous studies showing that ninhydrin
reacts with amines via a Strecker type transamination reaction
(19, 20). According to the obtained results, amine degradation
produced by epoxyalkenals depended on the pH, the tempera-
ture, the amount of epoxyalkenal, and, mainly, the amine
involved.
The reaction yield depended on the reaction pH, and different
optimum pH values were obtained for the two assayed amines.
However, although the reaction yield was reduced to the third
part when the reaction was carried out at a pH far from the
optimum, the Strecker degradation of the amine was always
observed within the pH range studied (pH 2-8). Therefore, this
reaction should be produced to some extent with independence
of the pH if epoxyalkenals and suitable amines are in close
contact.
Figure 3. Time-course of Strecker aldehyde formation in the reaction of
,5-epoxy-2-decenal (1) with (A) benzylamine (3) and (B) 2-phenylglycine
methyl ester (4), in acetonitrile/50 mM sodium citrate buffer (2:1) at 37
) or 60 C ( ). The reaction pH values were 4 (A) and 6 (B). The
Strecker aldehydes determined were benzaldehyde (17) (A) and methyl
4
(
0
°
O
2-oxo-2-phenylacetate (18) (B).
For both amines, their corresponding Strecker aldehydes were
produced to a higher extent at 60 than at 37 °C. Thus, when
the epoxyalkenal/amine ratio was 1:1, the 2.4 or 4.3% of
benzylamine (3) was converted into benzaldehyde (17) after 21
h of reaction at 37 or 60 °C, respectively. The conversion of
2
-phenylglycine methyl ester (4) into methyl 2-oxo-2-phenyl-
acetate (18) was much higher, and 35.1 or 48.9% of the amine
was converted into its Strecker aldehyde after 21 h at 37 or 60
°
C, respectively.
Effect of Incubation Time on Strecker Aldehyde Produc-
tion in the Reaction of 4,5-Epoxy-2-decenal (1) with Primary
Amines. Strecker aldehydes were produced in the reaction of
The temperature of the reaction also influenced the reaction
yield to some extent, but it was determinant for the reaction
rate. Thus, the rate constant for benzaldehyde (17) formation
at 37 °C was 0.077 µmol benzaldehyde/100 µmol benzylamine
h, and it increased 9.5 times at 60 °C. In addition, the rate
constant for methyl 2-oxo-2-phenylacetate (18) formation at 37
4,5-epoxy-2-decenal (1) with the assayed amines at a reaction
rate that depended on the amine employed and the reaction
temperature (Figure 3). Thus, benzaldehyde (17) formation at
3
0
7 °C followed zero-order reaction kinetics (r ) 0.999, p <
.0001) and was described by eq 1:
°
C was 1.62 µmol methyl 2-oxo-2-phenylacetate/100 µmol
2
6
-phenylglycine methyl ester h, and it increased 6.5 times at
0 °C. Nevertheless, both assayed amines were degraded at 37
BA ) BA + kt
(1)
0
°
C. Because lipid oxidation is produced at low temperatures
(
21) and the reaction of lipid oxidation products with amines,
where BA0 represents the concentration of benzaldehyde in the
intercept, k is the rate constant, and t is the time. The values
for BA0 and k were 0.77 µmol benzaldehyde/100 µmol
benzylamine and 0.077 µmol benzaldehyde/100 µmol benzyl-
amine h, respectively.
Benzaldehyde (17) formation at 60 °C also followed zero-
order reaction kinetics (r ) 0.98, p ) 0.02) during the first 3 h
and was also described by eq 1. After that, benzaldehyde
concentration increased much more slowly until a maximum
value was achieved. The values for BA0 and k at 60 °C were
amino phospholipids, amino acids, and proteins is the last step
of the lipid peroxidation process (22-25), the Strecker degrada-
tion of suitable amines should be produced at low temperatures
as a common process within the lipid peroxidation pathway.
The amount of the epoxyalkenal determined the amount of
Strecker aldehyde produced because a linear relationship was
observed between 4,5-epoxy-2-decenal (1) concentration in the
reaction mixture and the amount of the produced aldehyde.
Therefore, even small amounts of epoxyalkenals should be
expected to degrade suitable amines to some extent.
0.47 µmol benzaldehyde/100 µmol benzylamine and 0.731 µmol
Although pH, temperature, and the amount of the aldehyde
played a significant role in the Strecker degradation of amines,
the yield of the Strecker aldehyde produced was mostly a
consequence of the structure of the amine involved. Thus,
octylamine, which has only one alkyl chain at the R-position
of the amino group, only produced trace amounts of octanal.
However, the change of an alkyl chain by a phenyl ring
increased considerably the amount of the Strecker aldehyde
produced. Thus, benzylamine (3) was converted into benzal-
dehyde (17) with a yield of almost 5%. This increase in the
reaction yield should be related to the easiness of conversion
benzaldehyde/100 µmol benzylamine h, respectively.
Methyl 2-oxo-2-phenylacetate (18) formation also followed
zero-order reaction kinetics (r ) 0.998, p < 0.0001) at 37 °C
and was described by eq 2:
MO ) MO + kt
(2)
0
where MO0 represents the concentration of methyl 2-oxo-2-
phenylacetate in the intercept, k is the rate constant, and t is the
time. The values for MO0 and k were 1.15 µmol methyl 2-oxo-
2-phenylacetate/100 µmol 2-phenylglycine methyl ester and 1.62

Amine Degradation by Epoxyalkenals
J. Agric. Food Chem., Vol. 54, No. 6, 2006 2403
Scheme 2. Alternative Pathways for the Strecker Type Degradation of
Amino Acids Produced by Epoxyalkenalsa
this study has been carried out with epoxyalkenals, it may be
hypothesized that R-dicarbonyl compounds produced in the
Maillard pathway will also be able to degrade suitable amines
in a similar way. In fact, when ribose and methyl 2-phenyl-
glycine (4) were incubated overnight at 60 °C under the same
conditions that are above-described for the epoxyalkenal 1,
methyl 2-oxo-2-phenylacetate (18) was produced as a major
reaction product (data not shown). Therefore, this reaction may
constitute a new alternative pathway for flavor formation in the
Maillard reaction.
ACKNOWLEDGMENT
We are indebted to Jos e´ L. Navarro for technical assistance.
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